Magnetism as seen with X Rays Elke Arenholz Lawrence Berkeley - - PowerPoint PPT Presentation

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Magnetism as seen with X Rays Elke Arenholz

Lawrence Berkeley National Laboratory and Department of Material Science and Engineering, UC Berkeley

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+ 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:

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Magnetic Materials Today

Magnetic materials for energy applications Magnetic nanoparticles for biomedical and environmental applications Magnetic thin films for information storage and processing

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Permanent and Hard Magnetic Materials

Engineering magnetic anisotropy

• n the

atomic scale Controlling grain and domain structure

• n the micro-

and nanoscale

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Magnetic Nanoparticles

Tailoring magnetic nanoparticles for environmental applications Optimizing magnetic nanoparticles for biomedical applications

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GMR Read Head Sensor

Magnetic Thin Films

Magnetic domain structure

• n the

nanometer scale Unique magnetic phases at interfaces Ultrafast magnetization reversal dynamics Magnetic coupling at interfaces

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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

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sample intensity wavelength, photon energy intensity wavelength, photon energy

Spectroscopy

Sample Light/Photon Source Monochromator

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Sample Electron Storage Ring Monochromator

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

770 780 790 800 0.6 0.7 0.8 0.9 1.0 photon energy (eV)

transmitted intensity It / I0

EY_TM.opj

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770 780 790 800 2 4 6

EY_TM.opj

electron yield Ie / I0

photon energy (eV) 770 780 790 800 0.6 0.7 0.8 0.9 1.0 photon energy (eV)

transmitted intensity It / I0

EY_TM.opj

Photons absorbed Electrons generated 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

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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 It / I0 photon energy (eV) Fe

luminescence_BL402_Apr2012.opj

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

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0.8 1.0 0.8 1.0 0.8 1.0 0.8 1.0

Mn Ni Co

electron yield (arb. units) It / I0

Fe

640 650 660 0.9 1.0 photon energy (eV)

luminescence_BL402_Apr2012.opj

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

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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

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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

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core level 2p3/2 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.

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core level 2p3/2 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.

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2p3/2 L3 2p1/2 L2 valence states

‘White Line’ Intensity

Co core level 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.

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2p3/2 L3 2p1/2 L2 valence states

‘White Line’ Intensity

Ni core level 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.

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2p3/2 L3 2p1/2 L2 valence states 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.

‘White Line’ Intensity

Cu core level

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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

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• 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

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• J. Stöhr, H.C. Siegmann,

Magnetism (Springer)

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

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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

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455 460 465 470

PZT_XA.opj

photon energy (eV) XA (arb. units) PbZr0.2Ti0.8O2

Sensitivity To Site Symmetry: Ti4+ L3,2

+ Electric dipole transitions: d02p53d1 + Crystal field splitting 10Dq acting on 3d orbitals: Octahedral symmetry: e orbitals towards ligands  higher energy t2 orbitals between ligands  lower energy Tetragonal symmetry: e orbitals  b2 = dxy, e = dyz, dyz t2 orbitals  b1 = dx2y2, a1 = d3z2r2

• J. Stöhr, H.C. Siegmann,

Magnetism (Springer)

t2 e e t2

3d orbitals

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x- ray absorption 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

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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

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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

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+ 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-up spin-down

empty states, “holes”

) n (n μ m| |

min e maj e B

 

Stoner Model For Ferromagnetic Metals

• J. Stöhr, H.C. Siegmann,

Magnetism (Springer)

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Origin of X-ray Magnetic Circular Dichroism

2-Step Model for photoexcitation of electrons from 2p3/2, 2p1/2 to 3d states by absorption

• f 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 split valence shell acts as detector for spin of excited electrons. 3d

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

Origin of X-ray Magnetic Circular Dichroism

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Origin of X-ray Magnetic Circular Dichroism

initial state |i initial state energy i final state |f final state energy f + Wave functions describe electronic and photon states Energies 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

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+ 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

Origin of X-ray Magnetic Circular Dichroism

↓ ↓ ↓ ↓

• J. Stöhr, H.C. Siegmann,

Magnetism (Springer)

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Origin of X-ray Magnetic Circular Dichroism

q = +1 q = 0 q = 1 q=1 q=+1 q=0

L3 L2

50 30 10 30

L3: X rays with q = +/1 excite 62.5%/37.5% of the spin up electrons L2: X rays with q = +/1 excite 25%/75% of the spin up electrons

• J. Stöhr, H.C. Siegmann,

Magnetism (Springer)

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Origin of X-ray Magnetic Circular Dichroism

