magnetism as seen with x rays - spallation neutron sourceneutrons.ornl.gov/sites/default/files/elke...

1

Magnetism as seen with X Rays

Elke ArenholzLawrence Berkeley National Laboratory

andDepartment of Material Science and Engineering, UC Berkeley

2

+ Magnetic Materials Today

+ Magnetic Materials Characterization Wish List

+ Soft X-ray Absorption Spectroscopy – Basic concept and examples

+ X-ray magnetic circular dichroism (XMCD)

- Basic concepts

- Applications and examples

- Dynamics: X-Ray Ferromagnetic Resonance (XFMR)

+ X-Ray Linear Dichroism and X-ray Magnetic Linear Dichroism (XLD and XMLD)

+ Magnetic Imaging using soft X-rays

+ Ultrafast dynamics

What to expect:

3

Magnetic Materials Today

Magnetic materials for energy applications

Magnetic nanoparticles for biomedical andenvironmental applications

Magnetic thin films for

information storage and processing

4

Permanent and Hard Magnetic Materials

Engineering magnetic anisotropy

on the atomic scale

Controlling grain and

domain structure on the micro-

and nanoscale

5

Magnetic Nanoparticles

Tailoring magnetic nanoparticles for

environmentalapplications

Optimizing magnetic nanoparticles for biomedical applications

6

GMR Read Head Sensor

Magnetic Thin Films

Magnetic domain structure on the nanometer scale

Unique magnetic phases at interfaces

Ultrafast magnetization

reversal dynamics

Magnetic couplingat interfaces

7

Magnetic Materials Characterization Wish List

+ Sensitivity to ferromagnetic and antiferromagnetic order

+ Element specificity = distinguishing Fe, Co, Ni, …

+ Sensitivity to oxidation state = distinguishing Fe2+, Fe3+, …

+ Sensitivity to site symmetry, e.g. tetrahedral, Td; octahedral, Oh

+ Nanometer spatial resolution

+ Ultra-fast time resolution

Soft X-Ray Spectroscopy and Microscopy

8

sample

inte

nsi

ty

wavelength, photon energy

inte

nsi

ty

wavelength, photon energy

Spectroscopy

Sample

Light/Photon Source

Monochromator

9

SampleElectron Storage Ring Monochromator

Soft X-Ray Spectroscopy ( h 500-1000eV, 1-2nm)

770 780 790 8000.6

0.7

0.8

0.9

1.0

photon energy (eV)

tran

sm

itte

d i

nte

nsit

y I

t / I

0

EY

_T

M.o

pj

10

770 780 790 8000

2

4

6

EY

_T

M.o

pj

ele

ctr

on

yie

ld I

e /

I0

photon energy (eV)

770 780 790 8000.6

0.7

0.8

0.9

1.0

photon energy (eV)

tran

sm

itte

d i

nte

nsit

y I

t / I

0

EY

_T

M.o

pj

Photonsabsorbed

Electronsgenerated

Electron yield: + Absorbed photons create core holes subsequently filled by Auger electron emission+ Auger electrons create low-energy secondary electron cascade through inelastic scattering+ Emitted electrons probability of Auger electron creation absorption probability

J. Stöhr, H.C. Siegmann,Magnetism (Springer)

X-Ray Absorption Detection Modes

mean free path 5nm

11

10-20 nm layer thick films supported by substrates transparent to soft X rays

640 660

0.8

1.0

700 720 780 800 860 880

I t /

I0

photon energy (eV)

Fe

lum

inescence_B

L402_A

pr2

012.o

pj

Co

//////

////Ni

Mn//

Ru

MnIr

CoFe

Ru

NiFe

Ru

Soft X-Ray Absorption – Probing Depth

3nm

15nm

5nm

3nm

15nm

15nm

Element 10eV below L3

1/ [nm]

at L3

1/ [nm]

40 eV above L3

1/ [nm]

Fe 550 17 85

Co 550 17 85

Ni 625 24 85

12

0.8

1.0

0.8

1.0

0.8

1.0

0.8

1.0

Mn NiCoe

lectr

on

yie

ld (

arb

. u

nit

s)

I t /

I0

Fe

640 650 660

0.9

1.0

photon energy (eV)

lum

inescence_B

L402_A

pr2

012.o

pj

710 720

1.0

1.1

photon energy (eV)

