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Doctoral Thesis

Electronic and magnetic ordering phenomena in multiferroic andcolossal magnetoresistive manganites

Author(s): Garganourakis, Marios

Publication Date: 2013

Permanent Link: https://doi.org/10.3929/ethz-a-009900674

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Diss. ETH No. 21109

ELECTRONIC AND MAGNETIC ORDERING PHENOMENA IN

MULTIFERROIC AND COLOSSAL MAGNETORESISTIVE

MANGANITES

Abhandlung zur Erlangung des Titels

DOKTOR DER WISSENSCHAFTEN

der

ETH Zurich

vorgelegt von

MARIOS GARGANOURAKIS

Physiker, Aristotle University of Thessaloniki, Greece

geboren am 1 Juli 1982

aus Griechenland

Angenommen auf Antrag von

Prof. Dr. J. F. van der Veen, Referent

Prof. Dr. M. Fiebig, Korreferent

Dr. U. Staub, Korreferent

2013

To the memory of my mother

I

Summary

This doctoral thesis presents studies on the magnetic and electronic properties of manganites.

Resonant soft X-ray diffraction experiments are used to directly probe the different electronic

order phenomena occurring in this class of materials. In doped perovskite manganites, the strong

interactions between spin, orbital and charge degrees of freedom induce several extraordinary

phenomena. For example, the colossal magnetoresistance effect and the charge and orbital order,

which are related to metal-insulator transitions. Studying these effects is the main part of this

thesis. Undoped manganite materials can belong to the group of multiferroics, materials which

simultaneously possess two or more primary ferroic properties. We studied the interplay between

the magnetic and electronic structure of orthorhombic perovskite manganites, which exhibit largest

value of electric polarization.

The first chapter describes how X-rays can imprint magnetic bits on an epitaxial thin film

of Pr0.5Ca0.5MnO3. To achieve that, the film was illuminated with a synchrotron soft X-ray

beam. During the X-ray illumination, a magnetic/orbital reflection was measured with the same

X-ray beam. These results were compared with in-situ resistance measurements of the thin film.

It is shown that the exposure of the film to X-rays reduces the resistance of the material. In

addition, the illumination increases the magnetic/orbital reflection’s intensity. We even observe

that antiferromagnetic order is improved upon X-ray illumination. Both effects are found to be

persistent in time. The change in magnetic scattering intensity and in conductivity is attributed

to the tilting of the manganese magnetic moments. These tilts are caused by X-ray photodoping

of the Pr0.5Ca0.5MnO3 thin film.

It is still a matter of debate whether or not application of an external electric field can switch

the high-resistive phases of manganites (or other strongly correlated semiconductors) and induce

a metal-insulator transition. To understand such an effect, a high voltage was applied to the

Pr0.5Ca0.5MnO3 thin film and resonant soft X-ray diffraction was used to probe the electronic

II

states. Measuring the resistance and a magnetic/orbital reflection in-situ, shows that resistive

heating plays a significant role and is, most probably, the mechanism for the large hysteresis

observed in the resistance of the film.

In the last chapter we present experiments on orthorhombic (o)RMnO3 perovskites with

R=Tm, Y, Lu. These three multiferroic materials are interesting since they exhibit strong cou-

pling between magnetism and ferroelectricity and have higher electric polarization values than

other (o)RMnO3 perovskites. The magnetic and orbital order of the Mn and R sublattices are

investigated by resonant soft X-ray diffraction in polycrystalline TmMnO3 and LuMnO3 and in

an epitaxial single-crystal YMnO3 film. The results show that for the perovskite with R=Tm,

the Tm magnetic moments already order in the non-ferroelectric incommensurate magnetic phase.

Additionally, deviations from the suggested collinear E-type Mn magnetic structure at low tem-

peratures are found for both the TmMnO3 and the LuMnO3 compounds. Finally, in the epitaxial

YMnO3 film, coexistence of an E-type and a cycloidal magnetic state was observed as predicted

by theory.

III

Zusammenfassung

In dieser Doktorarbeit wird beschrieben, wie durch resonante Streuung von Rontgenstrahlen die

magnetischen und elektronischen Eigenschaften von Manganaten charakterisiert werden konnen.

Dies wird durch die direkte Untersuchung von magnetischer und orbitaler Ordnung ermoglicht. In

Perovskit-Manganat-Materialien fuhren die Wechselwirkungen von Spin, Orbitalen und Ladungs-

freiheitsgraden zu diversen bemerkenswerten Phanomenen. Diese sind unter anderem eine starke

Magnetfeldabhangigkeit des elektrischen Widerstands (“colossal magnetoresistance”) und Metall-

Isolatorubergange, welche den Schwerpunkt dieser Arbeit bilden. Manganate gehoren zur Gruppe

der Multiferroika, d.h. Materialien, die zwei oder mehr ferroische Eigenschaften besitzen. Ausser-

dem wird das Wechselspiel der magnetischen und elektronischen Struktur orthorhombischer Per-

ovskit - Manganate untersucht, welche eine grosse elektrische Polarisation aufweisen.

Im ersten Kapitel wird erklart, wie mit Hilfe von Rontgenstrahlung binare magnetische und

elektronische Information auf eine epitaktisch gewachsene Pr0.5Ca0.5MnO3 Dunnschicht geschrieben

werden kann. Wahrend der Bestrahlung mit weicher Rontgenstrahlung, wurde mit demselben

Rontgenstrahl die magnetische/orbitale Reflexion gemessen. Anschliessend wurden die Resul-

tate mit in-situ Widerstandsmessungen an der Dunnschicht verglichen. Es zeigte sich, dass der

Widerstand der Materials sich wahrend der Bestrahlung verringert und gleichzeitig die Inten-

sitat der magnetischen/orbitalen Reflexion zunimmt. Es konnte sogar beobachtet werden, dass

sich die antiferromagnetische Ordnung durch die Bestrahlung mit Rontgenlicht verbessert. Un-

sere Untersuchungen zeigen, dass die beiden beschriebenen Effekte zeitlich von Bestand sind.

Die Anderung der magnetischen Streuintensitat kann auf eine Verkippung der magnetischen Mo-

mente der Manganatome zuruckgefuhrt werden, welche durch eine photoninduzierte Dotierung

der Pr0.5Ca0.5MnO3 Dunnschicht bei der Bestrahlung mit Rontgenlicht verursacht wird.

Es wird weiterhin noch eine andere Problemstellung angesprochen. Dies ist die Frage, ob ein

externes elektrisches Feld die hochresisitive Phase von Pr0.5Ca0.5MnO3 (oder anderer stark kor-

IV

relierter Halbleiter) in eine niedrigresistive umschalten kann. Die Ergebnisse einer direkten Un-

tersuchung der elektronischen Zustande durch die Beobachtung einer magnetischen/orbitalen Re-

flexion mit Hilfe von resonanter Beugung weicher Rontgenstrahlung zeigen, dass die Deponierung

von Warme durch die Strahlung eine signifikante Rolle spielt. Sie ist hochstwahrscheinlich der

Schlusselmechanismus fur die beobachtete grosse Widerstandshysterese der Dunnschicht.

Im letzten Kapitel dieser Arbeit werden Experimente an orthorhombischen Perovskiten,

(o)RMnO3 mit R=Tm, Y, Lu, angesprochen. Diese drei multiferroischen Materialien sind sehr

interessant, da sie eine starke Kopplung zwischen Magnetismus und ferroelektrischer Ordnung

aufweisen und weiterhin eine starke elektrische Polarisation im Vergleich zu anderen (o)RMnO3

Perovskiten zeigen. Die magnetische und orbitale Ordnung von Mn und R Untergittern werden in

polykristallinem TmMnO3 and LuMnO3 untersucht, sowie in einer epitaktischen dunnen Schicht

von YMnO3. Die Experimente zeigen, dass im Perovskit mit R=Tm, die magnetischen Momente

des Thulium sich bereits in einer nicht-ferroelektrischen inkommensurablen magnetischen Phase

ordnen. Weiterhin werden Abweichungen von der kollinearen Typ E Manganatstruktur bei tiefen

Temperaturen in TmMnO3 und LuMnO3 beobachtet. Schliesslich wird die Koexistenz eines Typ

E Zustands und eines zykloidalen Zustands in YMnO3 gezeigt, entsprechend den theoretischen

Vorhersagen.

CONTENTS V

Contents

Summary I

Zusammenfassung III

1 X-ray scattering 2

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Synchrotron radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3 Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.3.1 Single-crystal diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.3.2 One-atom diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.3.3 Powder diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.4 Resonant X-ray scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.5 Magnetic resonant X-ray scattering cross section . . . . . . . . . . . . . . . . . . . 13

1.6 Experimental set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2 Colossal magnetoresistive (CMR) manganese oxides 20

2.1 Fundamental properties and theory of CMR manganites . . . . . . . . . . . . . . 20

2.1.1 Correlated electron physics and the physics of manganites . . . . . . . . . 20

2.1.2 The colossal magnetoresistance effect . . . . . . . . . . . . . . . . . . . . . 23

2.1.3 The properties of Pr1−xCaxMnO3 . . . . . . . . . . . . . . . . . . . . . . . 25

2.2 Imprinting magnetic bits in Pr0.5Ca0.5MnO3 with X-rays . . . . . . . . . . . . . . 26

2.3 Switching of resistive phases in Pr0.5Ca0.5MnO3 using electric field . . . . . . . . . 43

2.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

3 The E-type ordered orthorhombic RMnO3 systems 60

CONTENTS VI

3.1 Introduction to multiferroics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

3.2 Resonant soft X-ray powder diffraction from RMnO3 (R=Tm, Lu) . . . . . . . . . 63

3.2.1 Tm M 5 edge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

3.2.2 Mn L2,3 edges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

3.3 Resonant soft X-ray diffraction from single-crystalline YMnO3 film . . . . . . . . . 73

3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

4 Conclusions and outlook 84

Acknowledgments 87

Curriculum vitae 89

CONTENTS 1

2

Chapter 1

X-ray scattering

1.1 Introduction

On the evening of 8 November 1895, Wilhelm Rontgen (Figure 1.1 left) observed something awk-

ward in his darkened room; a barium platinocyanide plate fluoresced due to a high-voltage dis-

charge tube enclosed in thick black cardboard two meters away from it. He concluded that the

new unknown form of radiation could escape the glass tube as well as the covering. It was the

unidentified nature of this new type of radiation which led him to name it X-rays [1].

Several experiments followed that discovery. One of the most famous is the image of his Swiss

colleague’s hand, the anatomist Albert von Kolliker taken at Wurzburg (Figure 1.1 right). He

placed the hand of his colleague in the path of the X-rays showing that objects with different

thicknesses have different transparency. He concluded that X-rays are the product of the interac-

tion of high-energy electrons with a material. The first Nobel Prize in the history of physics was

awarded to him in 1901.

X-rays turned out to be quite useful; they provided a powerful technique to study the structures

of crystals and were widely used in medicine. One of the greatest discoveries in the history of

X-rays was the determination of the structure of deoxyribose nucleic acid (DNA) (Figure 1.2) and

its replication mechanism (Nobel Prize in medicine to James Watson, Francis Crick and Maurice

Wilkins in 1953).

X-ray physics has allowed for a major insight in many disciplines and has influenced the twen-

tieth century through numerous discoveries and studies. Table 1 (reproduced from [2]), presents

the variety of disciplines in which X-ray science was applied and Nobel prizes were awarded.

1.1 Introduction 3

Figure 1.1: Left hand side: Wilhelm Rontgen. Right hand side: The hand of Albert von Kollikerplaced in the path of the X-rays.

Figure 1.2: X-ray diffraction images of DNA salt crystals recorded by Rosalind Franklin [3].

Table 1 Nobel Prizes in X-ray science

Year Recipient(s) Discipline

1901 W.C. Rontgen Physics; discovery of X-rays

1914 M. von Laue Physics; X-ray diffraction from crystals

1915 W.H. Bragg and W.L. Bragg Physics; crystal structure derived from X-ray diffraction

1917 C.G. Barkla Physics; characteristic radiation of elements

1924 K.M.G. Siegbahn Physics; X-ray spectroscopy

1927 A.H. Compton Physics; for scattering of X-rays by electrons

1936 P. Debye Chemistry; diffraction of X-rays and electrons in gases

1962 M. Perutz and J. Kendrew Chemistry; structures of myoglobin and haemoglobin

1962 J. Watson, M. Wilkins and F. Crick Medicine; structure of DNA

1979 A. McLeod Cormack and G. Newbold Hounsfield Medicine; computed axial tomography

1981 K.M. Siegbahn Physics; high-resolution electron spectroscopy

1985 H. Hauptman and J. Karle Chemistry; direct methods to determine X-ray structure

1988 J. Deisenhofer, R. Huber and H. Michel Chemistry; determining the structure of proteins crucial to photosynthesis

2003 R. MacKinnon and P. Arge Chemistry; structure and operation of ion channels

2006 R.D. Kornberg Chemistry; atomic description of DNA transcription

2009 V. Ramakrishnan, T.A. Steitz and A.E. Yonath Chemistry; structure and function of the ribosome

1.1 Introduction 4

An X-ray is a transverse electromagnetic wave whose electric (E) and magnetic (H) fields are

perpendicular to each other and to the direction of propagation k (Figure 1.3).

Figure 1.3: X-ray: a transverse electromagnetic wave [4].

It is identified by a wavelength λ, or a wavenumber k=2π/λ. In three dimensions the electric

field is given by

E(r, t) = εE0ei(k·r−ωt), (1.1)

where r is the position vector, t is the time, ε the polarization unit vector of X-ray electric field

and ω the angular frequency.

The relation between the wavelength λ and the energy of the photon is given by

λ[A] =hc

~ω=

12.398

~ω[keV ], (1.2)

where c=3·108m·s−1 is the speed of light, h=6.6·10−34J·s is the Planck constant with ~ =h/2π

and ~ω the photon energy.

The wavelength of an X-ray is typically 1 Angstrom (10−10m). An interatomic distance in a

solid is also of the order of an Angstrom. Thus, X-rays probe atomic length scales.

The first X-ray diffraction experiments were performed by W. H. and W. L. Bragg in 1913

[5]. Over the years, the X-ray diffraction technique became indispensible to determine crystal

1.2 Synchrotron radiation 5

structures. The development of powerful synchrotron radiation sources followed in order to meet

the requirements of a fast developing field.

1.2 Synchrotron radiation

Nowadays, synchrotron light sources play a leading role in investigating crystal structures of

materials. The tunability of the photon energy, as well as the high flux provided, make these

sources essential tools for studies of matter. Modern synchrotron sources generate beams of

electromagnetic radiation with wavelengths extending from the IR to the hard X-ray region.

Their main advantage is their orders-of-magnitude greater intensity compared to laboratory based

sources.

For the resonant soft X-ray scattering experiments presented here, the use of synchrotron light

is crucial. Various types of ordering can be probed: magnetic, charge and orbital ordering. This is

achieved by using the tunability of the X-ray beam energy, which enables the selection of particular

signals from ions, through their corresponding atomic resonances. Moreover, different polarizations

of the incident radiation in combination with polarization analysis of the diffracted beam allows

us to distinguish between the different contributions to the scattering. For detecting the weak

magnetic and orbital signals the high flux of synchrotron light is needed since the polarization

analysis may reduce the diffracted intensity even more.

Figure 1.4: Left hand side: layout of a third-generation synchrotron light source. Right hand side:aerial view of the Paul Scherrer Institut, with the Swiss Light Source (indicated by the orangearrow). Figure reproduced from [2].

