140 Ce probe nuclei used to measure magnetic hyperfine fields in rare-earth magnetic compounds A. W. Carbonari Hyperfine Interactions Laboratory

Magnetic hyperfine interactions on 140Ce in rare-earth magnetic compounds A. W. Carbonari

140Ce probe nuclei used to measure magnetic hyperfine fields in rare-earth

magnetic compounds

A. W. Carbonari

Hyperfine Interactions Laboratory


Introduction


Introduction

Pelletronaccelerator

HelenaPetrilli

Nuclearreactor

IPEN

PhysicsInstitute

PAC Lab


HFI Laboratory

Two Closed-cycle He gas refrigerator: 8 K to 325 K

Two PAC machines with 4-BaF2 detectors

One PAC machine with 4-LSO detectors

(lutetium oxyorthosilicate, Lu2SiO5)

+


HFI Laboratory

Liquid He device: 1.2 K to 400 K

One Tubular graphite furnace: 295 K to 1700 K


HFI LaboratoryPAC spectrometer with 6 BaF2

detectors

With closed-cycle He refrigerator

With furnace


HFI Laboratory

Chemical laboratory for sample preparation

- Arc-melting for intermetallic samples- sol-gel preparation of oxides- Hydrothermal decomposition for magnetic nanoparticles- electron beam evaporation and spin-coating for thin films


Introduction

The main objective of our group is to develop PAC as a tool for experimental investigations in condensed matter and materials physics

We have been using PAC to study

- Heusler alloys based on Co: Co2XY

- Rare-earth intermetallic compounds

1. Magnetism in

2. Electric quadrupole and/or magnetic dipole interactions in

- Pure and doped semiconductor oxides: ZnO, In2O3, TiO2, SnO, and SnO2

- Oxides (RTMO3)

- Magnetic nanoparticles: Fe3O4, XFe2O4 (X = Mn, Co, Ni)

3. Dynamic and static hyperfine interactions in biomolecules: DNA and Antibodies


Microscopic measurements are important not only to study the origin of phenomena but also to investigate the properties of materials from an atomic view.

Several techniques can be used to Microscopically investigate materials, among them those which are based on hyperfine interactions (HFI) show higher efficiency and precision.

HFI are interactions between the atomic nucleus and charges and spins in its neighborhood.

- Electric interaction: between the electric quadrupole moment (Q) and the electric field gradient (EFG).

- Magnetic interaction: between the magnetic dipole moment (m) and the local magnetic hyperfine field (MHF).

Hyperfine interactions


Two examples of hyperfine interactions techniques which use radioactive nuclei are:

- Mössbauer Spectroscopy (ME)- Perturbed Angular Correlation (PAC) spectroscopy

In both techniques gamma-rays are emitted from the radioactive nuclei.

ME is based on the resonance of gamma energies and the radioactive source is often used as gamma-rays emitter.

In PAC the radioactive nuclei must be inserted into the samples to be studied.

source sample

HFI techniques


PAC method

PAC measures the time dependence of the g-ray emission pattern caused by hyperfine interactions

The angular distribution of the emission pattern is given by the correlation function

W(q,t) = 1 + A22G22(t)P2(cosq)

Hyperfine interactions cause a precession of the emission pattern described by the perturbation factor

g1

g2

Ii , Mi

If , Mf

I , M

PAC method is based on the angular momentum conservation between the nuclear momenta I and M and the gamma-ray direction (pg), emitted by a cascade


PAC method

hfB

coincidence

hfN

L Bg Magnetic interaction:

Electric quadrupole interaction: )12(4

II

eQVzzQ

)exp()(22 tatG ii

Dynamic interaction


Magnetic hyperfine field

Measurements of the magnetic hyperfine field (Bhf) at nuclear sites in magnetically ordered compounds are a useful source of information on:

• exchange interactions leading to spontaneous magnetic order,• on the order of magnetic phase transitions, • on spin wave excitations,• on relaxation processes, • other parameters characterizing a magnetic system.

Many experimental and theoretical hyperfine interaction works have been focused on magnetic systems involving the rare earth (R) elements Ce to Tm.

isostructural series of R compounds are interesting because a large number of them, for different R constituents, differ only slightly in the crystallographic properties, but strongly in the magnetic properties.