780 790

0.9

1.0

photon energy (eV)

850 860 870

1.0

1.2

1.4

photon energy (eV)

Ru

MnIr

CoFe

Ru

NiFe

Ru

e

3nm

15nm

5nm

3nm

15nm

15nm

X-Ray Absorption Detection Modes and Probing Depth

+ Electron sample depth: 2-5 nm in Fe, Co, Ni 60% of the electron yield originates form the topmost 2-5 nm

13

Experimental Concept:Monitor reduction in X-ray flux transmitted through sample as function of photon energy

Charge state of absorber Fe2+, Fe3+

Symmetry of lattice site of absorber: Oh, Td

Sensitive to magnetic order

Core level Atomic species of absorber Fe, Co, Ni, …. 2p3/2 L3

2p1/2 L2

Valence states

Absorption probability: X-ray energy, X-ray polarization,experimental geometry

X-Ray Absorption Fundamentals

E

14






2p1/2 L2

Valence states



E

15

core level2p3/2 L3

2p1/2 L2

valence states

‘White Line’ Intensity

Fe

Intensity of L3,2 resonances is proportional to number of d states above the Fermi level, i.e. number of holes in the d band.

16

core level2p3/2 L3

2p1/2 L2

valence states


Fe


17

2p3/2 L3

2p1/2 L2

valence states


Co

core level


18

2p3/2 L3

2p1/2 L2

valence states


Ni

core level


19

2p3/2 L3

2p1/2 L2

valence states



Cu

core level

20


Charge state of absorber Fe2+, Fe3+, … Symmetry of lattice site of absorber: Oh, Td



2p1/2 L2

Valence states



E

21

N. Telling et al.,Appl. Phys. Lett. 95, 163701 (2009)

Influence of the charge state of the absorber

J.-S. Kang et al., Phys. Rev. B 77, 035121 (2008)

X-ray Absorption – Valence State

22


X-Ray Absorption – Configuration Model

Ni2+ in NiO: 2p6 3d8 → 2p5 3d9

Configuration model, e.g. L edge absorption :

+ Excited from ground/initial state configuration, 2p63d8

to exited/final state configuration, 2p53d9

+ Omission of all full subshells (spherical symmetric)

+ Takes into account correlation effects in the ground state as well as in the excited state

+ Leads to multiplet effects/structure

http://www.anorg.chem.uu.nl/CTM4XAS/ 2S+1L

23






2p1/2 L2

Valence states



E

24

455 460 465 470

PZ

T_X

A.o

pj

photon energy (eV)

XA

(arb

. u

nit

s)

PbZr0.2Ti0.8O2

Sensitivity To Site Symmetry: Ti4+ L3,2

+ Electric dipole transitions: d02p53d1

+ Crystal field splitting 10Dq acting on 3d orbitals:

Octahedral symmetry: e orbitals towards ligands higher energyt2 orbitals between ligands lower energy

Tetragonal symmetry:e orbitals b2 = dxy, e = dyz, dyz

t2 orbitals b1 = dx2y

2, a1 = d3z2r

2


t2

e e

t2

3d orbitals

25

x-

ray a

bs

orp

tio

n

TiO2

Oh

D2h G. Van der Laan

Phys. Rev. B 41, 12366 (1990)

Influence of lattice site symmetry at the absorber

X-Ray Absorption Lattice Symmetry

rutile

anatase

TiO2

26






2p1/2 L2

Valence states



E

27






2p1/2 L2

Valence states



E

28

+ Magnetic moments in Fe, Co, Ni well described by Stoner model:d-bands containing up and down spins shifted relative to each other by exchange splitting

+ Spin-up and spin-down bands filled according to Fermi statistics

+ Magnetic moment |m| determined by difference in number of electrons in majority and minority bands

empty states,

“holes”

exchange

splitting

3d shell

spin-upspin-down

empty states,

“holes”

)n(nμ m|| mine

majeB

Stoner Model For Ferromagnetic Metals


29

Origin of X-ray Magnetic Circular Dichroism

2-Step Model for photoexcitation of electrons from 2p3/2, 2p1/2 to 3d states by absorption of circularly polarized X rays:

+ Transfer of angular momentum of incident circular polarized X rays to excited electrons (angular momentum conservation, selection rules)

+ Spin polarization of excited electrons opposite for incident X rays with opposite helicity.