One of the most important characteristics of synchrotron light is its brilliance. It is defined as

1.2 Synchrotron radiation 6

brilliance[ph s−1 (0.1%bandwidth)−1 mm−2 mrad−2] =flux

(2π)2εxεy, (1.3)

which is the flux per unit source area, unit solid angle and 0.1% energy bandwidth. εx and εy

are the horizontal and vertical emittances of the storage ring, respectively, with emittance being

a measure for the average spread of particle coordinates in position-and-momentum phase space.

For an undulator of a third-generation synchrotron source, the brilliance is approximately 1020

photons/s/mrad2/mm2/0.1% bandwidth.

A synchrotron light source has the following components (Figure 1.5):

1. A source of electrons generated in an electron gun. The electrons are accelerated in a linac.

2. A booster ring in which the electrons are accelerated further.

3. The electron storage ring with magnets maintaining the electrons on a closed path. X-rays

are generated in bending magnets (BMs) or insertion devices (IDs). The latter generate

more intense synchrotron radiation.

4. A radio frequency (RF) supply to re-accelerate the electrons, compensating the energy loss

due to the emission of synchrotron radiation.

5. The beamlines, which are tangential to the storage ring and have either a BM or an ID as

a source.

Figure 1.5: Sketch of a synchrotron light source. Figure reproduced from [2].

1.3 Diffraction 7

1.3 Diffraction

1.3.1 Single-crystal diffraction

A crystal is defined as a periodic arrangement of unit cells, containing molecules or atoms. The

primitive unit cell is the smallest volume of a parallelopipedon defined by the crystal axes, which

when repeated over the unit cell translation vectors a1, a2 and a3, creates the crystal. The corners

of the unit cells, the lattice points, lie on the end points of the vectors

Rn1 ,n2 ,n3 = n1a1 + n2a2 + n3a3, (1.4)

where n1, n2 and n3 are integer numbers. The volume of the unit cell is given by

vuc = a1 · (a2 × a3). (1.5)

We consider now the family of parallel lattice planes (hkl). These crystallographic planes are a

set of parallel equally distant planes which fulfill the following conditions: one plane goes through

the origin of the unit cell (“0 ”) and the next plane cuts the vectors at the distances a1/h, a2/k

and a3/l (Figure 1.6).

Figure 1.6: The parallel lattice planes (hkl), with n the unit vector perpendicular to the (hkl)planes.

1.3 Diffraction 8

In 1913 W.H. Bragg and his son W.L. Bragg used a broad spectrum of light for their diffraction

experiment. They explained the peaks of an X-ray diffraction pattern considering a crystal lattice

with planes separated by a distance d [5]. They expressed this observation in the now famous

equation:

λ = 2dhklsinθ, (1.6)

where dhkl is the distance between lattice planes (hkl), h, k and l are the Miller indices and θ the

angle between the incident beam and the lattice planes (Figure 1.7).

Figure 1.7: Bragg diffraction from lattice planes, separated by a distance d, where the pathdifference for rays reflected from adjacent planes is given by 2dsinθ.

We introduce the momentum transfer vector Q=kout-kin, with kout and kin the wavevectors of

the diffracted and incident beams (kout = kin = 2π/λ). Bragg’s law is satisfied for Q perpendicular

to the lattice planes and

Q =2π

dhkl. (1.7)

We also define the set of translation vectors b1, b2 and b3 in reciprocal space,

b1 = 2πa2 × a3

vuc,b2 = 2π

a3 × a1

vuc,b3 = 2π

a1 × a2

vuc. (1.8)

These vectors form the reciprocal lattice and satisfy the condition

ai · bj = 2πδij, (1.9)

1.3 Diffraction 9

where δij is the Kronecker delta function defined as δij = 1 for i=j and δij = 0 for i 6= j.

Bragg’s law can now be reformulated as

Q = Ghkl, (1.10)

where Ghkl is perpendicular to the (hkl) planes and is given by

Ghkl = hb1 + kb2 + lb3. (1.11)

If rj are the positions of the atoms within the unit cell, the amplitude scattered from the

crystal can be written as

Fcrystal(Q) =∑j

fj(Q)eıQ · rj∑n1

∑n2

∑n3

eıQ ·(Rn1,n2 ,n3), (1.12)

where the first sum represents the unit cell structure factor and the second the interference factor.

The fj(Q) are the atomic form factors for the different atoms j in the unit cell. They are introduced

in the next section. For more details, see [6] and [7].

1.3.2 One-atom diffraction

We will consider now elastic scattering by an atom with Z electrons. Using a classical description,

we define here the density probability of electron in orbit i, ρ(r) [8]. The radiation field that is

scattered can be viewed as a superposition of the contribution of the different volume elements of

this charge distribution (Figure 1.8). One has to track the change in phase of the incident wave

as it interacts with the volume element at the origin and the one at a given position r. This phase

difference between the two volume elements is 2π multiplied by the ratio of r, projected onto the

incident direction, and the wavelength. Considering an observation point Ξ, the scattered wave is

locally a plane wave with kout as a wave vector.

The phase difference between the scattered wave from a volume element around r and around

the origin is [8]

∆(φ) = (kout − kin) · r = Q · r. (1.13)

At a position r, a volume element dr will contribute the amount r 0ρ(r)dr to the scattered

field with a phase factor eiQ·r. r 0 is the Thomson scattering length

1.3 Diffraction 10

r0 =e2

4πε0mc2= 2.82× 10−5A, (1.14)

where e is the electron charge, m the mass of electron and ε0 the permittivity of free space. The

total scattering length of the atom is [8]

r0f0(Q) = r0

∫ρ(r)eiQ·rdr, (1.15)

where f 0(Q) is the atomic form factor.

However, the scattering process in an atom is in reality more complex. We have to add more

terms to the atomic form factor when the incident photon energy is close to one of the absorption

edges. In this case the atomic form factor is given by the following equation [8]:

f(Q, ~ω) = f 0(Q) + f ’(~ω) + if”(~ω), (1.16)

where f ’(~ω) and f”(~ω) are the dispersion corrections to f 0(Q).

Figure 1.8: Sketch of one-atom diffraction.

The dispersion corrections to the atomic form factor have a negligible Q dependence and

are maximal when the X-ray energy equals the energy of an absorption edge of an atom. The

imaginary term f”(~ω) is proportional to the atomic absorption and represents the dissipation in

the system [8]:

f”(~ω) =ω

4πr0cσA(ω), (1.17)

1.3 Diffraction 11

where σA is the atomic absorption cross section. The real part, f ’(~ω), is related to the imaginary

f”(~ω) part via the Kramers-Kronig relation:

f ’(~ω) =2

πP

∫ ∞0

~ω′f ′′(~ω′)(~ω′)2 − (~ω)2

d(~ω′), (1.18)

where P is the Cauchy principal value.

1.3.3 Powder diffraction

Powder diffraction, or the Debye-Scherrer method, has become more powerful over the years.

Compared with other methods of analysis, powder diffraction is a non-destructive method to ana-

lyze multi-component mixtures created without the need for extensive sample preparation. It can

be applied to different fields, such as metallurgy, mineralogy, archeology, biology, pharmaceutical

sciences and condensed matter physics (as shown in Chapter 3.2). For some materials, the growth

of single crystals could be either challenging or impossible, thus diffraction from a powder is an at-

tractive alternative. On the other hand, reflections may overlap in a powder diffraction experiment

due to the randomly oriented microcrystals. In a powder diffraction experiment, the detector may

cover a large 2θ range (depending on the experimental set-up) with respect to the incoming X-ray

beam direction (Figure 1.9). It will detect diffracted signal only when the diffraction conditions

are met, i.e. only when there are crystal planes (hkl) at an angle θ = arcsin(λ/2dhkl) relative to

the incoming beam (red circle in Figure 1.9).

Figure 1.9: Condition for powder diffraction. Only a few crystals (red here) are oriented such thatthe Laue condition is fulfilled.

The cylindrical symmetry of the set-up about the incident beam axis (Figure 1.10, top) pro-

duces cones of diffracted signal, known as Debye-Scherrer cones (Figure 1.10, bottom).

1.4 Resonant X-ray scattering 12

Figure 1.10: Top: the cones of diffracted signal are produced about the incident beam axis χ,in a cylindrical symmetry. Bottom: fractions of Debye-Scherrer cones from a powder diffractionexperiment. Figure reproduced from [2].

1.4 Resonant X-ray scattering

Resonant X-ray scattering (RXS) offers a unique way to investigate electron localization in mate-

rials. Of particular interest is the soft X-ray energy regime (0.09 to 2.5 keV), since it contains the

L absorption edge of e.g. Mn, Ni, Fe, Cu and other important elements. The magnetic properties

of these elements are largely determined by the 3d valence electrons. Since RXS is governed by

dipole selection rules the d -shell properties are best probed by L-edge studies (2p-3d dipole tran-

sitions). In this way ”element specific” experiments can be carried out when the energy of the

incoming X-ray beam is tuned to a specific absorption edge of an ion contained in the material.

Resonant elastic scattering is a two-step process. When an X-ray photon is absorbed by an

electron occupying a core level |µ >, it is excited to an unoccupied intermediate state |η > which

afterwards decays to the initial core level |µ > by emitting an X-ray photon with the same energy

as the incoming one. Hence this scattering is called elastic (Figure 1.11). In the soft X-ray regime

the probability of absorption or emission is dominated by the dipole transition (E1) between the

lower (core level) and upper (intermediate) states as determined by selection rules.

As aforementioned, there are three specific types of ordering which can be studied by resonant

scattering: charge, orbital and magnetic. Non-resonant magnetic and orbital scattering is several

orders of magnitude weaker than Thomson scattering, and was thus not widely applied until recent

1.5 Magnetic resonant X-ray scattering cross section 13

Figure 1.11: The second-order resonant elastic scattering process. An incoming photon withenergy ~ω excites a core level electron and after relaxation an outgoing photon with the sameenergy is emitted.

years. According to second-order perturbation theory, the resonant magnetic scattering amplitude

can be expressed in terms of operators as [9]

f = −(r0

m

)∑η(∆)

〈[〈µ|ε′ · J(kout)|η〉〈η|ε · J(kin)|µ〉]〉~ω −∆ + iΓ/2

, (1.19)

where ∆=En-Eµ is the resonant energy and Γ the lifetime of the state |η〉. The resonant energy

and the width of the resonance have a weak dependence on the intermediate states and thus

the energy profile can be often well described by a single damped harmonic oscillator. J(k)=∑(pj + isj × k)eik·Rj is the current operator with pj the electron linear momentum, Rj the

position and sj the spin operator, with ε and ε′ the polarization vectors of the incoming and

outgoing X-rays, respectively.

1.5 Magnetic resonant X-ray scattering cross section

Linear light polarization lying in the scattering plane (defined by kin and kout) is called π polar-

ization. When the polarization is perpendicular to the scattering plane, it is called σ polarization.

In Figure 1.12 the experimental scattering geometry is illustrated in a Cartesian system. With

this choice of axes, the σ polarization is parallel to the z -axis and Q is along the x -axis.

Close to an absorption edge, the scattering factor can be expressed in the dipole approximation

as [10]

fXRESnE1 = (ε′ · ε)F (0)n − i(ε′ × ε) · ζnF (1)

n + (ε′ · ζn)(ε · ζn)F (2)n , (1.20)

1.5 Magnetic resonant X-ray scattering cross section 14

Figure 1.12: Directions of π and σ light polarization in a Cartesian coordinate system.

where the first term, F(0)n , contributes to the charge Bragg peak and contains no dependence on

the magnetic moment, the second, F(1)n , produces first-harmonic magnetic satellites and the third,

F(2)n , produces second-harmonic magnetic satellites. ζn is a unit vector in the direction of the nth

ion’s magnetic moment. One can write Equation (1.20) as a 2 × 2 matrix. The first term connects

only states for which the polarization is not changed. For example, an ε|| photon is scattered into

an ε′|| (ππ′ scattering) and ε⊥ into an ε′⊥ (σσ′ scattering). This matrix is diagonal with ε′⊥ ·

ε⊥=1 and ε′|| · ε|| = cos2ϑ = kin· kout, so

(ε′ · ε) =

1 0

0 kout · kin

. (1.21)

The second term allows σπ′ scattering and ππ′ scattering, but σσ′ scattering is forbidden, so

the matrix representation is

(ε′ × ε) · ζn =

0 kin

−kout kout × kin

· ζn, (1.22)

where ε′|| × ε⊥ = -kout and ε′⊥ × ε|| = kin.

Finally, we can express the resonant dipole scattering amplitude as [10]

fXRESnE1 = F (0)n

1 0

0 cos2θ

− iF (1)n

0 ζn,ycosθ + ζn,xsinθ

ζn,xsinθ − ζn,ycosθ −ζn,zsin2θ

1.6 Experimental set-up 15

+F (2)n

ζ2n,z −ζn,z(ζn,ysinθ − ζn,xcosθ)

ζn,z(ζn,zsinθ + ζn,xcosθ) −cos2θ(ζ2n,ytan2θ + ζ2

n,x)

, (1.23)

where θ is the Bragg angle and ζn,x, ζn,y, ζn,z are the components of the vector ζn along the

Cartesian x,y and z axes. Equation 1.23 is useful for determining which components of a magnetic

moment contribute to the scattered signal for the different polarization channels.

1.6 Experimental set-up

The experiments were performed at the resonant soft X-ray diffraction (RESOXS) end-station,

connected to the Surface Interface Microscopy (SIM) beamline at the Swiss Light Source, Paul

Scherrer Institut, Switzerland. The SIM beamline features two 3.8 m long Apple II type undulators

and a plane grating monochromator (Figure 1.13) [15]. The Apple II undulator can produce linear

and circular polarization. The ability to vary the polarization is crucial for the study of magnetic

and orbital ordering phenomena. The energy range of the delivered photons is 90-2000 eV and the

energy resolution of the beamline is in the order of 0.1 eV. The light coming out of the undulators

is collimated in the vertical direction by a horizontally deflecting mirror. Downstream, a toroidal

mirror is used for astigmatic focusing, horizontal to a mechanical chopper and vertical to the exit

slit. The first toroidal refocusing mirror images the intermediate foci to the photoemission electron

microscope mounted on the SIM beamline, where a second toroidal refocusing mirror images to

the end-station connected to the beamline (e.g. RESOXS). This enables a match between the

beam size and the sample’s dimensions.

The RESOXS end-station is an in-vacuum diffractometer, in which the top flange holds the

cryostat and the sample manipulator. For the rotation of the sample and the detector arm,

differentially pumped rotary feedthroughs are used. The rotary feedthrough for the detectors is

mounted on the bottom part of the main UHV chamber.

A full 3600 rotation can be achieved for the sample and detector rotations. Several detectors

are mounted on the detector arm: an in-vacuum watercooled CCD camera, a photo-diode and

a set-up for polarization analysis. The CCD camera is the most sensitive detector and is ideal

for weak signals and for powder diffraction experiments. The polarization analyser consists of a

graded W/C multilayer with varying bi-layer thicknesses and a photo-diode to collect the diffracted

X-rays. This multilayer can be translated with a stepper motor to adjust the d -spacing of the


Figure 1.13: Optical layout of the SIM beamline in Paul Scherrer Institut, Switzerland. Figurereproduced from [15].

multilayer Bragg reflection to the chosen X-ray energy (Figure 1.14) [12].

Figure 1.14: Left hand side: sketch of the 2θ arm with the detectors mounted. Right hand side:the inner part of the RESOXS end-station.

The sample manipulator moves horizontally (xx,zz motions) and vertically (yy motion) (Figure

1.14), and is mounted above the sample rotation axis. In order to perform azimuthal angle scans,

three pins spaced 1200 degrees apart are attached to the sample holder. They allow a manual

azimuthal rotation (ϕ) as well as a replacement of the sample using the in-vacuum sample transfer

system. In addition, the sample holder can be tilted ± 60 with respect to the horizontal level.