R compounds offer favorable conditions for the separation of the magnetic from other solid-state parameters.


PAC nuclei

111InT1/2 = 2.83 d

EC

111Cd

7/2+

5/2+

1/2+

t1/2 = 85 ns

m(5/2+) = – 0.7656 mN

Q(5/2+) = + 0.83 b

171 keV

245 keV

111In

181Hf T1/2 = 42.4 d

b–

181Ta7/2+

5/2+

1/2+

t1/2 = 10.8 ns

m(5/2+) = + 3.24 mN

Q(5/2+) = + 2.36 b

133 keV

482 keV

181Hf


111Ag(111Cd) and 111mCd

111Ag: T1/2 = 7.45 days

The level used for PAC measurements is fed only by 5 % of the decay.

111Ag is obtained from the decay of 111m,gPd, which is produced by neutron irradiation of natural Pd:

110Pd(n,g)111m,gPd111mPd and 111gPd decay to 111Ag with short half-lives.

e-capture

111mCd (Т

1/2=48,3 min.)

94%

5%

111Ag

247 keV

172 keV

If=1/2

I=5/2, T1/2=84 ns

Ii=7/2

111Cd

111In

111mCd is obtained by neutron irradiation of natural Cd:

110Cd(n,g)111mCd


140La(140Ce)

140La

3+

4+

2+

0+

328 keV

486 keV

1596 keV

140Ce

T1/2 = 40,2h

N 68,4nsT 4,32/1

037,0

094,0

44

22

A

A

b-

bQ 3,0)4(

Natural lanthanum (99.9% of 139La) is irradiated with neutrons in a nuclear reactor to produce 140La via 139La(n,g)140Ce nuclear reaction

Advantages of 140ce probe:

1) It can be use to investigate Ce compounds2) No other techniques use Ce as probe.3) It measures pure magnetic dipole interactions


Ce-based intermetallic compounds have shown a wide variety of interesting phenomena, in particular, different magnetic as well as superconducting behavior.

Ce3+: one 4f electronitinerant

localized

140La(140Ce)

Ce compounds: Kondo effect, superconductor, magnetic, etc.

140Ce as probe nuclei for magnetic measurements

4f spin contributes to Bhf


Temperature dependence of Bhf for 111Cd (red circles) and 140Ce (blue circles)

140La(140Ce)


Experimental measurements of NMR frequency (nT ) in 1.5% Mn diluted in Fe matrix using 55Mn as probe[1].

Theoretical interpretation: Jaccarino et. al. [2] who considered a localized moment on Mn atoms and proposed a simple model based on the molecular field calculations in which the magnetic coupling between Mn and Fe atoms is weaker than the magnetic coupling between two Fe atoms.

[1] Y. Koi, A. Tsujimura and T. Hihara, J. Phys. Soc. Japan 19 (1964) 1493[2] V. Jaccarino, L. R. Walker and G. K. Wertheim, Phys. Rev. Lett. 13 (1964) 752

Results for the temperature dependence of nT showed a sharp deviation from the Brillouin-likecurve.

140La(140Ce)


The unusual behavior of Bhf with temperature is due to the presence of one 4f electron in the Ce+3 impurity ion, which may contribute to the total hyperfine field:

Bhost is the contribution from the R ions of the host, which scales with the host reduced magnetization s(T) . Bimp is proportional to the thermal average of the impurity magnetic moment Ji, which is localized.

Bhf = Bimp + Bhost

)()0()()0()( TByBBTB hostJimphf imp

kT

TBJgy

excimpJB imp)()1(

)()1)(1(

3)( 0 T

Jg

kTTB

BhostJexc

host

BJimp (y) is the Brillouin function with the argument:

T0 is the magnetic transition temperature. x takes into account the fact that the host–impurity exchange interaction strength may be different from that of the host–host exchange.

Bexc (T) is the exchange field, which also scales with s(T)

140La(140Ce)


why does the same impurity in different hosts show different behaviour? Which parameters, such as distance between magnetic ion of the host and impurity, number of conduction electrons or crystal structure are important in inducing this phenomenon?

A systematic investigation is needed to answer these questions

The best way to successfully achieve a good physical explanation of this phenomenon is to study a family of compounds.