+ Unequal spin-up and spin-down populations in exchange splitvalence shell acts as detector for spin of excited electrons.

3d

30

+ Calculate transition probabilities from filled 2p3/2 and 2p1/2 states to empty states in d-bandfor circularly polarized X raysusing Fermi’s Golden Rule


31


initial state |iinitial state energy i

final state |ffinal state energy f

+ Wave functions describe electronic and photon statesEnergies include electronic and photon energies

+ Calculate transitions probabilities Tif

considering photon as time-dependent perturbation,i.e. an electromagnetic (EM) field

ρ(εf ) = density of final states per unit energy

Tif Dimension [time−1]

Fermi’s Golden Rule

32

+ Consider strong ferromagnet with one filled spin band: All spin down d states filled Spin up d states partially filled

+ This specific case:Only spin up electron excited

X-ray absorption of circularly polarized photons with angular momentum q = ±1 in units of ħ


↓

↓

↓

↓


33


q = +1 q = 0q = 1 q=1q=+1 q=0

L3

L2

50 30 10 30

L3: X rays with q = +/1 excite 62.5%/37.5% of the spin up electronsL2: X rays with q = +/1 excite 25%/75% of the spin up electrons


34


↓

↓

↓

↓


Taking into account 2x higher population of 2p3/2 state as compared to 2p1/2 state:

Identical magnitude XMCD at L3 and L2 with opposite sign

= 25% = -50%

35

Magnitude of XMCD depends on+ expectation value of 3d

magnetic moment+ degree of circular photon

polarization, Pcirc

+ geometry

IXMCD = I I

0.0

0.5

1.0

770 780 790 800

-0.4

-0.2

0.0

photon energy (eV)

XA

(a

rb.

un

its

)

FeC

o_G

aA

s_X

MC

D.o

pj

XM

CD

(arb

. u

nit

s)

L3L2

Co

X-Ray Magnetic Circular Dichroism (XMCD)

36

0.0

0.5

1.0

700 710 720 730 780 790 800

-0.2

0.0

XA

(a

rb.

un

its

)

photon energy (eV)

XM

CD

(arb

. u

nit

s)

C

oF

e2O

4.o

pj

L3 Fe Co

L2

L3

L2

CoFe2O4

Fe2+,Oh

Fe3+, Oh

Fe3+, Td

Co2+


H

circularlypolarized

+ XMCD provides magnetic information resolving elements Fe, Co, …valence states: Fe2+, Fe3+, …lattice sites: octahedral, Oh, tetrahedral, Td,

X-Ray Magnetic Circular Dichroism (XMCD)

37

+ Geobacter sulfurreducens bacteria form magnetite nanocrystals (15nm)via extracellular reduction of amorphous Fe(III)-bearing minerals

Magnetic Bionanospinels

Fe3O4

Co-ferrite-1 6 at% Co

Co-ferrite-2 23 at% Co

XM

CD

photon energy (eV)

V. Cocker et al.,

Eur. J. Mineral. 19, 707–716 (2007)2 0 +2

60

0

+60

H (T)

M (

em

u g

1)

Fe2+,Oh

Fe3+, Oh

Fe3+, Td

38

V. Cocker et al.,

Eur. J. Mineral. 19, 707–716 (2007)2 0 +2

60

0

+60

H (T)

M (

em

u g

1)

Co2+ Oh

Magnetic Bionanospinels

+ Geobacter sulfurreducens bacteria form magnetite nanocrystals (15nm)via extracellular reduction of amorphous Fe(III)-bearing minerals

39

+ Comparing XMCD spectra with model compounds and/or calculations Identifying magnetic phases

XA

, X

MC

D

Co metal

XA

XA

XMCD

XMCD

photon energy (eV)

Co2+

in CoFe2O

4

780 790 800

0

0

XA

, X

MC

D

Co_C

o2+

.opj

Co-doped TiO2X

MC

D (

arb

. un

its)

XA

(ar

b. u

nit

s)

Co-doped TiO2

dilute magnetic semiconductors

J.-Y. Kim et al., Phys. Rev. Lett. 90, 017401 (2003)

780 800790 810photon energy (eV)

40

Cu

Co

Cu

Co

…

+ The element-specificity makes XMCD measurements an ideal tool to determine induced moments at interfaces between magnetic and non-magnetic elements.