Finally the sample holder is mounted on a gold coated cold-finger, supported by a He-flow cryostat


to achieve temperatures down to ' 10 K.

BIBLIOGRAPHY 18

Bibliography

[1] J. Maddox, “The Sensational Discovery of X-rays.”, Nature, 375, 183, 1995.

[2] P. Willmott, “An Introduction to Synchrotron Radiation. Techniques and Applica-

tions”, Wiley, 2011.

[3] J.D. Watson and F.H.C. Crick, “Molecular structure of nucleic acids - A structure for

deoxyribose nucleic acid.”, Nature, 171, 737, 1953.

[4] http://chempaths.chemeddl.org/services/chempaths/?q=book/General

%20Chemistry%20Textbook/Spectra%20and%20Structure%20of

%20Atoms%20and%20Molecules/1917/spectramolecules

&title=CoreChem:The Nature of Electromagnetic Radiation

[5] W.L. Bragg, “The diffraction of short electromagnetic waves by a crystal.”, Proceeding

of the Cambridge Philosophical Society, 17, 43, 1913.

[6] B.E Warren, “X-ray Diffraction.”, Dover Publications, 1990.

[7] J.F. van der Veen and B. Schonfeld, “Materials Research using Synchrotron Radiation”,

http://people.web.psi.ch/vanderveen/MarRes HS12/SynchAppl JFvdV BS.pdf.

[8] J. Als-Nielsen and D. McMorrow, “Elements of Modern X-Ray Physics.”, Wiley, 2001.

[9] S.W. Lovesey, E. Balcar, K.S. Knight and J.F. Rodriguez, “Electronic properties of

crystalline materials observed in x-ray diffraction.”, Phys. Reports, 411, 233, 2005.

[10] J.P. Hill and D.F. McMorrow, “X-ray Resonant Exchange Scattering: Polarization

Dependence and Correlation Functions.”, Acta Cryst. A, 52, 236, 1996.

BIBLIOGRAPHY 19

[11] U. Flechsig, F. Nolting, A. Fraile Rodrıguez, J. Krempasky, C. Quitmann, T. Schmidt,

S. Spielmann and D. Zimoch, “Performance measurements at the SLS SIM beamline.”,

AIP Conference Proceedings 1234, 319, 2010.

[12] U. Staub, V. Scagnoli, Y. Bodenthin, M. Garcıa-Fernandez, R. Wetter, A.M. Mulders,

H. Grimmer and M. Horisberger, “Polarization analysis in soft X-ray diffraction to

study magnetic and orbital ordering.”, J. Synchr. Rad., 15, 469, 2008.

20

Chapter 2

Colossal magnetoresistive (CMR)

manganese oxides

2.1 Fundamental properties and theory of CMR mangan-

ites

2.1.1 Correlated electron physics and the physics of manganites

The term “manganites” appeared for the first time in a 1950 publication by Jonker and Van Santen

[1]. In that work, this term is used for compounds that contain the transition metal ion manganese.

They observed that when replacing La with Ca, Sr or Ba in polycrystalline LaMnO3, the material

exhibits ferromagnetism. The appearance of ferromagnetism was attributed to indirect exchange

interactions, which were later called double exchange interactions. Moreover, they found that the

manganese ion is in the centre of a perovskite cell, with lattice constant ap surrounded by oxygens

in an octahedral configuration (Figure 2.1).

The electrons localized at neighbouring atomic sites give rise to a crystal field potential due to

the Coulomb interaction. This lifts the degeneracy of the d electron levels of the central Mn atom.

The splitting of the manganese 3d states results in orbital wavefunctions pointing towards the

O2− ions (the eg orbitals) with higher energy than the ones pointing between them (t2g orbitals)

(Figure 2.2). A further deformation of the MnO6 octahedron is observed in manganites due to the

Jahn-Teller distortion which induces elongation of the crystal within the x,y plane and shrinking

along the z -axis. This results in a further removal of the t2g and eg degeneracy.

2.1 Fundamental properties and theory of CMR manganites 21

Figure 2.1: A cubic perovskite structure ABO3 in which the transition metal Mn ion (B-site) issurrounded by six oxygen ions. The cations A are at the edges of the cube (A-site).

Figure 2.2: Sketch of the 3d levels in spherical (left) and cubic symmetry (middle) with thecorresponding wavefunction representation of the two eg and three t2g states (right).

To characterize the stability and distortion level of the perovskite crystal structure, one often

uses the tolerance factor f which is defined as

f = (rA + rO)/√

2(rB + rO), (2.1)

where rA, rB and rO represents the average ionic radius of the element at site A,B or O (Figure

2.1). There are four ranges for the tolerance factor in which different crystal structures are formed.

For f > 1 the structure is hexagonal (e.g. BaNiO3), for 0.96 < f < 1 is cubic (e.g. SrTiO3), for

0.71 < f < 0.96 the structure is orthorhombic or rhombohedral (e.g. GdFeO3, CaTiO3). For f <

0.71 the structure can be of different type (e.g. Al2O3, FeTiO3). A distortion of the lattice results


in a decrease of the O-Mn-O bond angles. This in turn affects the electron hopping between the

different sites (i.e., the resistivity) as well as the magnetic superexchange interactions.

In the first experimental results obtained on manganites, anomalous effects were observed in

the magnetoresistance [2]. The resistivity of manganites in the ferromagnetic state decreased

significantly in applied magnetic fields. Neutron scattering experiments by Wollan and Koehler

[3] showed that these materials possess not only ferromagnetic phases, but also antiferromagnetic

ones. The proposed basic magnetic structures of transition metal perovskites are visualized in

Figure 2.3.

Figure 2.3: Magnetic structures of transition metal perovskites obtained by neutron scattering [3].

The A-type structure is an antiferromagnetic (AFM) structure in which the in-plane spins are

coupled ferromagnetically and neighbouring planes antiferromagnetically. The B-type structure

is ferromagnetic. The C-type is antiferromagnetic, with the spins ferromagnetically aligned along

chains (z -axis) but antiferromagnetically within the planes. The E-type is also antiferromagnetic,

with the spins of neighbouring planes along the z -axis coupling antiferromagnetically. The G-

type is antiferromagnetic in all three directions. A coherent stacking of octants of the C- and

E-type structures leads to the CE-type structure [3]. This magnetic structure occurs in doped

manganites, which exhibit spin, orbital, and charge order. An example is Pr1−xCaxMnO3 (see

2.1.3). Particularly, we will present experiments on the mixed-valence manganite Pr0.5Ca0.5MnO3

and show how its spin, orbital and charge order are affected by X-ray radiation and the application

of an electric field.


2.1.2 The colossal magnetoresistance effect

Applying an external magnetic field to a single crystal of La0.75Ca0.25MnO3 results in a huge

decrease of its resistance (Figure 2.4) [2]. This phenomenon, known as colossal magnetoresistance

(CMR), is also observed in other manganese based transition metal oxides and it is believed to

be caused by the conductive d electrons. In doped manganites charge ordered (CO) and orbitally

ordered (OO) insulating states compete with the ferromagnetic (FM) metallic states and this

strong interaction leads to CMR, which is believed to occur at the boundaries between these two

different phases.

Figure 2.4: Temperature profile of the resistivity of La0.75Ca0.25MnO3 under the application ofvarious external magnetic fields showing a colossal magnetoresistance effect at T =250 K. Figurereproduced from [2].

The mechanism of magnetoresistance in manganites can be understood with the double ex-

change model. In this model, an electron is transferred from a filled eg orbital to an empty one

on a neighbouring atomic site (Fig. 2.5 (d)). Zener suggested a double exchange process between

two Mn sites with an intervening O 2p (Figure 2.5) [4]. According to this process, the hopping

depends on the orientation of the magnetic moment of the t2g state. The conductivity in the

colossal magnetoresistive manganites is then determined by the hopping of the eg electrons, with

the latter being described by the transfer integral

tij = t0ijcos(θij/2), (2.2)

where t0ij is the effective hopping interaction and θij is the relative angle between the magnetic

moments of the neighbouring i and j ions.


Therefore, the hopping probability of the eg electron depends on the relative spin alignment

between the two sites. The probability is maximal for a perfect ferromagnetic alignment of the

neighboring spins. It is believed that in the colossal magnetoresistance effect, the magnetic field

aligns the manganese magnetic moments (Figure 2.5(c)), activating the electron hopping from site

to site. This leads to a ferromagnetic metallic state.

Figure 2.5: Sketch of the double exchange mechanism in magnetoresistive La1−xSrxMnO3 belowTC (a), around TC (b), with spin aligning under an external magnetic field H (c) and the transferintegral (d). Figure reproduced from [5].

Recently, another mechanism has been introduced to explain the CMR effect, the so-called

Nanoscale Phase Separation, NPS [6]. This mechanism may be important for both cuprates and

manganites. It refers to inhomogeneities created by two competing “phases”. In manganites

these two phases are ferromagnetic and charge-ordered antiferromagnetic. Here, “phases” are not

to be understood as homogenous thermodynamic phases but rather as “clusters” or “stripes”.

In the latter sense, “phase separation” means states with nanoclustered coexistence of different

electronic properties (for details, see [6]). Nanoclusters can also be found above TC in low and

intermediate bandwidth manganites and are believed to play a crucial role in the CMR effect [7].

This may be explained as follows: electrons with spin up are mobile within the ferroclusters having

their magnetization up. Magnetization-down clusters restrict the mobility of these electrons and

effectively act as insulators. When an external field is applied, the magnetic moment of these

clusters is rotated, leading to a metallic state [6].

Neutron diffraction studies by Kajimoto et al. [8] reported ferromagnetic and antiferromagnetic

spin fluctuations in the temperature range between TCO and TN in Pr1−xCaxMnO3, implying

coexistence of FM and AFM states. Radaelli et al. [9] found mesoscopic and microscopic phase

segregation by neutron diffraction in La0.5Ca0.5MnO3.


2.1.3 The properties of Pr1−xCaxMnO3

Pr1−xCaxMnO3 has a rich phase diagram (Figure 2.6) [10]. The material is a ferromagnetic

insulator for 0.15 ≤ x ≤ 0.3 and temperatures lower than T∼125 K. For x≥0.3 it has an antifer-

romagnetic ground state and below TCO/OO ' 240 K it is a charge and orbital order insulator.

Above TCO/OO it is a paramagnetic semiconductor. An antiferromagnetic spin order sets in below

TN ' 140 K. Lowering the Ca concentration causes extra doping with electrons on the Mn4+

sites. A neutron diffraction experiment on Pr1−xCaxMnO3, performed by Jirak et al. [11] in 1985,

revealed its magnetic structure. They found that in the region 0.3 ≤ x ≤ 0.5 the CO/OO and the

magnetic/orbital structures are of AFM CE-type, as shown in Figure 2.7 [11]. They also found

a distinct difference between the magnetic structure for x=0.5 and the ones for x < 0.5. For Ca

concentration x=0.5, the spins lie in the xy-plane with collinear arrangement and the coupling

along the z -axis is antiferromagnetic. For lower Ca levels (x < 0.5) a canting of the spins in

the z -axis direction is found, leading to a non-collinear magnetic structure. This pseudo CE-type

AFM structure is depicted in Figure 2.8.

Figure 2.6: The phase diagram of Pr1−xCaxMnO3. Figure reproduced from [10].

2.2 Imprinting magnetic bits in Pr0.5Ca0.5MnO3 with X-rays 26

Figure 2.7: Antiferromagnetic CE-type structure with a unit cell of 2√

2ap×2√

2ap×2ap. The filledcircles represent Mn4+ ions and the blue lobes represent Mn3+ ions possessing eg orbital order.The green arrows indicate the directions of the magnetic moments.

Figure 2.8: The pseudo antiferromagnetic CE-type structure with a ferromagnetic spin couplingalong z -axis and antiferromagnetic in-plane spin arrangement. The red lines represent the directionof the magnetic moments.

2.2 Imprinting magnetic bits in Pr0.5Ca0.5MnO3 with X-

rays

Incident electromagnetic radiation has been found to change the properties of manganites. For

example, terahertz pulses were found to excite vibrational excitations, which in turn induced metal-

insulator transitions [12]. Intense laser pulses induce structural phase transitions on the ultrafast

time scale [13], some of which are long lived [14]. Those experiments revealed the ability to induce

structural phase transitions by electromagnetic radiation but for most cases these transitions are

not permanent. Permanent phase transitions were found to occur in manganite materials after

exposure to X-ray radiation. Kiryukhin et al., observed a hard X-ray (8 keV) induced metal


insulator transition in Pr0.7Ca0.3MnO3 at T =4 K [15]. They observed changes not only in the

resistance of the material but also in the crystal lattice structure. They noticed that upon X-ray

illumination, the intensity of a superlattice reflection decreases continuously. This superlattice

reflection is a measure of the periodic lattice distortion caused by the underlying charge and

orbital order. No further change is observed when the X-ray beam is switched off (Figure 2.9).

They attributed the persistent photoconductivity to the presence of defect centres which are

associated with large sized lattice distortions. The trapped charge carriers in these centres cause

an increase in conductivity. They also claimed that this mechanism resembles the one occurring

in DX centres in III-V semiconductors. However the microscopic origin of these defects was not

investigated.

The destructive power of X-rays is well known. One can argue that X-ray exposure destroys

the charge and orbital order and allows the localized charges to become mobile, which in turn

removes the lattice distortion. In protein crystallography for example, exposure of a sample to

X-rays easily breaks bonds and destroys the crystal structure [16]. Electron radiation was also

found to cause such an effect in Bi1−xCaxMnO3 [17]. Interestingly, in some cases X-ray radiation

was found to induce persistent photoconductivity as well as percolation of metallic clusters in

charge ordered manganites (Figure 2.10) [18]. Casa et al. [18] showed that X-ray illumination of

Pr0.65Ca0.245Sr0.105MnO3 at T =100 K results in a microscopically phase-separated state in which

the charge ordered insulating regions act as barriers for the transfer of electrons between the

metallic clusters. They proposed that these barriers are reduced or even removed when the clusters

are illuminated by X-rays, inducing photoconductivity. Thus, a metallic thin film is created on

the surface of the insulating substrate. While the photoconductivity of the material increased,

measurement of a superlattice reflection showed [18] that its peak intensity remains constant under

illumination by X-rays. This observation implies that the CO phase remains intact. Casa et al.

attributed the persistent photoconductivity only to the increase in mobility of the eg electrons.

We study here an epitaxial 40 nm thick film of Pr0.5Ca0.5MnO3 deposited onto a (LaAlO3)0.3-

(SrAl0.5Ta0.5O3)0.7 substrate (LSAT) (0 1 1)C by pulsed laser deposition (Figure 2.11). The laser

pulse frequency was 2 Hz, and the sample was annealed at 850 0C in an oxygen pressure of 1.5

mTorr (for details, see [20]). The film has been previously characterized by different methods,

including resonant soft X-ray diffraction [19].

To investigate the interaction between the X-rays and the manganite, we use resonant soft


Figure 2.9: X-ray exposure dependence of the (2 32

0) (cubic notation) Bragg reflection at 4 Kusing an X-ray energy of 8 keV and an X-ray flux of 5 · 1010 ph/s. Figure reproduced from [15].

Figure 2.10: Electrical resistance (above) and intensity of a superlattice reflection (below) ofPr0.65Ca0.245Sr0.105MnO3 at T =100 K. Figure reproduced from [18].

X-ray diffraction as a probe to directly access the magnetic and orbital states. For measuring the

electrical conductivity of the sample, three pairs of gold contacts were deposited on the surface

of the thin film (Figure 2.12). Each pair had a 100 µm gap and the pairs were spaced 700 µm

apart. Indium wires were attached to the gold electrodes and were connected through the cryostat

to a Keithley 2400 Sourcemeter to measure the resistance. This resistance measurement set-up is

sufficiently precise, as the contact and wire resistances are always much smaller than the resistance

of the film.