It is possible to separately study the influence of one specific parameter by varying the composition of the compound by changing the constituent elements within the same crystalline structure: - to vary the magnetic ion of the host, - the distance between the magnetic ion and the impurity, the number of

conduction electrons, - and the position of the localized impurity band relative to the Fermi level.

Localized moments


Intermetallic compound family: RAg (R = rare earth element)

RAg compounds

Crystal structure: BCC, CsCl-prototype Space group:

mPm3The antiferromagnetic structure in

these compounds is built up by ferromagnetic (1,1,0) planes coupled

antiferromagnetically


Sample preparation

RAg compounds

• Samples were prepared by repeatedly melting the constituent elements (Gd 99.99% and Ag 99.9985%) under argon atmosphere purified with a hot titanium getterer in

an arc furnace.

• Radioactive La substituting 0.1% of R atoms in samples used for PAC measurements.

• Samples were annealed under an atmosphere of ultra pure Ar for 48 h at 700 ºC


20 40 60 80 100 120-200

0

200

400

600

800

1000 GdAg

Inte

nsity

(a.

u.)

2

0 10 20 30 40 50 60 70 80 90 100

1,2

1,4

1,6

1,8

2,0

2,2

2,4

2,6

2,8

0 10 20 30 40 50 60 704

5

6

7

(x

10-4

em

u/O

e.g)

T (K)

HoAg

-1 (g

.kO

e/e

mu)

T (K)

RAg compounds

Sample characterization

X-ray diffraction

Magnetic susceptibility


0 5 10 15-0,02

0,00

0,02

0,04

0,06

-0,02

0,00

0,02

0,04

0,06

-0,02

0,00

0,02

0,04

0,06

-0,02

0,00

0,02

0,04

0,06

-0,02

0,00

0,02

0,04

0,06

-0,02

0,00

0,02

0,04

0,06

0,08

5 10 15 20

10 K

30 K

60 K

80 K

-R(t

)

100 K

125 K

50 K

t (ns)

60 K

70 K

80 K

-R(t

)

90 K

100 K

• Conventional fast-slow coincidence setup

• 4 BaF2 detectors, 12 coincidence

spectra

• time resolution for 329 keV- 487

keV: 0.6 ns

• measurement temperatures: 295

K and in the range of 10 K to 130

K.

PAC measurements

R(t) for GdAg (left) and TbAg (right) at temperatures below their respective TN.

RAg compounds


0 10 20 30 40 50 600

20

40

60

80

100

0 20 40 60 80 100 1200

10

20

30

40

50

60

70

80

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

0 5 10 15 20 25 30 350

20

40

60

80

100

T [K]

T [K]

T [K]

DyAg

TbAg

Bhf(T

) [

T ]

Bhf(T

) [

T ]

Bhf(T

) [

T ]

Bhf(T

) [

T ]

T [K]

GdAg

HoAg

Temperature dependence of Bhf in RAg

It is observed a strong departure from a Brillouin-like curve for all

studied compounds. It is a consequence of the influence of

140Ce probe

RAg compounds


0 10 20 30 40 50 600

20

40

60

80

100

120

140

0 20 40 60 80 100 1200

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 350

20

40

60

80

100

120

140

160

T [K]

T [K]

T [K]

DyAg

TbAg

Bhf(T

) [

T ]

Bhf(T

) [

T ]

Bhf(T

) [

T ]

Bhf(T

) [

T ]

T [K]

GdAg

HoAg

hhf

ihfhf BBB

)()0()()0()( TByBBTB hhfJ

ihfhf imp

kT

TBJgy

excimpJB imp)()1(

)()1)(1(

3)( 0 T

Jg

kTTB

Bh

J

exch

RAg compounds


Gd

Tb

Dy

Ho

Spin exchange is the dominant mechanism in the polarization:

h

j

hhf JgB h )1(

RAg: host contribution

0 50 100 150 200 250 300 350 400-10

-5

0

5

10

15

20

25

30GdCo

2

GdAg

GdNiIn

GdNi2

Bhf (

T)

T (K)

RKKY theory: magnetic ordering temperature (To) scales de Gennes factor:

)1()1( 2 hh

jo JJgT h

1)1)(1(/

hjo

hhf JgTB h


The reduced Bhf is written as:

hhf

ihfhf BBB

RAg: impurity contribution

eff

hhf

eff

ihf

eff

hf BBB

The host reduced contribution is very small when compared to impurity reduced contribution