Induced Moments At Co/Cu Interfaces

M. G. Samant et al.,Phys. Rev. Lett. 72, 1112 (1994)

41

Mn L3,2

Co L3,2

XA

(a

rb.

un

its

)

630 640 650 660 770 780 790 800 810

XM

CD

(a

rb.

un

its

)

photon energy (eV)

x200

20 Å Co / 200 Å Ir20Mn80

+ Weak Mn XMCD signal

Uncompensated Mn at Co/IrMn interface

+ Same sign of XMCD signal for Co and Mn

Parallel coupling of Co and Mnmoments

+ Nominal thickness of uncompensated interface moments: (0.50.1)ML

IrMn

Co

Magnetic Interfaces

H. Ohldag et al.,

Phys. Rev. Lett. 91, 017203 (2003)

FM

AFM

H = +/-0.5T

42

705 710 715 720 725

-0.50

-0.25

0.00

0.25

710 712-0.02

0.00

0.02

0.0 0.2 0.4 0.6

-0.50

-0.25

0.00

0.0

0.5

1.0

X

MC

D (

arb

. u

nit

s)

photon energy (eV)

photon energy (eV)

XA

(a

rb.

un

its

)

XM

CD

(a

rb.

un

its

)

Fe L

3 X

MC

D

(arb

. u

nit

s)

x

0.0 0.2 0.4 0.6

-0.50

-0.25

0.00

Fe

L3 (

eV

)

x

Band Filling In GaxFe1−x

+ Fe majority-spin filled for x=0.3+ x 0.3: Fe moment decrease strongly,

Formation of D03 precipitates


GaxFe1−x

Fe L3,2

E. Arenholz et al.,Phys. Rev. B 82, 180405 ( 2010)

43

Composition Dependence of Fe moment in Fe1-xMnx

Drop of magnetic moment in bcc Fe1-xMnx, i.e. independent of structural transition

44

A < 0

B > 0

+ Theoretically derived sum rules correlate XMCD spectra with spin and orbital moment providing unique tool for studying magnetic materials.

mS = B–A + 2B / C

mL = –2BA + B / 3CNh = IL3 + IL2 /C

Sum Rules


45

+ Strong variation of orbital and spin magnetic moment observable as change in relative L3 and L2 intensity in XMCD spectrum.

+ Co atoms and nanoparticles on Pt have enhanced orbital moments up to 1.1 B

P. Gambardella et al.,Science 300, 1130 (2003)

Orbital Moment Of Co Nanoparticles

46

Element-specific Magnetization Reversal

Co L3,2

+/- circ. pol.

H = -0.5T

X

A (

arb

. u

nit

s)

780 800

0

I+ -

I- (

arb

. u

nit

s)

photon energy (eV)

+ Monitoring field dependence of XMCD Element-specific information on magnetization reversal in complex magnetic

nanostructures.

47

XA

(arb

. u

nit

s)

Co L3,2

+/- circ. pol.

H = +0.5T

780 800

0

I+ -

I- (

arb

. u

nit

s)

photon energy (eV)

Co L3,2

+/- circ. pol.

H = -0.5T

X

A (

arb

. u

nit

s)

780 800

0

I+ -

I- (

arb

. u

nit

s)

photon energy (eV)



nanostructures.

48

-0.4 -0.2 0.0 0.2 0.4

0

XM

CD

magnetic field (T)


nanostructures.

Co L3,2

+/- circ. pol.

H = -0.5T

X

A (

arb

. u

nit

s)

780 800

0

I+ -

I- (

arb

. u

nit

s)

photon energy (eV)

XA

(arb

. u

nit

s)

Co L3,2

+/- circ. pol.

H = +0.5T

780 800

0

I+ -

I- (

arb

. u

nit

s)

photon energy (eV)


49

x3

H=+0.2 TH=-0.2 T

XA

(a

rb.

un

its

)

700 750 800 850

0

XM

CD

photon energy (eV)

Fe

Co

Ni

5 ML Co

8,10 ML Fe

18 ML Ni

-40 -20 0 20 40

-10

-5

0

5

10

XM

CD

(%

)

field (mT)

NiFeCo

8 ML Fe


50

-40 -20 0 20 40

-10

-5

0

5

10

XM

CD

(%

)

field (mT)

NiFeCo

x3

H=+0.2 TH=-0.2 T

XA

(a

rb.

un

its

)

700 750 800 850

0

XM

CD

photon energy (eV)

Fe

Co

Ni

8 ML Fe

5 ML Co

8,10 ML Fe

18 ML Ni

-200 -100 0 100 200-10

-5

0

5

10

XM

CD

(%

)field (mT)

NiFeCo

10 ML Fe


51

X-Ray Ferromagnetic Resonance

+ XMCD is the difference in X-ray absorption between antiparallel and parallel orientation of magnetic moment and photon spin.