In order to measure the magnetic/orbital (14

14

0) reflection, a 300 wedge was used to bring the


Figure 2.11: Three-dimensional view of the Pr0.5Ca0.5MnO3 thin film onto [011] oriented LSATsubstrate. The eg orbitals (lobes) are on Mn3+ sites and the closed circles represent the Mn4+

ones. Figure reproduced from [20].

Figure 2.12: Picture of the epitaxial Pr0.5Ca0.5MnO3/LSAT (0 1 1) thin film mounted on a Cuwedge showing the indium wires connected to the gold contacts.

reflection of the [1 1 0] oriented film in the scattering plane for all chosen azimuthal angles. In

this mount we choose the thin film’s [-1 1 2] axis to be perpendicular to the scattering plane. The

sample’s orientation with respect to the scattering plane and the linear polarization of the X-rays

are shown in Figure 2.13. We refer here to a simple cubic structure as a reference for the axis

system, even though the true crystal symmetry is expected to be monoclinic at low temperatures.


Figure 2.13: The geometry for the diffraction experiment on Pr0.5Ca0.5MnO3, with Q=(14

14

0)lying in the horizontal scattering plane.

The temperature dependence of the (14

14

0) reflection taken at 642.5 eV photon energy is

shown in Figure 2.14. For this measurement no X-ray focusing was used and the beam size was

approximately 2x2 mm2. This is important, as it reduces the X-ray flux density on the sample by

more than two orders of magnitude compared to the focused beam. We measured with incident

X-rays of both π and σ polarization. The polarization dependence of reflections sensitive to CE-

type orbital order is well understood [21]. A signal occurs only in the rotated (πσ′) and (π′σ)

polarization channels, with the intensity of both channels being equal. For magnetic scattering

at resonance, the σσ′ channel is forbidden (as discussed in 1.5). Hence, the magnetic scattering

signal is on average larger in the π incident channel than in the σ incident channel.

Below TN' 125 K, one observes that the intensity collected with π polarization increases

much more than the one probed with σ polarization. This indicates that the additional intensity

is of magnetic origin. The intensity between TN and TOO ' 210 K is equal for both incident

polarizations, and is therefore dominated by orbital scattering as anticipated from the phase

diagram (Figure 2.6). These observations and their interpretation are consistent with recent soft

X-ray powder diffraction experiments on Pr0.5Ca0.5MnO3 [19].

The sequence of our measurements is illustrated in Figure 2.15. An X-ray beam of size 100x50

µm2 (∼ 6x1012 photons/s) was centered in the gap between the two gold electrical contacts. This

allows simultaneous exposure of the sample to the X-rays and collection of the diffracted Bragg

intensities, as will be described in the next paragraph. After the initial alignment performed at

50 K, we heated the sample above the TOO, in order to erase any possible effect of the X-ray

illumination on the sample (which is shown later). Subsequently we cooled down the film to

50 K without X-rays impinging on the sample. By returning to the same temperature (50 K),


Figure 2.14: Temperature dependence of the magnetic/orbital (14

14

0) reflection collected on aPr0.5Ca0.5MnO3 film with σ and π incident X-ray polarization. The shading represents approxi-mately the individual orbital and magnetic contributions to the signal.

possible temperature induced movements of the sample are avoided. The magnetic/orbital (14

14

0)

reflection and the in-situ X-ray induced photoconductance of the film between the electrodes were

recorded alternately. The temperature of 50 K (well below TN) and π incident light with E=642.5

eV was chosen for all experiments presented (unless otherwise stated). Under these conditions

the intensity of the reflection is primarily caused by magnetic scattering. Note that most of the

incoming X-ray intensity is absorbed and only a small fraction is scattered.

Initially, the sample was exposed to X-rays for 6.6 s. During this time the diffracted intensity

of the (14

14

0) reflection was collected. Afterwards, the beam was turned off to record the con-

ductivity. This is important, as the X-ray beam creates additional photocurrents, disturbing the

measurement of the conductivity of the film. The beam was then turned on again to simultaneously

expose and collect the scattered intensity, and turned off again to record the conductivity.

Repeating these steps leads to the key observation shown in Figure 2.16: the X-ray exposure

increases the electrical conductivity as well as the intensity of the diffracted X-rays. The increase in

conductivity is qualitatively similar to that reported earlier for Pr1−xCaxMnO3 with a significantly

lower doping (x=0.3) [15]. However, compared with Figure 2.9, our observed increase is orders of

magnitude weaker [15].


Figure 2.15: Sketch of the experimental procedure for recording the X-ray induced changes ofboth the diffracted intensity and the conductivity of the film.

Figure 2.16: X-ray exposure dependence of the (14

14

0) reflection (left axis, black points) andin-situ measured electrical conductivity (right axis, red points) of an epitaxial Pr0.5Ca0.5MnO3

thin film.

Both effects are persistent in time since no changes occurred when the X-ray beam was turned

off for hours. The change in the magnetic/orbital Bragg peak intensity in Pr0.5Ca0.5MnO3 under

X-ray illumination is remarkable. Despite the suppression of the structural superlattice reflection

observed in Pr0.7Ca0.3MnO3 [15], the intensity of the (14

14

0) reflection increases.

In Figure 2.17 the reciprocal space (h -h 0) scan of the diffracted X-ray intensity of the (14

14

0) reflection is illustrated. It shows an increase in the intensity for increasing exposure times. By

normalizing the peak maximum to unity, the sharpening of the Bragg peak (a decrease in its full


width at half maximum) can be observed as well. This result implies an increase in the correlation

length of the magnetic and/or orbital order (inset in Figure 2.18). We can extract its behaviour

from a Lorentzian fit as the width is much smaller than the instrumentation resolution. This

indicates that the illumination of the sample with X-rays did not cause any observable destructive

effects, as has been observed to crystalline structure on the lower doped Pr0.7Ca0.3MnO3 [15].

On the contrary, X-ray illumination improved the antiferromagnetic and/or orbital order of the

system.

Figure 2.17: Reciprocal space (h -h 0) scan of the (14

14

0) reflection for various exposure times.


Figure 2.18: Reciprocal space (h -h 0) scan of the (14

14

0) reflection for different exposure timeswith maximum intensity normalized to unity. The inset displays the evolution of the correlationlength with respect to X-ray exposure time.

The intensity changes can be caused by changes either in the magnetic or in the orbital structure

of the material. One way to investigate this is by repeating this experiment in σ polarization. The

CO/OO manganites do not exhibit large X-ray linear dichroism at these energies [23]. Therefore,

any changes of the scattered intensity as a function of exposure time can be attributed solely to

changes in the scattering strength caused by the absorbed X-rays. The variation of the intensity

recorded with σ polarization with increasing exposure time is much smaller than for π polarization

(Figure 2.19). This result implies that illumination with X-rays mainly changes the magnetic

structure.


Figure 2.19: Exposure time dependence of the (14

14

0) reflection collected on a Pr0.5Ca0.5MnO3

film with π and σ incident X-ray polarization.

Clearly, irradiation with synchrotron X-rays enhances the intensity of the magnetic reflection

and improves the magnetic order of the material. Simultaneously, the X-rays also induce an in-

crease of the conductivity, which is persistent. Is it possible not only to imprint magnetic and

electronic bits but also to control their density and to erase them again? In order to answer this

question, contour plots (Figure 2.20) of the (14

14

0) reflection were made by moving the sample

with respect to the X-ray beam position. In order to reduce the effect of X-rays on the sample,

half of the X-ray photon flux and short exposure times (1 s/point) were used compared to the

earlier X-ray exposure experiments. The different colours in the contour plot represent different

intensities of the (14

14

0) reflection. The low intensity (dark blue) regions are positions at which

the sample is covered by the gold contacts and/or the X-ray beam is blocked by the indium wires.

The red colour regions are positions at which the Bragg peak intensity is the highest. In order

to write magnetic bits, we have exposed two different regions of the sample to the X-ray beam,

indicated by the arrows in Figure 2.20. A clear increase in the scattered intensity is observed at

the position of the X-ray exposure (Figure 2.20b). This result implies that the effect is local. After

heating the sample above the transition temperature TOO and cooling to T =50 K the magnetic

bits are erased. Indeed, we observe an intensity distribution identical to that before the X-ray


exposure (Figure 2.20c).

Figure 2.20: Intensity map of the (14

14

0) Bragg reflection from the Pr0.5Ca0.5MnO3 thin film.Before (a), and after (b) exposure to the X-rays in the two positions (indicated by the arrows) forapproximately 60 minutes and the pattern collected after the heating/cooling cycle (c).

In the next step, we studied the effect of beam size and X-ray flux density. In Figure 2.21,

the contour plots of the film are shown, before and after the illumination at four positions (i - iv)

with different beam properties. For position (i) and (ii) the vertical beam size was 100×50 µm2,

but the flux for (ii) was doubled compared to position (i). The change in X-ray flux density has

been achieved by using either one or both undulators of the beamline. For positions (ii) to (iv)

the exit slit was increased, leading to constant X-ray flux density but with vertical beam sizes of

50, 100 and 200 µm, respectively. Figure 2.22 shows the time dependence of the Bragg intensity

for the four exposure conditions (i)-(iv). The exposure dependence of the positions (ii)-(iv) is

qualitatively similar, which is expected since the flux density is constant. This is in contrast to

the behaviour of the position (i), which shows a clear saturation behaviour after 100 s. As this

difference is caused by a lower flux density, it indicates that the interaction between the X-rays

and the magnetic structure has a strongly non-linear dependence on X-ray flux density.


Figure 2.21: Mesh scans of the sample before (a) and after (b) the illumination with the X-raybeam at four different positions using different X-ray beam parameters.

Figure 2.22: Time dependence of the Bragg intensity of the (14

14

0) reflection at different positionsusing different beam parameters for the X-ray exposure.

Next we address the question: what is the microscopic origin of the effect? The (14

14

0) reflection

has a magnetic and an orbital contribution in most of the CE-type order manganites [24], [25], but

as shown before, the scattering is dominated by the magnetic contribution. For resonant magnetic

scattering, dominated by electric dipole transitions (E1), the magnetic scattering amplitude is

given by Equation 1.22. It can be simplified to the following Equation (2.3) [26],


Fεε′ ∝ −i(ε′ × ε) · Fm, (2.3)

where Fm is the magnetic structure factor.

We calculate the magnetic reflection’s intensity by considering the magnetic structure factor

Fm of the (14

14

0) reflection for different doping concentrations in Pr1−xCaxMnO3:

Fm =∑j

mjeirj ·Q, (2.4)

where mj is a tensor representing the magnetic moment at site j identified by rj.

First we shall consider a doping of x=0.5. According to neutron scattering experiments, the

magnetic structure for this doping level is as shown in Figure 2.7 [11]. In Figure 2.23 we present

the magnetic structure in the z=0 and z=12

planes.

Figure 2.23: The magnetic and orbital structure of Pr0.5Ca0.5MnO3. On the left the structure inthe z=0 plane is illustrated, on the right the one in the z=1

2plane.

In this structure, [11] the magnetic moments are antiferromagnetically aligned in the xy plane,

as well as out of plane, along the z -axis. We choose to calculate the (0 1 0) reflection in the

magnetic unit cell 2√

2ap×2√

2ap×2ap, which is equivalent to the (14

14

0) reflection in the cubic

representation. This orthogonal unit cell consists of two planes, each having four magnetic ions.

Applying Equation 2.4 to the z=0 plane we obtain


F(010) = −m4ei2π(010)( 1

400) + m4e

i2π(010)( 34

00)

+m3ei2π(010)(0 1

40) + m3e

i2π(010)( 12

14

0)

+m4ei2π(010)( 1

412

0) −m4ei2π(010)( 3

412

0) −m3ei2π(010)(0 3

40) −m3e

i2π(010)( 12

34

0) = 4im3, (2.5)

where m4 represents the magnetic moment of Mn4+ ions and m3 that of the Mn3+ ions. Depending

on the direction of the magnetic moment with respect to the x -axis, the sign of m3,4 is either plus

or minus compared to sites (i) and (iii) for the Mn4+ and Mn3+, respectively. Applying the same

procedure for the z=1/2 plane we obtain

F(010) = m4ei2π(010)( 1

400) −m4e

i2π(010)( 34

00)

−m3ei2π(010)(0 1

40) −m3e

i2π(010)( 12

14

0)

−m4ei2π(010)( 1

412

0) + m4ei2π(010)( 3

412

0)

+m3ei2π(010)(0 3

40) + m3e

i2π(010)( 12

34

0) = −4im3. (2.6)

Hence, only the Mn3+ ions contribute to the structure factor in a single plane. If we now add

the magnetic structure factors for both planes, we obtain

F(010) = 4im3 − 4im3 = 0. (2.7)

Considering the structural phase factors and the CE-AFM configuration of the moments of

the magnetic structure of Pr0.5Ca0.5MnO3 (Figure 2.23), all contributions to the (14

14

0) reflection

add up to zero (F=0).

We shall now examine the magnetic structure at lower doping, the pseudo CE-type antifer-

romagnetic structure [11]. For this structure, there is a small canting of the magnetic moments

leading to additional z components of the individual magnetic moments, which are ferromagnet-

ically coupled (Figure 2.24). The x -components remain in the CE-AFM ordering contributions

and the in-plane coupling remains antiferromagnetic.


Figure 2.24: The magnetic and orbital structure of Pr0.6Ca0.4MnO3 with the unit cell definedas 2√

2ap×2√

2ap×2ap. Left: structure of Pr0.6Ca0.4MnO3. Right: solely the component of themoments along the z -axis is illustrated.

Since the in-plane projection of the magnetic moments form the same CE-AFM configuration

as for x=0.5, the magnetic structure factor remains zero for these components. On the other hand,

the magnetic structure factor for the z components (Figure 2.24) equals for the z=0 plane

F (010)z = +mz

4ei2π(010)( 1

400) −mz

4ei2π(010)( 3

400)

−mz3ei2π(010)(0 1

40) −mz

3ei2π(010)( 1

214

0)

−mz4ei2π(010)( 1

412

0) +mz4ei2π(010)( 3

412

0)

+mz3ei2π(010)(0 3

40) +mz

3ei2π(010)( 1

234

0) = −4imz3, (2.8)

and for z=12

plane

F (010)z = +mz

4ei2π(010)( 1

400) −mz

4ei2π(010)( 3

400)

−mz3ei2π(010)(0 1

40) −mz

3ei2π(010)( 1

214

0)

−mz4ei2π(010)( 1

412

0) +mz4ei2π(010)( 3

412

0)

+mz3ei2π(010)(0 3

40) +mz

3ei2π(010)( 1

234

0) = −4imz3. (2.9)

Thus,

F (010)z = −4imz

3 − 4imz3 = −8imz

3. (2.10)


The previous calculations show that the structure factor contains a single component that is

proportional to the magnetic moment components along the z -axis, Fm=(0,0,Fm) and that only

the Mn3+ ions contribute to the magnetic signal. Therefore, the enhancement of the magnetic

peak intensity arises from the ferromagnetically coupled z -axis components of the Mn3+ spins in

the CE-type antiferromagnet.

How does the increase of the magnetic moment component along the z -axis relate to the

increase in the conductivity? The conductivity directly relates to the creation of defects in the

charge and orbital order, which is consistent with the previously observed destruction of the

crystallographic superstructure of the x=0.3 compound [15]. The defects act as impurities and give

rise to deep donor levels in the band. This mechanism is similar to the behaviour of the DX centres

in III-V semiconductors [27]. The X-ray-induced defects will be ionized and the corresponding

electrons are excited to the conduction band, resulting in photodoping of the material. The strong

coupling of the electrons to the manganite lattice leads to a relaxation of the lattice around the

defect. This creates a metastable state consistent with the fact that when the beam is turned off

no recovery occurs. This doping is equivalent to a decrease in Ca concentration.