RAg: impurity contribution

eff

ihf

eff

hf BB

Therefore the reduced magnetic hyperfine field is approximately the reduced impurity contribution:

For GdAg at 10 K:

Beff

hf TB

/ 1.8

Beff

ihf TB

/ 6.7

Calculated at 0 K:


Conclusions

140Ce probe can be used to measure the contribution to Bhf from magnetic hosts, and, consequently to study their magnetic properties

140Ce probe can be used to study localized magnetic moments as a dilute impurity in magnetic hosts.

We propose that the unusual behavior of the temperature dependence of Bhf measure with 140Ce probe in some magnetic hosts is due to the hybridization of 4f band of Ce with d band of the host. As the Ce 4f band is located very close to the Fermi level, when temperature increases, more and more of d electrons are promoted to the conduction band, weakening the contribution from 4f spins to Bhf


Magnetic dilute impurities in magnetic hosts

The behavior of magnetic moments at dilute magnetic atoms in metallic hosts is an interesting topic which is related to fundamental properties in condensed matter physics such as the Kondo effect.

Localized moments

Non-magnetic metal

Magnetic impurities in non-magnetic metallic host


localized magnetic moment in metals was first observed in the early 1960s from susceptibility measurements of iron atoms diluted in 4d metals[1,2],

Theoretical description was given by Anderson [3], who emphasized the nature of these moments by considering a localized resonance at the impurity

[1] B. T. Matthias, M. Peter, H. J. Williams et al., Phys. Rev. Lett. 5 (1960) 542[2] A. M. Clogston, B. T. Matthias, M. Peter M et al., Phys. Rev. 125 (1962) 541[3] P. W. Anderson, Phys. Rev. 124 (1961) 41

Localized moments


Localized moments

Magnetic dilute impurities in magnetic hosts

No physical description of the origin of the anomalous behavior of the Bhf has been given so far, which makes this an open issue yet to be resolved.

Hyperfine interactions are ideal to study this problem: anomalous temperature behavior of Bhf at 140Ce


Collaborators

Artur W CarbonariRajendra N. Saxena Jose Mestnik Filho Andre Luiz LapolliClaudio Domienikan

Permanent staff

Fabio H. M. CavalcanteGabriel A. Cabrera PascaLuciano F. D. PereiraMessias S. Costa Emiliano L. MuñozLaura C. OliveiraFernando Effenberg

Pos-doc fellows

Brianna B. Santos Juliana M. Ramos Isabela Teles Matos

Ph.D. students

Sarah D. Pinheiro Thiago Martucci Caio O. Salutte Ademir Stanoviff

MSc. students

Collaborators

Helena M. Petrilli – Physics Dept. - Univ. of São Paulo Elisa M Baggio-Saitovitch and H. Saitovitch – CBPFManfred Forker and Reiner Vianden – HISKP – Univ. of Bonn Mario Renteria, Alberto Pasquevitch and Leonardo Errico – Univ. de La PlataMichael Uhrmacher – Univ. of Göttingen


Funding

FAPESP – Research Foundation of the state of São Paulo

CNPq – National Science Foundation of Brazil

CAPES –National Academic Foundation of Brazil


THANK YOU


No physical description of the origin of the anomalous behavior of the Bhf has been given so far, which makes this an open issue yet to be resolved.

Localized magnetic moment in magnetic metallic hosts

First example: experimental data andits interpretation

NMR frequency (nT ) in 1.5% Mn diluted in Fe matrix using 55Mn as probe.

Localized moments


RRh2Si2, R = Ce, Pr, Nd, Gd, Tb, DyR

Rh

Si

ThCr2Si2-type structure, I4/mmm spatial group

RRh2Si2


Ruderman–Kittel–Kasuya–Yosida (RKKY) theory predicts that the magnetic ordering temperature is proportional to the de Gennes factor:

)1()1( 22 JJgJT sf

RRh2Si2


(a) GdRh2Si2 and (b) TbRh2Si2 at indicated temperatures.

(a) PrRh2Si2 and (b) NdRh2Si2 at indicated temperatures.

