Andreas Scholl,1 Marco Liberati,2 Hendrik Ohldag,3 Frithjof Nolting,4 Joachim Stöhr3

1Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA2Department of Physics, Montana State University, Bozeman, MT 59717, USA

3Stanford Synchrotron Radiation Laboratory, Stanford, CA 94309, USA4Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland

The role of planar and vertical domain walls and uncompensated interface spins in exchange bias.

X-RAY SPECTROSCOPY & IMAGING OF ANTIFERROMAGNETS VERTICAL DOMAIN WALLS

PLANAR WALLS UNCOMPENSATED INTERFACE SPINS

Malozemoff, PRB 35, 1987

Random field problem:

Competition between domain wall and interface energy:

Meiklejohn & Bean, Phys. Rev. 102 1956

Ideal interface:

Experiment:

AFM

FM

INTRODUCTION

Observations about exchange bias:

• Exchange bias is an interface effect.

• Exchange bias is a result of symmetry breaking.

• Exchange bias appears in a variety of materials.

Basic physics understood – Exchange interaction

X-ray microscopy and spectroscopy bring forth a microscopic understanding.

Co domain switching

LaFeO3Local loops

-223 Oe 92 Oe

103 Oe 223 Oe

XMCD

XMLD

[100]

[010]

10 mm-200 -100 0 100 200

Field (Oe)

1 2

12

+30 Oe

-30 Oe

LaFeO3

F. Nolting et al., Nature 2000

Co/LaFeO3

Co

A. Scholl et al., APL 2004

Domain area distributionLocal exchange bias

Bias field vs. domain area

Local, remanent hysteresis loops of Co/LaFeO3 show

a dependence of the variance of the local bias field

with the domain size. Large domains show a small

variance. For small domains a large width of the

domain size distribution was observed. The

sample was measured as-grown and did not

possess a macroscopic bias, explaining that both

directions of the bias occurred with equal

probability. An approximately linear dependence of

the width of the bias distribution with the inverse

domain diameter is in accordance with predictions

of the model proposed by Malozemoff.

CONCLUSIONS

Coferromagnetic

NiO interfaceferromagnetic

NiOantiferromagnetic

XMCDXMCDXMLD

NiO Co

[010]

[100]

~1 monolayer

E

Ohldag et al. PRL 2001

-3k -2k -1k 0 1k 2k 3k

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

Applied Field (Oe)

-15

-10

-5

0

5

10

15

Mn

Co

XM

CD

Asy

mm

etry

(%

)

0.2 0.3

0.1

0.2

3nmCo/NiO

2nmCo/IrMn

2nmCoFe/PtMn[a]

1nmCoFe/PtMn[a]

3nmCoFe/PtMn[b]

Spinned (mBohr)

Inte

rfac

ial E

nerg

y (m

J/m

2 )

Ohldag et al. PRL 2003

Redox reaction atCo/NiO interface

AFM Interface loop compared with FM loop

Coupling energy scales with density of pinned spins

NiO(001)

NiO/Si

Stöhr et al., PRL 1999Scholl et al., Science 2000Ohldag et al., PRL 2001

s

XMCDCoL3/L2Co

LaFeO3

E

LaFeO3

B/AXMLD

Ferromagnet

Antiferromagnet

[100] [110]

X

h

H

rightE

leftE

NiO XMLD

Co XMCD

5 m

[011]

[011]

HAB

FM

A FM

H

FM

A FM

A B

H

H

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

1.25

L 2 ra

tio

Field (T)

HE H||E

0

10

20

30

40

50

Wall Angle Fit Error

An

gle

(°)

[110]

[100]

X

h

H

horE

verE

Y

Z

PEEM imaging shows that the exchange

coupling of Co to a NiO(001) substrate

results in a uniaxial anistropy. Two classes

of Co domains possess easy axes along in-

plane <011> directions. A magnetic field

applied along one <011> direction leads to

switching of one class of domains at low

field (A) and rotation of the other class of

domains at high field (B).