+ The XMCD magnitude reflects the magnetic moment aligned parallel to the X ray beam.

0.0

0.5

1.0

770 780 790 800

-0.4

-0.2

0.0

photon energy (eV)

XA

(a

rb.

un

its

)

FeC

o_G

aA

s_X

MC

D.o

pj

XM

CD

(arb

. u

nit

s)

L3L2

Co

H

circularlypolarized

52

0.0

0.5

1.0

770 780 790 800

-0.4

-0.2

0.0

photon energy (eV)

XA

(a

rb.

un

its

)

FeC

o_G

aA

s_X

MC

D.o

pj

XM

CD

(arb

. u

nit

s)

L3L2

Co

H

circularlypolarized

M(t)

+ In fact: Magnetic moments are not fully aligned with applied fields but precess around them.


53

0.0

0.5

1.0

770 780 790 800

-0.4

-0.2

0.0

photon energy (eV)

XA

(a

rb.

un

its

)

FeC

o_G

aA

s_X

MC

D.o

pj

XM

CD

(arb

. u

nit

s)

L3L2

Co

H

circularlypolarized

M(t)

H

M x H

+ In fact: Magnetic moments are not fully aligned with applied fields but precess around them.

+ Is it possible to measure the pression of magnetic moments making use of the pulsed nature of synchrotron radiation and XMCD? (!)


54

Pulsed nature of synchrotron radiationExample: Advanced Light Source

bunch spacing

Pulse length 70 ps

+ 256-320 bunches for 500mA beam current

+ Bunch spacing: 2 ns (500MHz)

+ Pulse length 70ps


55

Dynamic XMCD measurement, i.e. synchronize X-ray pulses with FMR precession

M(t)

HB

X-ray pulses

microwave

driving field

~70 ps2 ns

Mz(t)

X-ray pulse repetition signal, ~500 MHz

f = n500 MHz


56

0 200 400 600 800 1000-4

-2

0

2

4

XM

CD

(arb

. units)

Microwave delay (ps)

M

X-rays

M

M(t)

Static XMCD Dynamic XMCD

0.0

0.5

1.0

770 780 790 800

-0.4

-0.2

0.0

photon energy (eV)

XA

(a

rb.

un

its

)

FeC

o_G

aA

s_X

MC

D.o

pj

XM

CD

(arb

. u

nit

s)

L3

L2

Co


57

+ Precession is resonantly excited in the NiFe layer with an 4 GHz RF field.

+ The resonance field of the CO layer is higher, i.e. no precession is excited in the Co layer.

+ Precession in Py, Cu75Mn25, and Co layers are probed by XMCD using left- and right-circularly polarized X-rays at Ni, Mn, and Co edges, respectively.

+ The Cu75Mn25 spin precession is a direct indicator of the AC spin current through the structure.


58

X-Ray Linear Dichroism:

+ Difference in X-ray absorption for different linear polarization direction relative to crystalline and/or spin axis.

+ Due to the anisotropic charge distribution about the absorbing atom caused by bonding and/or magnetic order.

+ “Search Light Effect”: X-ray absorption of linear polarized X rays proportional to density of empty valence states in direction of electric field vector E.

Cu

O

photon energy (eV)920 940 960

La1.85Sr0.15CuO4

L2

Cu

XA

(ar

b. u

nit

s)

L3

C. T. Chen et al. PRL 68, 2543 (1992)

X-Ray Linear Dichroism

E

59

Pb2+ O2 Ti4+, Zr4+

+

ferroelectric polarization

+ Spontaneous electric polarization due to off-center shift of Ti4+, Zr4+ associated with tetragonal distortion linear dichroism

+ Reversing ferroelectric polarization changes XA Change in tetragonal distortion

0.0

0.5

1.0

455 460 465 470

-0.05

0.00

XA

(arb

. u

nit

s)

dif

fere

nc

e

(arb

. u

nit

s)