↓ ↓ ↓ ↓

• J. Stöhr, H.C. Siegmann,

Magnetism (Springer)

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%

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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 (arb. units)

FeCo_GaAs_XMCD.opj

XMCD (arb. units)

L3 L2 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 (arb. units) photon energy (eV) XMCD (arb. units)

CoFe2O4.opj

L3 Fe Co L2 L3 L2 CoFe2O4 Fe2+,Oh Fe3+, Oh Fe3+, Td Co2+

• J. Stöhr, H.C. Siegmann,

Magnetism (Springer)

H circularly polarized

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

X-Ray Magnetic Circular Dichroism (XMCD)

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+ 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

XMCD photon energy (eV)

• V. Cocker et al.,
• Eur. J. Mineral. 19, 707–716 (2007)

2 0 +2

60

+60 H (T) M (emu g1)

Fe2+,Oh Fe3+, Oh Fe3+, Td

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• V. Cocker et al.,
• Eur. J. Mineral. 19, 707–716 (2007)

2 0 +2

60

+60 H (T) M (emu g1)

Co2+ Oh

Magnetic Bionanospinels

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

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

XA, XMCD Co metal XA XA XMCD XMCD photon energy (eV) Co

2+ in CoFe2O4

780 790 800 XA, XMCD

Co_Co2+.opj

Co-doped TiO2

XMCD (arb. units) XA (arb. units)

Co-doped TiO2 dilute magnetic semiconductors

J.-Y. Kim et al.,

• Phys. Rev. Lett. 90, 017401 (2003)

780 800 790 810 photon energy (eV)

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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 (arb. units) 630 640 650 660 770 780 790 800 810 XMCD (arb. units) photon energy (eV)

x20

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 Mn moments + Nominal thickness of uncompensated interface moments: (0.50.1)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 XMCD (arb. units) photon energy (eV) photon energy (eV) XA (arb. units) XMCD (arb. units) Fe L3 XMCD (arb. units) 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

• J. Stöhr, H.C. Siegmann,

Magnetism (Springer)

GaxFe1−x FeL3,2

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

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Composition Dependence of Fe moment in Fe1-xMnx

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

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A < 0 B > 0

+ Theoretically derived sum rules correlate XMCD spectra with spin and

• rbital moment providing

unique tool for studying magnetic materials.

mS = B–A + 2B / C mL = –2BA + B / 3C Nh =  IL3 + IL2 /C

Sum Rules

• J. Stöhr, H.C. Siegmann,

Magnetism (Springer)

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

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Element-specific Magnetization Reversal

Co L3,2 +/- circ. pol. H = -0.5T

XA (arb. units)

780 800

I

+ - I

• (arb. units)

photon energy (eV)

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

47

XA (arb. units)

Co L3,2 +/- circ. pol. H = +0.5T

780 800

I

+ - I

• (arb. units)

photon energy (eV)

Co L3,2 +/- circ. pol. H = -0.5T

XA (arb. units)

780 800

I

+ - I

• (arb. units)

photon energy (eV)

Element-specific Magnetization Reversal

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

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• 0.4 -0.2 0.0

0.2 0.4 XMCD magnetic field (T)

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

Co L3,2 +/- circ. pol. H = -0.5T

XA (arb. units)

780 800

I

+ - I

• (arb. units)

photon energy (eV)

XA (arb. units)

Co L3,2 +/- circ. pol. H = +0.5T

780 800

I

+ - I

• (arb. units)

photon energy (eV)

Element-specific Magnetization Reversal

49 x3 H=+0.2 T H=-0.2 T

XA (arb. units) 700 750 800 850 XMCD photon energy (eV)

Fe Co Ni

5 ML Co 8,10 ML Fe 18 ML Ni

• 40
• 20

20 40

• 10
• 5

5 10

XMCD (%)

field (mT) Ni Fe Co

8 ML Fe

Element-specific Magnetization Reversal

50

• 40
• 20

20 40

• 10
• 5

5 10

XMCD (%)

field (mT) Ni Fe Co

x3 H=+0.2 T H=-0.2 T XA (arb. units) 700 750 800 850 XMCD photon energy (eV)

Fe Co Ni 8 ML Fe

5 ML Co 8,10 ML Fe 18 ML Ni

• 200
• 100

100 200

• 10
• 5

5 10

XMCD (%)

field (mT) Ni Fe Co

10 ML Fe

Element-specific Magnetization Reversal

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 (arb. units)

FeCo_GaAs_XMCD.opj

XMCD (arb. units)

L3 L2 Co H circularly polarized

52 0.0 0.5 1.0 770 780 790 800

• 0.4
• 0.2

0.0 photon energy (eV) XA (arb. units)

FeCo_GaAs_XMCD.opj

XMCD (arb. units)

L3 L2 Co H circularly polarized

M(t)

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

X-Ray Ferromagnetic Resonance

53 0.0 0.5 1.0 770 780 790 800

• 0.4
• 0.2

0.0 photon energy (eV) XA (arb. units)

FeCo_GaAs_XMCD.opj

XMCD (arb. units)

L3 L2 Co H circularly polarized

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? (!)