The doping effect explains the enhancement of the magnetic intensity as well. Decrease in the

Ca concentration and photodoping both lead to an increase in the spin canting in Pr1−xCaxMnO3,

as visualized in Figure 2.25. The increase of the (14

14

0) magnetic reflection reflects the alignment of

the ferromagnetically coupled spin component along the z -axis. This is connected to an increase in

conductivity, because the double exchange model enhances the hopping of electrons for decreasing

θij angles between neighbouring spins i and j (Equation 2.2). The increase of the magnetic

correlation length for increasing X-ray exposure is more difficult to understand.

This interpretation might be related to recent observations of transient “hidden” phases in

manganites caused by the competing interactions which are in a delicate equilibrium [28]. A recent

femtosecond time-resolved spectroscopic study [29] found such a “hidden” phase in the CO/OO

compound Nd0.5Sr0.5MnO3. It represents a photoinduced, structurally ordered, homogenous and

transient state of matter. In that study, photoinduced changes in the lattice appeared due to a

change in the orbital polarization. This implies that the driving mechanism of the photoinduced

hidden phase is a rearrangement of orbital ordering in the CO/OO state. A 0.3% change of electron

occupation in the orbitals d3z2−r2/(d3x2−r2 or d3y2−r2) can explain the observed photoinduced

changes in the lattice constants on the basis of the model calculation for the ground state [29].


Figure 2.25: Pictorial view of the spin canting after X-ray illumination (orange arrows) ofPr1−xCaxMnO3 at different exposure times.

Figure 2.26: Photoinduced transient hidden phase caused by a rearrangement of orbital polariza-tion. Figure reproduced from [29].

In conclusion, in this section we described a method to write magnetic bits in strongly cor-

related manganites. X-ray illumination photodopes an epitaxial thin film of Pr0.5Ca0.5MnO3 by

creating defects in the magnetic and orbital ordered state. As a result, the manganese magnetic

moments re-align from a collinear antiferromagnetic structure to a canted structure with ferro-

magnetic magnetic components along the z -axis. In addition, this improves the long range order

associated with the antiferromagnetic state. The changes can be varied by either tuning the X-

ray flux density or the exposure time. This effect offers a route to control magnetism and write

magnetic and electronic bits.

2.3 Switching of resistive phases in Pr0.5Ca0.5MnO3 using electric field 43

2.3 Switching of resistive phases in Pr0.5Ca0.5MnO3 using

electric field

In this section, we study the effect of an applied electric field on the electronic structure of

Pr0.5Ca0.5MnO3. We are interested in a potential switching of the resistance caused by the de-

struction of the orbital order of the Mn ions by the application of electric fields.

This is motivated by the study of Asamitsu et al. [30], which has shown that application of up

to 1000 V to a single crystal of Pr0.7Ca0.3MnO3 induces a transition from a high- to a low-resistive

phase (Figure 2.27). The resistance versus applied voltage shows sharp transitions with a clear

hysteresis effect. This indicates a first-order metal-insulator transition caused by the destruction

of orbital and charge order.

Figure 2.27: Resistance of a single crystal of Pr0.7Ca0.3MnO3 at 20 K as a function of appliedexternal voltage [30].

Asamitsu et al. attributed this effect solely to the application of the voltage and to a corre-

sponding change in the electronic structure of the material. By in-situ measurement of the sample’s

temperature they verified that the observed result is not due to resistive heating. Such a dielectric

break-down, destroying the charge and orbital ordered state, would be an interesting novel effect

for which no microscopic explanation exists to date. They also observed this transition in applied

magnetic fields (magnetoresistance) and concluded that “the current-switching phenomenon may

be used for the fabrication of electromagnets on a micrometre or nanometre scale”.


Several studies on manganites followed, especially on Pr0.5Ca0.5MnO3, in order to study this

effect for possible memory applications [31] - [33]. Here, we want to address the generation of

metallic/conducting paths with the rest of the sample remaining insulating [34]. Moreover, we

want to investigate the microscopic origin of this voltage-induced dielectric breakdown by studying

its effect on the orbital order.

To address these points, we use spatially resolved resonant soft X-ray diffraction and in-situ

resistance measurements to investigate the region where the conducting part is created by the

application of the electric fields. We characterized the electronic and magnetic states of the

material under applied fields and established whether the applications of electric fields are directly

responsible for a switching between the two resistive phases. The energy of the X-ray beam was

643.25 eV throughout these studies.

The 40 nm thick epitaxial film of Pr0.5Ca0.5MnO3, previously used for the study of the X-ray

induced effects, was investigated (see section 2.2). To protect the film against overload of current

due to a field induced metal-insulator transition, we connected an external load resistor of 1 MΩ

in series. For the application and recording of voltage we used a Keithley 2400 SourceMeter

Instrument.

For the electrical characterization of the film, we determined the temperature dependence of

its resistance Rs (Figure 2.28). A clear jump is visible around TOO during heating (210 K, red

curve) and cooling (205 K, black curve) (see inset Figure 2.28). The observed hysteresis around

210 K is characteristic of a first-order phase transition. At low temperatures the material becomes

highly resistive with the resistance being in the MOhm regime.

A plot of the temperature dependence of the resistance on a linear scale (Figure 2.29) shows the

very fast drop of the resistance at low temperatures. We model this behaviour by the exponential

function

Rs = aebT , (2.11)

with best-fit values a=1.8935·107±9·10−4Ω and b=-0.0678±4·10−4 1/K.


Figure 2.28: Temperature dependence of the resistance of Pr0.5Ca0.5MnO3 during cooling andheating. Inset: temperature range around TOO.

Figure 2.29: Temperature dependence of the resistance of Pr0.5Ca0.5MnO3. The red curve repre-sents the best fit to Equation 2.11.

The electric field induced resistance changes and resistive switching are expected to be local

effects. Therefore, it is crucial to be able to focus the X-ray beam on the region of the sample

where the conducting path is created. Figure 2.30 (left) shows a contour plot of the intensity of

the (14

14

0) Bragg reflection as a function of the position on the sample. A clear saddle point of

intensity is observed, marked by a cross, where the gap between the electrodes is located. This

method of finding the position between the electrical contacts has already been demonstrated


in the previous chapter. Here we show an alternative method. Figure 2.30 (right), shows the

measured electric current flowing through the gap under a constant applied voltage of 10 V, while

scanning the sample through the beam. We observed that the current is large when the X-ray beam

is possitioned inside the gap. This is an X-ray induced electric current between the electrodes,

caused by the emitted photo-electrons, which change the electronic properties of the manganite.

Figure 2.30: Left: Intensity contour plot of the magnetic/orbital (14

14

0) reflection from theepitaxial Pr0.5Ca0.5MnO3 thin film. Right: contour plot of the X-ray induced current between theelectrical contacts taken with π incident polarization. The orange regions represent the electrodesand the indium wires attached on top of them.


We then measured the diffracted X-ray intensity of the magnetic/orbital (14

14

0) reflection

(Figure 2.31) as a function of applied voltage in the gap region. The measurement was performed

for both incident π and σ polarizations at 20 K. For voltages above 500 V the intensity drops.

The intensity drop is stronger for π incident than for σ polarization. The X-ray intensity as a

function of voltage shows a hysteresis loop that closes for both polarizations below 300 V.

Figure 2.31: Peak intensity of the magnetic/orbital (14

14

0) reflection as a function of appliedvoltage, measured in π polarization and σ polarization. The lines are to guide the eye.


In Figure 2.32 we present the resistance of the sample Rs between the electrical contacts as

a function of applied voltage, obtained from the measured induced current (I ) after correcting

for the external load resistance. The resistance was measured without X-rays impinging on the

sample as the X-ray beam significantly alters these measurements (as shown in Figure 2.30 right).

The observed hysteresis in the resistance is qualitatively similar to that found for the reflection

intensity (Figure 2.31). The decrease in resistance might be either directly attributable to the

application of a high electric field as proposed by Asamitsu et al [30] or to resistive heating. In

the first case, the hysteresis is caused by the onset of the first-order orbital and charge order

transition (as seen in inset of Figure 2.28), whereas in the second case, the voltage is not the

relevant physical quantity.

Figure 2.32: Resistance of Pr0.5Ca0.5MnO3 as a function of the applied voltage measured at 20 K.The lines are a guide to the eyes.

To investigate the possible role of resistive heating for the voltage induced metal-insulator

transition, we calculate the dissipated power in the sample (P s) using Equation 2.12 and present

the power dependence of Rs in Figure 2.33.

Ps = I2 · Rs. (2.12)


Figure 2.33: Sample’s resistance as a function of the dissipated power in the sample, for bothincreasing (black points) and decreasing (red points) electric fields.

After substituting the voltage with the power using Equation 2.12, no hysteresis is observed in

the resistance. However, we must still address the temperature change caused by the dissipated

power. Since we have no information about thermal conductivity in the crystal, we shall assume

that the change in resistance is caused only by resistive heating. This allows us to substitute

the resistance in Equation 2.12 with its temperature dependence from Equation 2.11, providing a

direct relation between the temperature and the power. This is shown in Equation 2.13.

Ps = I2 · Rs = I2 · aebT. (2.13)

Figure 2.34 shows the obtained temperature for different dissipated powers. For P≤10 mW

(V≤500 V) the induced temperature change in the sample is negligible (T increase≤10 K). The

material remains in a highly resistive phase (Rs≥20 MΩ) with no change observed in the X-ray

diffracted intensity (Figure 2.31). However, this changes rapidly for higher values of power. When

P>20 mW (corresponding to V>600 V) there is a significant increase in the temperature (∼ 70

K) with a decrease in the diffracted X-ray intensity (Figure 2.31).

The proposed temperature increase model can be used to explain the behavior of the (14

14

0)

reflection. To show this, Figure 2.35 presents the diffracted intensity as a function of the proposed

temperature increase. This is compared to the temperature dependence of the diffracted intensity

in Figure 2.14. The relation between intensity and temperature increase corresponds qualitatively


Figure 2.34: Calculated temperature increase in Pr0.5Ca0.5MnO3 thin film as a function of dis-sipated power. The induced temperature refers to the part of the sample between the electricalcontacts where the applied field increases its conductivity. For each point, the value of the appliedvoltage is shown. The sequence of the measurements is indicated by the line and arrows.

to the temperature dependence in the range 20-90 K. We conclude that electric field dependence

of the diffracted intensity is a direct result of local resistive heating in the sample.

Figure 2.35: The behaviour of the diffracted X-ray intensity as a function of the induced temper-ature in the sample after the application of the electric field in both polarizations. The lines area guide to the eye.

Due to the relation between resistance and intensity, this result indicates that the applied

electric field is not the direct mechanism for the transition between high and low resistive phases.

Instead, we interpret this as the result of resistive heating.

Is it still possible that a reduction of the orbital and magnetic order (as observed through the

reduced X-ray intensity) can lead to the same resistance reduction? To answer this question, we

should investigate the diffracted X-ray intensity around the electrical contacts, where the electric


field to the sample is small. Figure 2.36 shows such images of the diffracted X-ray intensity for

various applied voltages.

Figure 2.36: Contour plots of the magnetic/orbital (14

14

0) reflection in the region around theelectrical contacts. The upper left panel shows the electrical contacts in orange colour.

These measurements show that for increasing voltage changes do not occur only in the region

between the electrical contacts, where the conducting path is created by the applied voltage.

The X-ray intensity is similarly reduced for increasing voltage in a region further away from the

electrodes, where no electric field is present. In the case of a pure electric field induced effect,

we would expect only the narrow region between the contacts to be affected, which is clearly

inconsistent with the observations.

Figure 2.37 shows the IV characteristics of the film for three different data collection times

with waiting times of 0.1, 0.5 and 3 s after switching the X-ray beam off. These data can be

compared to the diffracted X-ray intensity of the magnetic/orbital (14

14

0) reflection measured

with the same data acquisition time (Figure 2.38).


Figure 2.37: IV characteristics measured for different data collection times (a,b and c). Thenumbers on the plot indicate the measurement sequence.


Figure 2.38: Peak intensity of the magnetic/orbital (14

14

0) reflection. The numbers on the plotindicate the measurement sequence.

In all plots of Figures 2.37 and 2.38 a clear hysteresis is observed, with the hysteresis being

larger for the slower measurements. This also shows that the presence of hysteresis is not directly

due to the applied voltage. One would expect that a first-order metal-insulator transition results

in larger hysteresis for the faster measurements compared to the slower measurements, but this is

contrary to our observations. This provides further evidence that the hysteresis in the resistance

curves is due to resistive heating, in contrast to the interpretation of Asamitsu et al [30].

2.4 Conclusions 54

2.4 Conclusions

In summary, we presented experiments describing the effect of X-rays and electric fields on charge

and orbitally ordering in Pr0.5Ca0.5MnO3 epitaxial films. We observed that intense X-ray exposure

changes the magnetic structure and simultaneously improves the material’s conduction properties.

Our results cannot be explained by X-ray radiation destroying chemical bonds. The latter could

be expected to also have a destructive effect on magnetic ordering phenomena. We attribute

the change in magnetic structure to a canting of the manganese magnetic moments leading to

ferromagnetically coupled magnetic components along the z -axis. With X-rays that are focused

down to a sub-micrometer spot size, one can write magnetic and electronic bits in a material.

This effect is not a result of the flipping of magnetic domains. Instead, it arises from a change

in the magnetic and orbital structure. Importantly, both effects are persistent in time. A clear

microscopic description cannot be given based solely on these results. Nevertheless we believe

that the lattice interacts with defects created by the X-rays, which prevent the material from

recovering. Furthermore, the increase in the correlation length can not be explained as annealing

of the sample. Such a process would improve the charge and orbital ordering. Therefore, it would

also suppress electron hopping, leading to an increase in resistance, which disagrees with our

observations.

In the second part of this chapter, we investigated the electric field switching of resistive phases

in Pr0.5Ca0.5MnO3. Our goal was to understand this effect at a microscopic level. Asamitsu et

al [30] attributed the switching of the resistive phases in Pr0.7Ca0.3MnO3 directly to the external

applied field. They excluded heating effects, claiming that an attached carbon resistor on the

sample’s surface did not show a change in temperature. However, the situation may be quite

different in the inner part of the sample, where a local increase in temperature cannot be excluded.

Our experiments on the epitaxial film can be explained by an increase in temperature caused by

the dissipated power. Applying high electric fields to the material creates electrical currents in

the manganite. The associated dissipated power increases the local temperature, which in turn

reduces the resistance and subsequently further increases the current. This coupling leads to a

first-order like phase transition observable as both a decrease in diffracted X-ray intensity and

a decrease in resistance. Even though there could be a direct influence of the applied fields on

the orbital and charge order of the material, our data do not support this scenario. In addition,

the magnetic/orbital reflection is very similarly reduced at positions outside the electrode gap,

2.4 Conclusions 55

where the electric field created by the applied voltage is negligible. This observation is fully

consistent with the resistive heating effect, which we conclude is responsible for the observed drop

in resistance.