// //

0.0

0.5

1.0

XA

(arb

. u

nit

s)

-0.2

0.0

0.2

photon energy (eV)

dif

fere

nc

e

(arb

. u

nit

s)

////

////

Structural Changes In PbZr0.2Ti0.8O3

as grown

reversed

t2

e et2

E. Arenholz et al.,Phys. Rev. B 82, 140103 (2010)

60

→→

+ IXMLD = I|| I m 2 , m 2 = expectation value of square of atomic magnetic moment

+ XMLD allows investigating ferri- and ferromagnets as well as antiferromagnets

+ XMLD spectral shape and angular dependence are determined by magnetic order and lattice symmetry

Isotropic d electron charge density No polarization dependence

Magnetically aligned system Spin-orbit coupling distorts

charge density Polarization dependence

SL

L3

Fe Co

L3L2

CoFe2O4

L2

= 0

= 45H

H

X-Ray Magnetic Linear Dichroism

[001]0.0

0.5

1.0

XM

LD

(arb

. u

nit

s)

XA

(arb

. u

nit

s)

Co

Fe

2O

4_

11

0_

Se

pt.o

pj

-0.02

0.00

0.02

photon energy (eV)

700 710 720 730 780 790 800

//

//

//

61

→

0.0

0.5

1.0

-0.05

0.00

0.05

850 855 860 865 870 875

Co

NiO

_4

.0.2

_N

i_X

ML

D.o

pj

XA

(a

rb.

un

its

)X

ML

D (

arb

. u

nit

s)

photon energy (eV)

→

+ IXMLD = I|| I m 2 , m 2 = expectation value of square of atomic magnetic moment

+ XMLD allows investigating ferri- and ferromagnets as well as antiferromagnets

+ XMLD spectral shape and angular dependence are determined by magnetic order and lattice symmetry

L3

L3

Isotropic d electron charge density No polarization dependence

Magnetically aligned system Spin-orbit coupling distorts

charge density Polarization dependence

antiferromagnetic NiO

= 0

S

= 45

L

[001]

[001]

X-Ray Magnetic Linear Dichroism

62

LSFO

640 650-0.2

0.0

0.0

0.5

1.0

XA

(a

rb.

un

its

)

photon energy (eV)

XM

CD

(arb

. u

nit

s)

0.0

0.5

1.0

710 720

-0.2

0.0

0.2

XA

(arb

. u

nit

s)

expt. calc.

photon energy (eV)

XM

LD

(arb

. u

nit

s)

0.0

0.5

1.0

710 720

-0.1

0.0

0.1

XA

(a

rb.

un

its

)

XM

LD

(arb

. u

nit

s)

photon energy (eV)

Magnetic Coupling At Interfaces

HMn XMCD

Fe XMLD

H

Perpendicular coupling

at LSMO/LSFO interface

LSMO

La0.7Sr0.3MnO3 (LSMO)ferromagnet

La0.7Sr0.3FeO3 (LSFO)antiferromagnet

H

0.0

0.5

1.0

0.0

0.5

1.0

0 100 200 300 4000.0

0.5

1.0

Fe

L3,2 X

ML

D (a

rb. u

nits

)

Mn

L3,2 X

MC

D

(arb

. u

nit

s)

Fe

L3,2 X

ML

D

(a

rb.

un

its

)

temperature (K)

E. Arenholz et al.,Appl. Phys. Lett. 94, 072503 (2009)

63

770 780 790 850 860 870

0.0

0.5

1.0L3

L2

L2

Ni

Co

CoN

iO_6.3

.1.o

pj

X

MC

D,

XA

(a

rb.

un

its

)

photon energy (eV)

Co/NiO

x50

L3

A. Scholl et al.,

Phys. Rev. Lett. 92, 247201 (2004)

Co

NiO

1

2

868 870 872

-0.2

0.0

photon energy (eV)

XM

LD

(arb

. u

nit

s)

NiO

Co/NiO

XA

(arb

. u

nit

s)

0.0 0.2 0.4 0.6

0.9

1.0

1.1

1.2

Ni

L2 r

ati

o

applied field (T)

EH E||H

0o

10o

20o

30o

40o

50o

wa

ll an

gle

H

-0.5 0.0 0.5

-10

0

10

Co

XM

CD

(a

rb.