X-Ray Ferromagnetic Resonance

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Pulsed nature of synchrotron radiation Example: Advanced Light Source bunch spacing Pulse length 70 ps + 256-320 bunches for 500mA beam current + Bunch spacing: 2 ns (500MHz) + Pulse length 70ps

X-Ray Ferromagnetic Resonance

55

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

M(t) HB X-ray pulses microwave driving field ~70 ps

2 ns

Mz(t)

X-ray pulse repetition signal, ~500 MHz

f = n500 MHz

X-Ray Ferromagnetic Resonance

56 200 400 600 800 1000

• 4
• 2

2 4

XMCD (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 (arb. units)

FeCo_GaAs_XMCD.opj

XMCD (arb. units)

L3 L2 Co

X-Ray Ferromagnetic Resonance

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.

X-Ray Ferromagnetic Resonance

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 (arb. units)

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

• ff-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. units) difference (arb. units)

// //

0.0 0.5 1.0

XA (arb. units)

• 0.2

0.0 0.2 photon energy (eV)

difference (arb. units)

// // // //

Structural Changes In PbZr0.2Ti0.8O3

as grown reversed

t2 e e t2

• 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 S L L3 Fe Co L3 L2 CoFe2O4 L2

 = 0  = 45 H H

X-Ray Magnetic Linear Dichroism

[001]

0.0 0.5 1.0

XMLD (arb. units) XA (arb. units)

CoFe2O4_110_Sept.opj

• 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

CoNiO_4.0.2_Ni_XMLD.opj

XA (arb. units) XMLD (arb. units)

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 (arb. units) photon energy (eV) XMCD (arb. units) 0.0 0.5 1.0 710 720

• 0.2

0.0 0.2

XA (arb. units)

expt. calc. photon energy (eV)

XMLD (arb. units) 0.0 0.5 1.0 710 720

• 0.1

0.0 0.1 XA (arb. units) XMLD (arb. units) photon energy (eV)

Magnetic Coupling At Interfaces

H

Mn 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 1.0 100 200 300 400 0.0 0.5 1.0 Fe L3,2 XMLD (arb. units) Mn L3,2 XMCD (arb. units) Fe L3,2 XMLD (arb. units) temperature (K)

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

63 770 780 790 850 860 870 0.0 0.5 1.0

L3 L2 L2 Ni Co

CoNiO_6.3.1.opj

XMCD, XA (arb. units) 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) XMLD (arb. units) NiO Co/NiO XA (arb. units) 0.0 0.2 0.4 0.6 0.9 1.0 1.1 1.2 Ni L2 ratio applied field (T) EH E||H

• 10
• 20
• 30
• 40
• 50
• wall angle

H

• 0.5

0.0 0.5

• 10

10 Co XMCD (arb. units) field (T)

NiO Co H

Planar Domain Wall

FM AFM

H

64

Magnetic Microscopy

65

10-50 nm spatial resolution

• J. Stöhr, H.C. Siegmann,

Magnetism (Springer)

Magnetic Microscopy

66

780 790 800 0.0 0.5 1.0 Co/NiO(001) photon energy (eV) XMCD, XA (arb. units)

070727_Co_XMCD.opj i070725_046_047_div.jpg

A / B

Imaging Magnetic Domains Using X-Rays

i070725_049_050_div.jpg

A / B A

i070725_046.jpg

B

i070725_050.jpg

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 vortex conventional curling vortex

Magnetic Vortices

69

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

2m 2m

+ 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 R T 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.

2m 2m

AFM PFM XMCD-PEEM

71

el-sp lat-sp el-lat

Ultrafast Magnetism

Spin-lattice relaxation time Electron-phonon relaxation time Electron-spin relaxation 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

• J. Stöhr, H.C. Siegmann,

Magnetism (Springer)

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 perpendicular magnetic 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-sp lat-sp el-lat Spin-lattice relaxation time Electron-phonon relaxation time Electron-spin relaxation time

Ultrafast Dynamics Of Spin And Orbital Moments

74

• J. Stöhr, H.C. Siegmann

Magnetism From Fundamentals to Nanoscale Dynamics Springer

References And Further Reading