BIBLIOGRAPHY 56

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60

Chapter 3

The E-type ordered orthorhombic

RMnO3 systems

3.1 Introduction to multiferroics

In 1820, Hans Christian Oersted accidentally noticed that the needle of the magnetic compass

held in his hand was deflecting after switching on and off the current in a nearby battery. 40 years

after that discovery, Andre Marie Ampere and Michael Faraday worked out the classical theory

of electromagnetism. In the 1860s, Maxwell arrived at his unified theory, combining electricity

and magnetism. Already at that early time, his article focused on the interplay of electricity

and magnetism in real materials. In 1959, many years after Maxwell’s contribution, Landau and

Lifshitz stated in their Course of Theoretical Physics : ”Let us point out two more phenomena

which, in principle, could exist. One is piezomagnetism, which consists of linear coupling between

a magnetic field in a solid and a deformation (analogous to piezoelectricity). The other is a linear

coupling between magnetic and electric fields in a media, which would cause, for example, a mag-

netization proportional to an electric field. Both these phenomena could exist for certain classes

of magnetocrystalline symmetry. We will not however discuss these phenomena in more detail

because it seems that till present, presumably, they have not been observed in any substance” [1].

Most interesting for the magnetoelectric effect would be materials which have a large magnetiza-

tion and a spontaneous electric polarization. This property could lead to the development of new

materials and technologies in the field of memory devices, hard discs etc. However, most ferromag-

netic materials are metallic, whereas ferroelectrics are insulators in order to sustain the electric

3.1 Introduction to multiferroics 61

polarization. Furthermore, in ferroelectric materials the polarization is most frequently related to

a shift of the cations away from the center of the surrounding anions. The non-centrosymmetry

of the crystal structure, which keeps apart the positive and negative charges, is the reason for

the appearance of the macroscopic electric dipole moment of the material (electric polarization).

In contrast, most magnetic materials form centrosymmetric structures, thus no electric dipole

moment can occur.

One can find numerous examples of magnetic or ferroelectric materials in perovskites. Many

common ferroelectrics contain transition metal ions (e.g. Ti4+, Ta5+, W6+) with an empty d shell.

The best known ferroelectrics are BaTiO3 and Pb(ZrTi)O3 (PZT). In these systems, an off-center

shift of the transition metal ion induces ferroelectricity. For ferroelectricity to occur, the d shell

should be empty, whereas a partially filled d shell creates magnetism [2]. That is the reason why

magnetism and electricity are mutually exclusive in such ferroelectrics.

The first discovery of a strong coupling of magnetic and electric degrees of freedom in insu-

lators can be traced back to Pierre Curie [3]. However, the situation in this field only began to

change rapidly when Dzyaloshinskii predicted [4] and Astrov observed [5] this type of coupling,

known as the linear magnetoelectric effect. In recent years, it has been proposed that coupling

the two properties may allow spins to control the material’s electronic properties. The challenge

in this is that it requires to create new materials in which ferro-magnetic and ferro-electric or-

dering are simultaneously present. This can be achieved by either having ”single phase materials

which simultaneously possess two or more primary ferroic properties”, which Schmid [6] named

multiferroics, or by bringing a ferromagnet in contact with a ferroelectric to create a multiferroic

composite compound. In this work we are only concerned with the first case.

In single-phase multiferroics, one can find four basic ferroic ordering types: ferromagnetism,

ferroelectricity, ferroelasticity and ferrotoroidicity (Figure 3.1). These properties are linked to

symmetry and can be characterized by space and time inversions. Space inversion reverses the

direction of polarization, leaving the magnetization invariant. Time inversion will reverse the di-

rection of magnetization, leaving the polarization invariant. Under these definitions, a ferroelastic

material is time and space invariant, while a ferrotoroidic material is time and space variant. A

ferromagnet is time variant and space invariant, while a ferroelectric is the opposite [2]. Most

materials combine ferromagnetism and ferroelectricity and can be divided into two groups: the

type-I group includes materials in which ferromagnetism and ferroelectricity have different origins,

3.1 Introduction to multiferroics 62

Figure 3.1: Sketch of different kinds of interactions in multiferroics. Ferroelectricity (P) can beswitched by electric field (E), ferromagnetism (M) by magnetic field (H), and ferroelasticity (ε)by stress (σ). A promising ferroic ordering is the order of the toroidal moments (T) switchable bycrossed magnetic and electric fields. In addition, spontaneous switchable orbital orders can occur(“O”), a prospect for future research. Figure reproduced from [7].

whereas in the type-II group ferromagnetism causes ferroelectricity, indicating a coupling between

the two.

There are many type-I multiferroics and they often have a large spontaneous polarization, P

(10-100 µC/cm2). However, the coupling between magnetism and ferroelectricity in them is weak.

Their ferroelectricity can originate from lone pairs, the geometry of the lattice, or charge order.

In “lone pairs”, ferroelectricity is caused by the two outer 6s electrons which do not participate

in chemical bonds. They mix with p orbitals causing electric polarization. Examples are BiFeO3,

BiMnO3 and PbVO3. Geometric ferroelectricity occurs for example in hexagonal YMnO3 [8],

where a tilting of the rigid MnO5 block is the driving mechanism. The tilt provides a closer-packed

structure with the oxygen moving towards the Y ion. Ferroelectricity due to charge ordering is

observed in compounds which have transition metal ions with different valences. In order for

the material to exhibit ferroelectricity, the charge order should give rise to inequivalent sites and

bonds. Charge order is predicted to induce ferroelectricity in compounds such as Pr0.5Ca0.5MnO3

or RNiO3 [2], but experimental verification remains ambiguous.

Type-II multiferroics are interesting because ferroelectricity is induced by magnetic order. It is

interesting on the one hand to address the microscopic mechanisms of such a rare effect and on the

other hand to develop new materials with novel electric and magnetic functions. One could further

3.2 Resonant soft X-ray powder diffraction from RMnO3 (R=Tm, Lu) 63

divide the type-II multiferroics into two subgroups: one in which ferroelectricity is caused by a

particular type of cycloid magnetic structure, and one in which it is caused by collinear magnetic

structures. Examples are TbMnO3, Ni3V2O6 and MnWO4. In these systems magnetic frustration

is released by moving magnetic ions. This leads to a shift of the oxygen ions perpendicular to the

Mn-Mn bonds, which induces polarization along the shift direction. Examples of collinear type-II

multiferroics are RMn2O5, Ca3CoMnO6 and orthorhombic RMnO3, with R being a lanthanide or

Y ion. We discuss experimental results of the latter in the sections 3.2 and 3.3.

3.2 Resonant soft X-ray powder diffraction from RMnO3

(R=Tm, Lu)

In this section we present experiments on orthorhombic (o) perovskites RMnO3, which are type-II

multiferroics. Note that the hexagonal RMnO3 perovskites are also multiferroics but of type-I.

We focus on (o)RMnO3 with the smaller ionic radii rR (R=Tm, Y, Lu) whose structure belongs

to the space group Pnma at room temperature. Our motivation to study these materials is the

larger electric polarization exhibited in the multiferroic phase compared to many other multiferroic

systems [9], [10].

The (o)RMnO3 perovskites exhibit different magnetic ground states as a function of the size of

the A-site R ion (Figure 3.2). For R=La it possesses an A-type AFM order below TN=140 K. For

R=Eu and Gd, a sinusoidal incommensurate (ICM) structure is below TN , which changes into an

A-type structure at lower temperatures, whereas for rR≤rTb the ICM structure transforms into a

spiral structure when cooled. Finally, for rR<rHo the magnetic structure is different again. The

ICM structure transforms into an E-type AFM structure upon cooling (Figure 3.2) [11].

Only two of these magnetic phases are ferroelectric, namely the cycloidal and the E-type struc-

tures. In general, the smaller rR, the larger the electric polarization (Figure 3.3) [10]. Furthermore,

for decreasing rR the transition temperature from the paraelectric phase (PE) to the ferroelectric

phase increases. Thus (o)RMnO3 with R=Lu,Tm are promising multiferroic materials since they

exhibit a strong magnetoelectric coupling and high electric polarizations compared to the other

(o)RMnO3 perovskites.

In this section, the magnetic order and the orbital order of the Mn and R sublattices are inves-

tigated in polycrystalline (o)TmMnO3 and (o)LuMnO3 by resonant soft X-ray powder diffraction


Figure 3.2: Magnetic phases of (o)RMnO3 as a function of T and rR. Inset shows a sketch of theMnO2 framework, d3x2−r2/d3y2−r2 orbitals and the magnetic order. Figure reproduced from [11].

Figure 3.3: Magnetic phases and electric polarization of (o)RMnO3 as a function of the rR andthe transition temperature T. TN1 is the temperature transition from the paramagnetic to ICM-sinusoidal AFM structure, TN2 a further transition to a bc cycloidal structure, where in the caseof R=Eu1−xYx a possible transition to ab cycloidal sets in TN3. Figure reproduced from [10].


(RSXPD). We specifically studied the magnetic order of the Mn ion in both systems, as well

as that of the Tm sublattice. This allows acquiring direct insight into the magnetic interaction

between the R and Mn ions in these multiferroic systems, which cannot be easily obtained from

neutron diffraction. Studies with neutrons on (o)TmMnO3 have found an ICM AFM phase with

an ordering wavevector of (h 0 0) that sets in at TN=40 K. Below TC=32 K a further magnetic

transition locks the system into a commensurate (CM) E-type AFM structure, with collinear Mn

magnetic moments. Additionally, Tm3+ magnetic moments were found to align along the c-axis

at 2 K [9]. (o)LuMnO3 is antiferromagnetic below TN=40 K, and below TC=35 K it possesses

an E-type antiferromagnetic order with the Mn moments antiferromagnetically aligned along the

a- and b- axes. This result was also obtained by neutron diffraction [12]. The latter study found

no satellite peaks down to T =8 K, indicating a single phase magnetic E-type structure.

For our study, RMnO3 samples were synthesized in the following way: first hexagonal RMnO3

was obtained from Mn2O3 and R2O3 by solid reaction. Then the powders were sealed in gold

capsules and heated to 1573 K for 1 h in a belt-type press and cooled down to room temperature

before the pressure was released [11]. High oxygen pressure (6 GPa) was used, in order to stabilize

the orthorhombic phase. These samples were also investigated by powder neutron diffraction [9],

[13].

RSXPD at low temperatures was performed at the RESOXS end-station [14] at the SIM

beamline [15] of the Swiss Light Source at the Paul Scherrer Institut in Switzerland. For the

experiments, polycrystalline pellets of 6-10 mm diameter were first pressed and then glued on a

copper sample holder mounted on the cold finger of a He flow cryostat. An in-vacuum, water-

cooled CCD camera (Roper Scientific, MPE II) was used to collect sections of the Debye-Scherrer

powder diffraction rings (Figure 3.4) as well as the X-ray fluorescence light.

3.2.1 Tm M 5 edge

Figure 3.5 shows the X-ray absorption spectra of (o)TmMnO3 taken in fluorescence mode, as well

as the photon energy dependence of the magnetic (12

1 0) reflection in the vicinity of the Tm M5

edge. Three main features appear in the X-ray absorption data exactly at the energies at which

there are local minima in the energy dependence of the (12

1 0) reflection. These features correspond

to multiplet transitions from the 3d5/2 core to 4f valence states at the Tm M5 edge. The inset of

Figure 3.5 shows an integrated section of the Debye-Scherrer ring of the (12

1 0) reflection (Figure


Figure 3.4: Image taken by the in-vacuum, water-cooled CCD camera of the magnetic (12

1 0)reflection of TmMnO3 in the vicinity of the Tm M5 edge. The vertical white stripe is a section ofthe (1

21 0) powder diffracted ring.

3.4) as a function of 2θ, taken in the E-type AFM phase of (o)TmMnO3 at 9 K with 1464.25 eV X-

ray energy. The shape of the reflection is well described by a Lorentzian function (solid line). The

diffraction measurements were performed both with π and σ incident light polarization. As shown

in Figure 3.5 (bottom), the diffracted signal measured with π incident light polarization is larger

than that measured with σ polarization. The large difference of the scattered intensity between

those two incident light polarizations in all the spectral features supports the interpretation of a

pure magnetic origin of the signal, a result which can be attributed to the zero magnetic intensity

in the σ - σ′ channel (as discussed in section 1.5). These data demonstrate for the first time that

resonant magnetic powder diffraction can be performed at the M -edges of 4f systems, even for

weakly ordered magnetic moments.


Figure 3.5: Top: X-ray absorption data collected in fluorescence yield mode with π polarizedlight. Bottom: energy dependence of (1

21 0) reflection for T =9 K and with σ and π incident

X-ray polarization. Inset: scattered intensity across the Debye-Scherrer diffraction ring for themagnetic (1

21 0) reflection in (o)TmMnO3. The scan was performed in the vicinity of the Tm M5

edge at 1464.25 eV.


Figure 3.6: Top: temperature dependence of the intensity of the (h 1 0) reflection, taken at twoof the multiplet features (1464.25 eV and 1467 eV) in the vicinity of Tm M5 edge with π incidentlight polarization. Bottom: temperature dependence of h for the magnetic (h 1 0) reflection.

In order to further verify the magnetic origin of the features found in the energy dependence of

the (12

1 0) reflection, the temperature dependence of the reflection (h 1 0) (with h=(12± δ)) has

been determined at two different energies (see Figure 3.6). The drop in intensity with increasing

temperature is direct evidence of the magnetic origin of these features. The concave shape of

the scattered intensity of (h 1 0) reflection below 32 K implies an induced magnetic order of

Tm3+ moments, possibly correlated with the onset of ferroelectricity. Induced 4f moments have

already been observed in TbMnO3 [16], in DyMnO3 [17] and other 3d -4f systems such as NdNiO3

[18]. Suprisingly, X-ray intensity is also observed in the ICM phase, above TC . This result is

not expected, since above TC no ordering of the Tm3+ magnetic moment occurs, according to

neutron scattering [9]. Contrary to neutron scattering, RSXPD is element selective. It enables us


to directly observe non-zero magnetic intensities caused by very weakly ordered Tm moments that

are otherwise not detectable. In order to verify that the observed non-zero intensity in the ICM

phase originates from magnetic order of Tm3+ ions, we determine the temperature dependence of

the position of the magnetic (h 1 0) reflection along h (Figure 3.6 bottom). A shift of the (h 1 0)

reflection is observed from h=0.5 to 0.46, where the structure enters the ICM phase at T≈32 K,

meaning that the diffracted intensity caused by the Tm moments above TC also departs from the

E-type magnetic order.

In addition to the (12

1 0) reflection, the space group forbidden (1 0 0) reflection is observed.

The energy dependence of this reflection is shown in Figure 3.7a. The spectral shape of this

reflection is completely different from that of the (12

1 0) reflection, shown in Figure 3.5 (bottom),

indicating that this reflection is not related to the magnetic order. The observed intensity is caused

by Templeton scattering [19], which is described by the anisotropic tensor of susceptibility (ATS).

It is a measure of the asphericity of the probed valence shell. It has been previously demonstrated

that the resonant diffraction intensity of this space-group-forbidden reflection is caused by a tilt of

the oxygen octahedra in Pr1−xCaxMnO3 and LaMnO3, which exhibit the same crystal structure.

The reflections were observed when the X-ray energy is tuned to the L absorption edge of the R

(Pr,La) ion [19]. The crystal field associated with the octahedra’s rotation induces splitting of

the unoccupied 5d states of the R ions, which produces the resonant intensity. In our study, the

resonant intensity of the (1 0 0) reflection is attributed directly to the orbital order of the Tm 4f

states. This order is also caused by the tilts of the oxygen octahedra. However, in this case the

order is a result of crystal field splitting acting on the open 4f valence shell. The orbital reflection

does not change with temperature across the transition, implying that the population of the 4f

ground state remains constant at these temperatures. This indicates the presence of an energy

gap of greater than 10 meV in the 4f excitation spectrum.