Linear dichroism spectroscopy (XMLD)

measures the rotation of the NiO anti-

ferromagnetic axis in response to the

rotation of the Co magnetization in an

external field. Spectra show no spin-flop

of NiO without Co cap layer but evidence

of a rotation of the surface magnetic

moments in the presence of a Co layer.

The layer acts like a lever that twists the

magnetic structure of the AFM. A planar domain wall parallel to

the interface is wound up. The data was fitted applying a model

developed by Mauri et al. Fitting parameters are the interface

exchange stiffness A12 and the antiferromagnetic domain wall

energy EAFM. For Co/NiO(001) we find EAFM = 0.66 mJ/m2 and A12 =

1.52·10-13 J/m.

AFM domain wall energy Interface energy

FM anisotropy Zeeman energy

Uncompensated spins at the

surface of the NiO single

crystal are magnetically

coupled to a Co layer,

indicated by the identical FM

domain images of Ni and Co.

The uncompensated spins

are the result of a chemical

reaction at the Co/NiO interface leading to a reduction of the

NiO surface and partial oxidization of the Co layer.

Uncompensated interface spins were also observed for other

combinations of ferromagnets and ferromagnets, for example

CoFe/PtMn and Co/IrMn. Element-specific hysteresis loops of

the uncompensated moments at the surface of the

antiferromagnet showed a pinned fraction, which was aligned

with the bias direction of the material. Rotation of the bias

direction led to a clear vertical loop shift. The amount of

pinned uncompensated spins in several materials was found

to be proportional to the exchange bias interfacial energy,

suggesting that pinned moments are responsible for the bias.

NiO/MgO(001)LaFeO3

PEEM-2 at BL 7.3.1.1 Octupole magnet at BL 4.0.2

PEEM images of

Antiferromagnets

X-ray absorption spectroscopy (XAS) is an element-specific technique that

measures the chemical state and the electronic structure of materials. The

magnetization of a ferromagnet relatively to the x-ray propagation direction can be

determined using circularly polarized x-rays (X-ray Magnetic Circular Dichroism).

Sum rules quantitatively determine the spin and orbital moment. The magnetic axis

of antiferromagnets can be sensed using linearly polarized x-rays (X-ray Magnetic

Linear Dichroism). In combination with microscopy techniques like Photoemission

Electron Microscopy (PEEM), x-ray techniques are uniquely capable of visualizing

domain structures of ferromagnets and antiferromagnets at high spatial resolution.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0.0

0.1

0.2

0.3

0.4

0.5

0.6

NiO(001) NiO/Ag(001) poly-NiO/Si RT poly-NiO/Si 400K

XM

LD

(re

l. to

1)

Field (T)

The domain wall rotation of different NiO materials in contact

with 2.5 nm Co is compared. A strong rotation is found in a

NiO single crystal. An epitaxial NiO film on Ag(001) shows an

intermediate rotation while a polycrystalline NiO film on Si

shows a very small rotation. Only the polycrystalline film

possesses a significant exchange bias. The decreasing

Mauri-Model:

-200 -150 -100 -50 0 50 100 150 200

-10

-5

0

5

10

L3

XM

CD

(a

.u.)

Field (mT)

poly-NiO/Si RT 400 K

structural quality going from a single crystal over an epitaxial film to a

polycrystalline film results in a higher defect density and better pinning

of domain walls in the soft, low-anisotropy antiferromagnet NiO. On one

hand this leads to a greatly reduced planar wall rotation, on the other

hand to much better biasing properties. We learn that good exchange

bias materials are characterized by a strong anisotropy or a high defect

density to prevent erasure of the bias state by the generation of a planar

wall. A planar wall will likely play no role in high-anisotropy

antiferromagnets but needs to be taken into consideration in soft

antiferromagnets, like NiO.

• Exchange bias is mediated by uncompensated spins at the surface of the antiferromagnet.

• Uncompensated spins are randomly distributed and lead to an enhanced bias in small,

lateral domains in accordance with Malozemoff’s model.

• Planar walls appear in soft, structurally perfect antiferromagnets but likely play no role in

hard, polycrystalline antiferromagnets with good exchange bias properties.

Scholl et al., PRL 2004

The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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