un

its

)

field (T)

NiO

Co

H

Planar Domain Wall

FM

AFM H

64

Magnetic Microscopy

65

10-50 nm spatial resolutionJ. Stöhr, H.C. Siegmann,Magnetism (Springer)

Magnetic Microscopy

66

780 790 800

0.0

0.5

1.0

Co/NiO(001)

photon energy (eV)

XM

CD

, X

A (

arb

. u

nit

s)

070727_C

o_X

MC

D.o

pj

i070725_046_047_d

iv.jp

g

A / B

Imaging Magnetic Domains Using X-Rays

i070725_049_050_d

iv.jp

g

A / B

A

i070725_046.jp

g

B

i070725_050.jp

g

left circ. right circ.

+ Images taken with left and right circularly polarized X-rays at photon energies with XMCD, i.e. Co L3 edge, provide magnetic contrast and domain images.

E. Arenholz et al.,

Appl. Phys. Lett. 93, 162506 (2008)

67

Co XMCD

Ni XMLD

probing in-plane

5 m

5 m

X rays

+ Taking into account the geometry dependence of the Ni XMLD signal

Perpendicular coupling of Co and NiO moments at the interface.

Magnetic Coupling At Co/NiO Interface

E. Arenholz et al.,

Appl. Phys. Lett. 93, 162506 (2008)

68

+ First direct observation of vortex state in antiferromagnetic CoO and NiO disks in Fe/CoO and Fe/NiO bilayers using XMCD and XMLD.

+ Two types of AFM vortices:- conventional curling vortex

as in ferromagnets- divergent vortex,

forbidden in ferromagnets- thickness dependence of

magnetic interface coupling

J. Wu et al.,

Nature Phys. 7, 303 (2011)

divergent vortexconventional curling vortex

Magnetic Vortices

69

Q. He et al., Nature Comm. 2, 225 (2011)

2m

2m

+ BiFeO3 – multiferroic = ferroelectric + antiferromagnetic+ Compressive strain on rhombohedral phase (R-phase)

induced by substrate tetragonal-like phase (T-phase) + Partial relaxation of epitaxial strain Formation of a nanoscale mixture of T- and R-phases

AFM

RT

AFM

Nanoscale Magnetic Phases

70

Q. He et al.,

Nature Comm. 2, 225 (2011)

Nanoscale Magnetic Phases

XMCD PEEM

+ Highly distorted R-phase is the source of enhanced magnetic moment in the XMCD image.

2m

2m

AFM PFM

XMCD-PEEM

71

el-splat-sp

el-lat

Ultrafast Magnetism

Spin-lattice relaxation time

Electron-phonon relaxation time

Electron-spinrelaxation time

+ Energy reservoirs in a ferromagnetic metal

+ Deposition of energy in one reservoir

Non-equilibrium distribution and subsequent relation through energy and angular momentum exchange


72

C. Boeglin, et al., Nature 465, 458 (2010)

+ Orbital (L) and spin (S) magnetic moments can change with total angular momentum is conserved.

+ Efficient transfer between L and S through spin–orbit interaction in solids

+ Transfer between L and S occurs on fs timescales.

+ Co0.5Pt0.5 with perpendicularmagnetic anisotropy

+ 60 fs optical laser pulses change magnetization

+ Dynamics probed with XMCD using 120fs X-ray pulses

+ Linear relation connects Co L3 and L2 XMCD with Lz and Sz using sum rules

Co0.5Pt0.5

Ultrafast Dynamics Of Spin And Orbital Moments

73

+ Thermalization: Faster decrease of orbital moment

+ Theory: Orbital magnetic moment strongly correlated with magnetocrystalline anisotropy

+ Reduction in orbital moment Reduction in magnetocrystalline anisotropy

+ Typically observed at elevated temperatures in static measurements as well

C. Boeglin, et al., Nature 465, 458 (2010)

el-splat-sp

el-lat

Spin-lattice relaxation time

Electron-phonon relaxation time

Electron-spinrelaxation time

Ultrafast Dynamics Of Spin And Orbital Moments

74

J. Stöhr, H.C. SiegmannMagnetism From Fundamentals to Nanoscale DynamicsSpringer

References And Further Reading

magnetism as seen with x rays - spallation neutron sourceneutrons.ornl.gov/sites/default/files/elke...

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