3.2.2 Mn L2,3 edges

In order to gain more insight into the magnetic structure of the Mn ions for both (o)TmMnO3 and

(o)LuMnO3, we measured the magnetic (12

0 0) reflection in the vicinity of the Mn L2,3 edges. For

both materials the temperature dependence of the magnetic (12

0 0) reflection is shown in Figure

3.8. The onset of magnetic order for both materials is at approximately TN=40 K. However, the

intensity increases sharply below TC for (o)TmMnO3 [9]. For (o)LuMnO3 the sharp increase in


Figure 3.7: a) Photon energy dependence and b) temperature dependence of the structurallyforbidden (1 0 0) reflection with σ light polarization and photon energy in the vicinity of the TmM5 edge. The red line guides the eye.

Figure 3.8: Temperature dependence of the magnetic (12

0 0) reflection in (o)TmMnO3 and(o)LuMnO3 taken at the Mn L3 edge. The red curves guide the eye.


Figure 3.9: The spin-canted magnetic structure of (o)TmMnO3 and (o)LuMnO3 below TC . Theblack arrows represent the magnetic structure as proposed by neutron scattering [9]. The redarrows represent the spin-canted structure obtained by the RSXPD experiments.

intensity starts at 35 K. In both systems, the onset of magnetic intensity is related to TC and

not to occurrence of antiferromagnetic order. This result is consistent with magnetoelectric mea-

surements [10], which showed a slightly higher ferroelectric transition temperature for (o)LuMnO3

compared to (o)TmMnO3. This difference in TC may arise from the different ionic radii rR, which

results in different octahedra tilts which themselves influences the Mn-O-Mn interactions.

To understand the appearance of the magnetic reflection at TC , we calculate the magnetic

structure factor (Equation 2.4) for the magnetic (12

0 0) reflection. Using the magnetic structure of

(o)TmMnO3 from [9] (determined by neutron scattering) results in F=0. The proposed magnetic

structure is therefore incomplete, and a spin canting along the b-axis should be introduced, as

in the case of multiferroic YMnO3. This produces a non-zero F, which is solely sensitive to the

b-axis component. This spin-canted magnetic structure is illustrated in Figure 3.9. The effect of

the proposed spin-canting on multiferroic properties remains to be investigated theoretically.

Interestingly, the intensity of the magnetic (12

0 0) reflection in (o)LuMnO3 is significantly

larger than that of the (o)TmMnO3. This is attributed to a larger canting angle of the manganese

magnetic moments towards the b-axis for (o)LuMnO3. The enhanced canting could be due to the


larger rotation of the oxygen octahedra, caused by the smaller ionic radius of Lu. It could also

be related to the absence of coupling between the Mn and Lu magnetic moments, since Lu has

a closed 4f - shell. In contrast, the Tm 4f - magnetic moment pointing along the c-axis might be

significantly coupled to the Mn moments, reducing their tilt towards the b-axis.

Also remarkable here is the vanishing of the (12

0 0) reflection’s intensity above TC , even though

the magnetic order of the Mn ions disappears at TN . As our experiment detects only the canted

component along the b-axis in the AFM E-type magnetic structure, it might be that the ICM

AFM structure above TC is fully sinusoidal without a moment component along the b-axis [10].

Another possible explanation is that there is intensity above TC , but the splitting of the peak

(12± δ 0 0) in the ICM phase ([9]) distributes the already weak intensity over a larger region in

2θ, making it much more difficult to separate it from the background within the limited 2θ range

of the CCD camera.

Finally, Figure 3.10 presents the photon energy dependence of the magnetic (12

0 0) reflection of

(o)LuMnO3 at 9 K, for both linear (π and σ, Figure 3.10a) and circular (Figure 3.10b) incident light

polarizations. The Mn moment component along the b-axis results in a ratio of I=Iσ/Iπ=0.277

in the powder average, obtained by integrating Equation 1.23 over the azimuthal angle. This

is consistent with the observed ratio in Figure 3.10a. Furthermore, the energy dependence with

incident circular light polarizations (Figure 3.10b) produces no difference between left- and right-

circular polarizations. This is expected since a helical or circular component is averaged out in a

polycrystalline material. The energy dependence of the (12

0 0) reflection on (o)LuMnO3 is very

similar to that of other RMnO3 compounds, such as TbMnO3 [20]. This indicates that additional

components of the magnetic moments along other directions (as in the case of TbMnO3 ([21], [22])

do not strongly affect the energy dependence of the magnetic reflection. This is in contrast to

recent theoretical predictions [23].

3.3 Resonant soft X-ray diffraction from single-crystalline YMnO3 film 73

Figure 3.10: Photon energy dependence scans of the magnetic (12

0 0) reflection from (o)LuMnO3

in linear a) and circular b) light polarizations at Mn L2 and L3 edges at 9 K.

3.3 Resonant soft X-ray diffraction from single-crystalline

YMnO3 film

We conclude this chapter with the study of the magnetic structure of another multiferroic of

the (o)RMnO3 family, the (o)YMnO3. This material belongs in the group of small ionic radii

multiferroics, together with TmMnO3 and LuMnO3 [11], but lies close to the transition to the

cycloidal magnetic state (Figure 3.3). For this study, we obtained a single crystal in form of thin

epitaxial film [24]. The 40 nm thick film of (o)YMnO3 was grown using PLD at 890 0C in an

oxygen pressure of 1 mTorr and it was in-situ annealed for 30 min at 430 0C in 760 Torr oxygen

pressure. It was found that the thin film of (o)YMnO3 exhibits a large electric polarization at 40

K (P=0.8 µC/cm2) [24]. The film showed an AFM transition at 45 K, a ferroelectric transition

at 40 K and a third transition at 35 K (Figure 3.11) [24].

Nakamura et al [24], proposed that the ferroelectric transition (TN1) at TC=40 K is induced

by either an E-type or an ab-cycloidal magnetic structure. They suggested that the magnetic

structure between 40 and 35 K is ab-cycloidal with a minor volume of coexistent E-type and


Figure 3.11: Temperature dependence of the electric polarization along the a-axis. The transitiontemperatures TN , TN1, and TN2 are indicated by vertical lines. Figure reproduced from [24].

below 35 K transforms entirely into the E-type, accompanied by an increase in polarization.

In this section, we present our results on the magnetic structure measured by RXD at the Mn 2p

−→ 3d edges. We have performed temperature and azimuthal angle dependence measurements of

the magnetic (0 qb 0) (qb≈0.5) reflection (using orthorhombic Pbnm symmetry) of the (o)YMnO3

thin film. The definition of the azimuthal angle ϕ is shown in Figure 3.12, in which we set ϕ=0

if the c-axis of the thin film lies in the scattering plane.


Figure 3.12: Diffraction geometry, defining the azimuthal angle ϕ with respect to the polarizationof the incident light.

Reciprocal space scans along the (0 qb 0) (qb≈0.5) reflection for various temperatures measured

at the 643.1 eV (at ϕ=0) are displayed in Figure 3.13. For the measurement, we used π and σ

incident polarization (Figure 3.13 (a) and (b), respectively). The reflection appears at 45 K,

exactly at the antiferromagnetic phase transition [24]. This is in contrast to previous results on

(Tm,Lu)MnO3 where a significant signal is observed only below TC . Satellite peaks are observed

on both sides of the main reflection’s intensity (known as Kiessig fringes). The intensity of the

peak and the fringes increases as the temperature decreases. The intensities for π and σ incident

polarizations are found to be equal for all temperatures for ϕ=0. Surprisingly, the peak position

shifts to higher qb values when the temperature is decreased. The peak shifts from qb values

0.457 to 0.491 in the temperature region between 11 and 44 K (Figure 3.14). This indicates

that the magnetic structure is ICM in the whole temperature range, whereas only the CM E-

type magnetic structure is observed in bulk YMnO3 at low temperatures [25]. We attribute this

different behaviour to strain effects caused by the substrate, which are important for magnetism

in epitaxial films [26].


Figure 3.13: The temperature dependence of the (0 qb 0) reflection in both linear incident polar-ization channels π and σ taken at 643.1 eV. The arrow indicates the decrease in temperature. Thesmall vertical bars indicate the maximum of the peak. Figure reproduced from [27].

Figure 3.14: Temperature dependence of the (0 qb 0) peak position (a) and intensity (b). Figurereproduced from [27].


Figure 3.15: Azimuthal angle dependence of the magnetic (0 qb 0) (qb ∼ 0.5) reflection. The solidline represents the fit. Figure reproduced from [27].

Measurement of the azimuthal angle dependence of the magnetic (0 qb 0) (qb ∼ 0.5) reflection

allows us to determine the orientation of the Mn spins (Figure 3.15). For ϕ=0 and 1800 the

intensities resulting from incident π and σ polarizations are equal. For increasing deviations of

ϕ angle from those values, the intensity for incident π polarization increases, whereas those for

incident σ polarization decreases.

To understand the azimuthal dependence we calculate the magnetic structure factor (Equation

2.4) for a canted E-type antiferromagnet (Figure 3.16a) and for an ab-cycloydal spin configuration

(Figure 3.16b). In both cases the structure factor is nonzero only due to the spin component

pointing along the c-axis. The calculation is shown by a solid line in Fig. 3.15 [27], and is in good

agreement with the measurement, even though the structure remains incommensurate.

The photon energy dependence of the magnetic (0 qb 0) (qb ∼ 0.5) reflection is presented in

Figure 3.17 at 44 and 11 K, with ϕ=0. The spectral shape is identical for both incident linear

light polarizations at both temperatures. A comparison of these spectra with those observed in

(o)TmMnO3, (o)LuMnO3 and in other studies [20], [21], indicates that magnetic reflections in all

(o)RMnO3 compounds have similar photon energy dependencies. It particularly shows that the

specular reflectivity of the film has little contribution to the observed reflection.

Finally we address the relation between the temperature dependencies of the reflection’s inten-

sity (Figure 3.14) and the electric polarization (Figure 3.11). The amplitude of the commensurate

(0 1 0) reflection, which is Pbnm space group forbidden, is directly related to the atomic displace-


Figure 3.16: The spin canted E-type AFM structure (left hand side), and the ab-cycloidal one(right hand side).

Figure 3.17: Photon energy dependence of the magnetic (0 qb 0) (qb ∼ 0.5) reflection measuredin both π and σ incident polarization light at 44 K (left hand side) and 11 K (right hand side) inϕ=0 azimuthal angle. Figure reproduced from [27].


ment induced by the commensurate E-type magnetic structure. The reflection appears below TN2

at 35 K and no incommensurability is found. This was measured by Wadati et al, with hard X-ray

diffraction (E=12 keV) [27]. The atomic displacement causing the occurrence of this reflection is

also responsible for the large electric polarization. Furthermore, the appearance of the reflection

below TN2 is consistent with the strong enhancement of the electric polarization shown in Figure

3.11. A clear picture of the low temperature behavior in the epitaxial (o)YMnO3 film can now

be formed. Combining the results of soft and hard X-ray diffraction with the macroscopic mea-

surements in [24], we obtain the following picture: at TN=45 K the material transforms from a

paramagnetic to an antiferromagnetic state. A ferroelectric transition occurs at 40 K. Between 45

and 40 K the magnetic structure is ICM and sinusoidal with a spin-canting but without electric

polarization. Below TN1=40 K the magnetic moments form a spin canted cycloidal phase. Below

TN2=35 K, both the ICM cycloidal magnetic and the commensurate lattice-distortion reflections

are present, revealing coexistence of E-type and cycloidal structures as predicted from theoretical

calculations by Mochizuki et al [28].

3.4 Conclusions 80

3.4 Conclusions

In summary, we presented results concerning orthorhombic RMnO3 multiferroics with the smaller

ionic radii of the R series. We studied this particular group of type-II multiferroics because of the

larger electric polarization values they exhibit. Our goal was to better understand the interplay of

R and Mn ions in the E-type magnetic sublattice and its role in electrically polarized multiferroics

such as (o)YMnO3. Moreover, we wanted to clarify the magnetic structure of (o)YMnO3 because

of theoretically predicted coexistence of phases and their influence on the material’s ferroelectric

properties.

We demonstrated that resonant soft X-ray powder diffraction can be used to study electronic

and magnetic ordering phenomena also in 4f polycrystalline materials. The extension of this

technique is important, since there is a shortage of single crystals of (o)RMnO3 and it is difficult

to synthesize them. The data on the (h 1 0) reflection taken at the Tm M5 edge of (o)TmMnO3

show that the Tm magnetic moments already order in the incommensurate magnetic phase. They

exhibit a clear induced characteristic in the temperature dependence below TC . The observation

of the (1 0 0) reflection at the same resonance is directly related to the orbital order of the Tm 4f

electrons, caused by the tilts of the oxygen octahedra. The observed (12

0 0) magnetic reflection

at the Mn L3 edge indicates that the magnetic structure is not purely collinear E-type, but that

there is an additional spin canting in the ferroelectric phase. This canting is significantly larger

for the (o)LuMnO3. It would be interesting to use ab initio methods to investigate if and how this

spin canting influences the exchange striction that produces the enhanced polarization in E-type

magnetic structures.

To complete the study on (o)RMnO3, we presented results on the magnetic structure and as-

sociated structural distortions in an (o)YMnO3 thin film. The incommensurate (0 qb 0) (qb ∼ 0.5)

magnetic peak appears below 45 K and corresponds to the ab-cycloidal phase. The commensu-

rate (0 1 0) lattice-distortion peak is caused by the E-type magnetic structure below 35 K. These

observations are consistent with the coexistence of spin canted E-type and cycloidal states, with

the large polarization values possibly originating from the E-type antiferromagnetic structure.

BIBLIOGRAPHY 81

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84

Chapter 4

Conclusions and outlook

In Chapter 1, the physics of synchrotron X-rays and resonant X-ray diffraction are described,

along with their application for revealing the magnetic and electronic properties of colossal mag-

netoresistive and multiferroic perovskite manganites.

In Chapter 2 we presented the effects of X-ray illumination and electric fields on the mag-

netic/orbital order of an epitaxial Pr0.5Ca0.5MnO3 film. We observed that X-ray illumination

photodopes the thin film by creating defects in the charge and orbital order state, improving the

long range order associated with the antiferromagnetic state. The manganese magnetic moments

rotate to a more canted structure with a ferromagnetic component along the z -axis. This enhances

the antiferromagnetic/orbital reflection, and increases the conductivity. The magnitude of these

changes can be varied by tuning either the X-ray flux density or by changing the exposure time.

The effect is persistent and local, allowing us to imprint magnetic and electronic bits with X-rays.

In the second part of this chapter, we investigated on a microscopic level the metal insulator

transition of Pr0.5Ca0.5MnO3 by applying electric fields. Even though the applied fields could in-

fluence the orbital and charge order of the material, our data does not support a direct interaction.

We observed that applying high voltages (up to 900 V) significantly increases the temperature

of the thin film due to resistive heating. This is supported by the fact that the magnetic and

orbital reflections are similarly affected at positions outside the electrode gap, where the electric

field created by the applied voltage is negligible.

In the third chapter, we presented experiments to study the electronic and magnetic ground

state of orthorhombic RMnO3 (R=Tm, Lu, Y). In TmMnO3 we observed that the Tm magnetic

moments already order in the incommensurate magnetic phase. The moments are induced by

85

the order of the Mn sublattice resulting in a concave temperature dependence below TC . We

were also able to directly measure the orbital order of the Tm 4f electrons, which exhibited no

change throughout the magnetic phase-transitions occurring at low temperatures. The magnetic

structures of both TmMnO3 and LuMnO3 are not purely of collinear E-type. There is an additional

spin canting along the b-axis in the ferroelectric phase. Measurements of a YMnO3 thin film

showed an ab-cycloidal phase below 45 K and an E-type magnetic structure below 35 K. These

observations are consistent with the coexistence of spin-canted E-type and cycloidal states, with

large polarization values originating from the E-type antiferromagnetic structure.