RRh2Si2


Spin-wave (SW) contribution to resistance (R)

R0 is the residual resistance at T = 0.

A is related to the spin wave velocity. D is the spin wave energy gap (minimum energy required to excite spin waves)

RRh2Si2


Anderson model: 4f band of the impurity is described as localized close to the Fermi level and hybridized with the conduction electron bands. The broadening of the f-level is given by:

)(2

Fkf NV

Vkf is the average of the matrix element of the impurity potential between the 4f orbital and the conduction electron k orbitals. N(eF) is the conduction electron density of states at the Fermi level.

The influence of hybridization on Bhf

Bhost is proportional to meff of R ions via conduction electrons.

Bhf (0)/meff

When V decreases, from R = Ce to Dy, the 4f band broadens: part of it becomes delocalized reducing the contribution of the orbital magnetic field of the impurity to Bhf.

RRh2Si2


GdRh2Si2 Ge XRD: 6.6% found for the volume ratio of GdRh2Ge2 to GdRh2Si2

Bhf: 7.1% found for the B4f ratio of GdRh2Ge2 to GdRh2Si2

This result completely follows theAnderson’s prediction: the larger is the volume, the weaker is the hybridization, and, consequently the larger is the B4f contribution.

RRh2Si2


The hybridization of the 4f band of the Ce impurity with d bands of the host, which are polarized by the magnetic Gd ions, is responsible for the exchange interaction between the spins of the magnetic ions (Gd) of the host and the Ce impurities.

The exchange interaction between the 4f electrons of Gd andCe is mediated by the wide d band, which extends beyond the Fermi level.

As temperature increases more and more of d electrons are promoted to the conduction band, which weakens the exchange interaction, and as a consequence decreases the B4f contribution, which is dominant in the total Bhf

This mechanism results in a strong deviation of the temperature dependence of Bhf from the standard Brillouin-like curve at higher temperatures.

RRh2Si2


140La(140Ce)

Compound TC (K) Tt (K) meff (mB)

GdNiIn 96 79 7.7

TbNiIn 70 - 9.5

DyNiIn 30 12.5 10.7

HoNiIn 20 - 10.7


Probe nuclei:

RNiIn

140La 140Ce

0.1% atomic added to samples during

preparation

0.04

0.08

0.00

0.04

0.08

0.04

0.08

0.00

0.04

0.08

0.00

0.04

0.08

0.00

0.04

0.08

0.04

0.08

0.00

0.04

0.08

0.04

0.08

0.00

0.04

0.08

0 5 10

0.04

0.08

100 200 300 4000.00

0.04

0.08

80 K

295 K

60 K

70 K

50 K

60 K

30 K

55 K

20 K

30 K

R(t

)

10 K

t(ns)

20 K

Carrier free 111In introduced into samples by

diffusion

111In 111Cd

PAC spectra for GdNiIn measured with 140Ce (left) and 111Cd (right)

140La(140Ce)


Temperature dependence of Bhf for 111Cd (red circles) and 140Ce (blue circles)

It was observed a significant difference in the transition temperature determined with 111Cd and 140Ce.

It was ruled out the sample dependence:

1. Three samples of GdNiIn presented same results. 2. Results for TbNiIn sample showed that 81% of 111Cd probes occupy In sites and 19% Tb sites. The transition temperatures are also different for these two fractions.

140La(140Ce)


a) The difference between TCe and TCd increases when TC increases.

b) TCe – TCd is proportional to (TC)2.

Difference in transition temperatures TCe and TCd.

140La(140Ce)


140La(140Ce)

A nonzero MHF appears at In sites despite the fact that no net magnetic moment is induced and the vector sum of the nearest-neighbor Ce magnetic moments is zero. The 5p subshells of In are spin polarized due to the hybridization with the extended Ce valence states. This polarization is essential for the appearance of a small MHF at the In nucleus, which has mostly a spin-dipolar character.

CeIn


140La(140Ce)

CeMn2Ge2 CeMn2Si2

TN = 415 KTC = 320 K

TN = 380 K


140La(140Ce)

The departure from a Brillouin-like curve is a consequence of the influence of Ce probe

Documents

140 Ce probe nuclei used to measure magnetic hyperfine fields in rare-earth magnetic compounds A. W. Carbonari Hyperfine Interactions Laboratory