We wish now to present an outlook for future work on the X-ray illumination effect, underlining

the immediate questions related to our results. The foremost question is whether this behavior

exists at other doping levels of Pr1−xCaxMnO3. It is likely that samples with x= 0.35, 0.4 and

0.45 will also exhibit such behavior, because their magnetic structures are also a pseudo-CE-type,

but with much larger canting angles. Exposing these materials to similar X-ray fluxes should lead

to an increase both in the reflection’s intensity and in the conductivity.

Secondly, it is of great importance to assess whether this effect can be observed by using lower

energies, such as ultraviolet and visible laser light. Such achievements could pave the way to

applications, for example through manipulation of spins with optical lasers. Another application-

oriented question is whether the illumination induces a ferromagnetic moment. One way to answer

this would be through X-ray magnetic circular dichroism, which would allow us to easily probe

the ferromagnetic moment of the Mn ions. Furthermore, this effect might be able to compete with

existing data storage densities if the experiment would include use of a zone plate to imprint bits,

which would focus the X-rays to a spot size of around 10 nm.

Also important is probing any structural changes induced by the X-rays. This can be done by

measuring the structural superlattice reflection as function of the X-ray exposure time.

To investigate what happens to the orbital orientation under the X-ray exposure (which could

give insight on the increase in conductivity), the orbital reflection of the material can be measured.

This can be done at the energy of 652 eV where only the contribution of the orbital order is probed.

Regarding the multiferroic perovskite manganites TmMnO3 and LuMnO3, a question we have

yet to address is whether there is a coexistence of two different ground states, as in the case of

YMnO3, and whether this is related to the high electric polarization. This thesis showed that

both these multiferroic manganites possess a spin-canted E-type magnetic structure. Does any

86

cycloidal structure exist? This question can be answered by measuring a structural superlattice

reflection of these two multiferroics with hard X-ray diffraction (e.g. a temperature dependence

measurement of such a reflection may reveal its existence).

87

Acknowledgments

I would like to take this opportunity to thank Dr. Urs Staub and Prof. Dr. Johannes Friso

van der Veen who gave me the opportunity to do my doctoral studies at the ETH Zurich and to

perform research at the Swiss Light Source of the Paul Scherrer Institut. In particular, I thank

my supervisor, Dr. Urs Staub, for his brilliant guidance and for the excellent collaboration we

had. In these three and a half years he was always willing to help me during experiments and to

explain concepts when they were not clear to me. His very friendly character and communication

skills were crucial for my understanding of the different concepts and I cannot recall any moment

of tension between us.

I should also give many thanks to my group colleagues, in particular Dr. Valerio Scagnoli for

his great support during the experiments, for discussions about science and modelling, for creating

a spectacular working environment, and for being my office mate. Many thanks are also due to

Winnie (Dr. S.W. Huang) for discussions about science and for collaborating in hard experiments

during the night. I would like also to thank my former group colleagues Dr. Yves Bodenthin, Dr.

R.A. de Souza for helping me in the beginning of my doctoral studies and Mr. Yoav Windsor for

fruitful discussions and English corrections.

I would also like to thank all the colleagues at the Swiss Light Source, Dr. Benjamin Watts

and Ms. Stephanie Stevenson (significant contribution to the English text style), Dr. Jan Dreiser

(for the german text style), Dr. Claude Monney, Dr. Souliman El Moussaoui, Ms. Ana Balan,

Mr. Michele Buzzi, Dr. Loic Le Guyader, Dr. Elena Mengotti and many more for the excellent

working environment and help whenever it was requested. Many thanks are due to Prof. Dr.

Frithjof Nolting, our group leader, for the very good group organization, to the beamline scientists

Dr. Armin Kleibert and Dr. Arantxa Fraile Rodriguez and to our technicians Mr. Juri Honegger

and Ms. Andrea Steiger.

At this point, I would like to thank my parents as well as my brother for the huge support

88

they provided me during all those years, particularly during my doctoral studies. Without their

vital help over many years, nothing would have been accomplished. I would like also to thank

my friends Dr. Ioannis Taxidis, Dr. Nikolaos Fanidakis, Mr. Anestis Haritidis, Dr. Nikolaos

Prasianakis and Dr. Christoforos Moutafis for the fun and the nice daily discussions we had.

Last but not least, I would like to thank Ms. Kathrin Meier for her love, her caring, her

understanding and daily support, especially during this last stressing period. I would like to

thank her for making my life beautiful every day.

89

Curriculum vitae

Personal information

Education

Doctor of Science, 11.2008-06.2012

Eidgenssische Technische Hochschule Zurich (ETHZ)

Paul Scherrer Institut (PSI), Switzerland

Thesis title: “Electronic and magnetic ordering phenomena in multiferroic and magnetoresistive

manganites.”

Master of Science in Nanoscience and Nanotechnology, 09.2006-11.2008

Physics Department, Aristotle University of Thessaloniki, Greece

Thesis title: “Growth and characterization of solar cells and OLEDs onto flexible polymeric sub-

strates.”

Bachelor in Physics, 01.2001-06.2006

Physics Department, Aristotle University of Thessaloniki, Greece

Thesis title: “Study of the magnetron sputtering plasma properties for the growth of carbon and

boron nitride thin films.”

Trainee Pilot in Greek Air Force Academy, 09.2000-01.2001

90

Employment

Doctoral researcher, 11.2008-06.2012

Paul Scherrer Institut (PSI), Switzerland

Research project: “Electronic and magnetic ordering phenomena in multiferroic and magnetore-

sistive manganites.”

Researcher, 09.2005-11.2008

Laboratory for Thin Films, Nanosystems and Nanometrology, Greece

Research project: “Growth and characterization of flexible Solar Cells and OLEDs.”

Researcher, 08.2004-09.2004

Hahn Meitner Institut, Germany

Research project: “Chemical Mechanical Polishing of a-Si thin films for the growth of Solar Cells.”

Air Traffic Controller, trainee, 10.2003-04.2004

Thessaloniki Airport, Greece

Computer Skills

Mac OS X, Microsoft Windows, Microsoft Office, C++, Matlab, OriginLab, Epics, Kaleida-

graph, Latex.

Languages

Greek: Native speaker

English: Fluent (Certificate of Proficiency, University of Michigan, USA)

German: Fluent (C1 level, Zertifikat-Goethe Institut)

91

Awards-Scholarships

Teaching Fellowship in X-ray Diffraction: awarded by Prof. Van der Veen and Prof. M.

Schnenfeld, ETH Zrich, Switzerland

Best Young Scientist: awarded in 4th International Workshop on Nanosciences & Nan- otech-

nologies (NN07), 2007 in recognition of the work “Monitoring of carbon plasma electrical properties

and the effect on the thin films structural properties.”

Master’s Program “Nanoscience & Nanotechnology”, awarded for entering in the second rank

among the students

“Charalambous Scholarship”, awarded by Physics Department, Aristotle University of Thes-

saloniki, Greece for having the highest marks in the academic year 2004-2005 in the major of Solid

State Physics and Material Science

Memberships

Member of Swiss Physical Society

Hobbies

Flying, sailing, cinema, reading, motorbikes

Publications

“Imprinting magnetic information in manganites with X-rays.”

M. Garganourakis, V. Scagnoli, S.W. Huang, H. Wadati, M. Nakamura, M. Kawasaki, V.

Guzenko, Y. Tokura and U. Staub.

Accepted in Phys. Rev. Lett. To be published soon.

“Magnetic and Electronic Orderings in Orthorhombic RMnO3 studied by Resonant Soft X-ray

Powder Diffraction.”

M. Garganourakis, Y. Bodenthin, R.A. De Souza, V. Scagnoli, A. Donni, M. Tachibana, H.

Kitazawa, E. Talayama-Muromachi and U. Staub.

92

Phys. Rev. B, 86, 054425, 2012.

“Resonant soft x-ray study on Pr0.5Ca0.5MnO3 to disentangle the origin of the electronic be-

havior in applied electric fields.”

M. Garganourakis, V. Scagnoli, S.W. Huang, H. Wadati, J. Okamoto, M. Nakamura, M.

Kawasaki, Y. Tokura and U. Staub.

on preparation

“Study of the growth of inorganic and organic electrodes onto polyethylene terephthalate sub-

strates.”

M. Garganourakis, S. Logothetidis, C. Pitsalidis, N.A. Hastas, K. Breza, A. Laskarakis, N.

Frangis.

Thin Solid Films, 518, 1124, 2009.

“Deposition and characterization of PEDOT/ZnO layers onto PET substrates.”

M. Garganourakis, S. Logothetidis, C. Pitsalidis, D. Georgiou, S. Kassavetis, A. Laskarakis.

Thin Solid Films, 517, 6409, 2009.

“Observation of orbital currents in CuO.”

V. Scagnoli, U. Staub, Y. Bodenthin, R. A. de Souza, M. Garcia-Fernandez, M. Garganourakis,

A. T. Boothroyd, D. Prabhakaran, and S. W. Lovesey.

SCIENCE, 332, 696, 2011.

“Ferrotype order of atomic multipoles in the polar ferrimagnetic GaFeO3.”

U. Staub, C. Piamonteze, M. Garganourakis, S. P. Collins, S.M. Koohpayeh, D. Fort, and S.

W. Lovesey.

submitted

93

“Origin of the Large Polarization in Multiferroic YMnO3 Thin Films Revealed by Soft- and

Hard-X-Ray Diffraction.”

H. Wadati, J. Okamoto, M. Garganourakis, V. Scagnoli, U. Staub, Y. Yamasaki, H. Nakao, Y.

Murakami, M. Mochizuki, M. Nakamura, M. Kawasaki and Y. Tokura.

Phys. Rev. Lett., 108, 047203, 2012.

“Magnetic structure and electric field effects in multiferroic YMn2O5.”

de Souza RA, Staub U, Scagnoli V, Garganourakis M, Bodenthin Y, Huang S.W, Garcia-

Fernandez M, Ji S, Lee SH, Park S, Cheong SW.

Phys. Rev. B, 84, 104416, 2011.

“Origin of the anomalous low-temperature phase transition in BaVS3.”

de Souza RA, Staub U, Scagnoli V, Garganourakis M, Bodenthin Y, Berger H.

Phys. Rev. B, 84, 014409, 2011.

“Magnetic order of multiferroic ErMn2O5 studied by resonant soft x-ray Bragg diffraction.”

Staub U, Bodenthin Y, Garcia-Fernandez M, de Souza R.A, Garganourakis M, Golovenchits

E.I, Sanina V.A, Lushnikov S.G

Phys. Rev. B, 81, 144401, 2010.

“Circularly polarized soft x-ray diffraction study of helical magnetism in hexaferrite.”

Mulders AM, Lawrence S.M, Princep A.J, Staub U, Bodenthin Y, Garcia-Fernandez M, Garganourakis

M, Hester J, Macquart R, Ling CD

Phys. Rev. B, 81, 092405, 2010.

“Parity- and time-odd atomic multipoles in magnetoelectric GaFeO3 as seen via soft x-ray

Bragg diffraction.”

Staub U, Bodenthin Y, Piamonteze C, Garcia-Fernandez M, Scagnoli V, Garganourakis M,

Koohpayeh S, Fort D, Lovesey SW.

Phys. Rev. B, 80, 140410, 2009.

94

“Orbital and magnetic ordering in Pr1−xCaxMnO3 and Nd1−xSrxMnO3 manganites near half

doping studied by resonant soft x-ray powder diffraction.”

Staub U, Garcia-Fernandez M, Bodenthin Y, Scagnoli V, De Souza R.A, Garganourakis M,

Pomjakushina E, Conder K.W.

Phys. Rev. B, 79, 224419, 2009.

Oral and poster presentations

Oral

Febr. 2012 Swiss Light Source Symposium, Switzerland

“Synchrotron X-rays able to probe hidden phases in manganites.”

Sept. 2011 JUMP Users Meeting, Paul Scherrer Institut, Switzerland

“Imprinting magnetic information in manganites with X-rays.”

Sept. 2011 Swiss and Taiwanese Light Source Meeting, Paul Scherrer Institut, Switzerland

“Imprinting information in manganites with X-rays.“

May 2011 ETH Zurich, Switzerland

“Metal Insulator Transition.”

June 2009 Swiss Physical Society and Austrian Physical Society Meeting, Innsbruck, Austria

“Resonant Soft X-ray Diffraction in RMnO3.“

Oct. 2008 Second international Symposium on Transparent Conductive Oxides, FORTH, Crete,

Greece

“Deposition and characterization of PEDOT/ZnO layers onto PET substrates.“

Poster

Febr. 2012 5th European School on Multiferroics, Ascona, Switzerland

“Writing magnetic bits in manganites with X-rays.”

Sept. 2011 Strongly Correlated Electron Systems, Cambridge, England

“Writing magnetic bits in manganites with X-rays.”

July 2011 MaNEP Meeting, Les Diablerets, Switzerland

“Enhancement of magnetic and orbital signal by X-ray illumination.”

June 2011 Swiss Physical Society and Austrian Physical Society Meeting, Lausanne, Switzerland

“Resonant Soft X-ray study on mixed-valent manganites to disentangle the origin of the electronic

95

behaviour in applied electric fields.”

Aug. 2010 Summer school in magnetism and correlated electron systems, Zuoz, Switzerland

“Resonant Soft X-ray Diffraction in TmMnO3 and LuMnO3.”

May 2008 European Material Research Society meeting, Strasbourg, France

“The role of PEDOT:PSS organic semiconductor on flexible OLED devices and flexible Solar

Cells.”


“Monitoring of plasma electrical properties and the bonding structure and properties of Carbon

and BN thin films.”

May 2007 4th International Workshop on Nanosciences & Nanotechnologies (NN07), Thessaloniki,

Greece

“Monitoring of carbon plasma electrical properties and the effect on the thin films structural

properties.”


“Study of the electrical properties of RF reactive carbon plasma.”

Teaching experience

2010-2011: Teaching Fellowship in X-ray Diffraction. Supervising doctoral, postgraduate and un-

dergraduate students in performing X-ray diffraction experiments in the Swiss Light Source (Paul

Scherrer Institut), Switzerland

February - June 2010: Teaching Fellowship in Physics I. Department of Informatics, ETH Zurich,

Switzerland

2007-2008: Supervisor of postgraduate and undergraduate students in the Laboratory of Thin

Films, Nanosystems-Nanometrology (LTFN), (Aristotle University of Thessaloniki (AUTH), Greece)

2005-2006: Private tutorials on Physics to undergraduate students and high school pupils.

Experimental Skills

User (as well as user support) at SIM Beamline at Swiss Light Source performing Soft X-ray

Diffraction experiments, Paul Scherrer Institut

Laue Diffraction and XRD measurements

Measuring magnetoelectric properties with SQUID

Measuring drain currents with Keithley

96

Capacitance measurements with LCR meter

Deposition of organic semiconductors on Transparent Conductive Oxides by spin coating

Deposition of Transparent Conductive Oxides by DC magnetron sputtering and Electron Beam

Evaporation techniques in an Ultra High Vacuum Physical Vapor Deposition (PVD) chamber

Deposition of a-C, a-C:H, Boron Nitride thin films by RF magnetron sputtering technique in a

High Vacuum Physical Vapor Deposition (PVD) chamber

Structural and optical analysis of thin films by Spectroscopic Ellipsometry

Monitoring of plasma electrical properties using the Langmuir Probe.

Chemical Mechanical Polishing system

Documents

Rights / License: Research Collection In Copyright - Non … · 2020-06-22 · MULTIFERROIC AND COLOSSAL MAGNETORESISTIVE MANGANITES Abhandlung zur Erlangung des Titels DOKTOR DER