X-Rays and Magnetism

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X-Rays and Magnetism
Joachim Stöhr Stanford Synchrotron Radiation
Laboratory
Past
Present
Future
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Present Size gt 0.1 mm, Speed gt 1 nsec Future
Size lt 0.1 mm, Speed lt 1 nsec
Ultrafast Nanoscale Dynamics
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Growth of X-Ray Brightness and Magnetic Storage
Density
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Non Resonant X-Ray Scattering
Relative Intensity (hn / mc2)2
Relative Intensity 1
hn 10 keV, mc2 500 keV
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Fe metal L edge
Kortright and Kim, Phys. Rev. B 62, 12216 (2000)
Soft X-Rays are best for magnetism!
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Core level binding energies give Element
specificity Chemical state specificity
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Magnetic Spectroscopy and Microscopy
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Part 1 Nanoscale Magnetism
Real Space Imaging
X-Rays have come a long way
1895
1993
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PEEM-II at ALS

• Full Field Imaging
• Electrostatic (30 kV)
• 20 - 50 nm Resolution
• Linear and circular polarization

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PEEM Contrast Mechanisms
Use soft x-rays L edges of Fe, Co, Ni
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Exchange Bias Exchange Coupling

• coercivity increase
• (uniaxial anisotropy)
• exchange bias, loop shift (unidirectional
  anisotropy)

A ferromagnet behaves different when in contact
with an antiferromagnet. No appropriate
understanding yet 45 years after
discovery Conventional techniques cannot study
the magnetic interface
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Co
NiO
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Co on NiO(001)
Analysis of dichroism contrast ? 3 dimensional
spin structure
s
s
s
010
2mm
2nm Co on NiO(001)
NiO after deposition
Bare NiO(001)
Ni spins near antiferromagnetic surface rotate
in-plane couple parallel to Co
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Spectromicroscopy of Ferromagnets and
Antiferromagnets
AFM domain structure at surface of NiO substrate
FM domain structure inthin Co film on NiO
substrate
NiO XMLD
Co XMCD
H. Ohldag, A. Scholl et al., Phys. Rev. Lett.
86(13), 2878 (2001).
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Interface Spectroscopy
XAS Line shape is sensitive to transition M ?
MxOy
Upon Co deposition on NiO 2ML NiO ? Ni 2ML Co ?
CoO Linear combination of metal and oxide
spectra possible
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Experiment 3 Interface Microscopy
Co
Co
CoNiO
NiO
NiO
AFM NiO
FM Co
FM Ni(O)
XMCD
XMLD
XMCD
Chemically induced interfacial spins provide the
magnetic link
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Only a small fraction of interfacial spins is
pinned
Method
Experiment
Co/IrMn
A small fraction (4) of interfacial spins is
pinned they are the origin of exchange bias!
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Exchange Bias A new x-ray look at an old
problem
4 crucial experiments 1.) The bare
antiferromagnetic surface. F. U. Hillebrecht, H.
Ohldag et al., Phys Rev. Lett. 86(15), 3419
(2001). 2.) The antiferromagnet surface coupled
to a ferromagnet. H. Ohldag, A. Scholl et al.,
Phys. Rev. Lett. 86(13), 2878 (2001). 3.) The
interface between both. H. Ohldag, A. Scholl et
al., Phys Rev Lett. 87(24), 7201 (2001). 4.)
Where are the pinned interfacial spins? To be
published.
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Exchange Bias Model from X-Rays
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The Future - PEEM-III

• Aberration corrected PEEM
• Estimated 2 nm resolution

• Picosecond dynamics

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Part 2 Ultrafast Magnetization Dynamics
Switching with Exchange Fields
Oersted fields
Exchange fields
Oersted fields are long range and weak
Exchange fields are short range and strong
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Magnetic Switching by Spin Injection

• Nanoscale lt1000 Å
• Both in-plane (black) and out-of-plane (red)

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Proposed Solution

• Use exchange field of injected spin current
• Strong field, short pulse
• Picosecond switching
• Optimal at small sizes

t0
t0 t
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The Ultrafast Worlds
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Creating spin current

• Spin moment relaxes into direction of bulk
  ferromagnet
• Spin polarization reaches maximum at
  approximately 10 nm
• Theoretically, spin polarization can be 100 in
  some metals

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Spin injection
? 1 nm for ferromagnets (or 10 fs) ? 1 µm for
noble metals ( or 10 ps) ? 100 µm for
semiconductors (or 1 ns)
X-ray experiments can observe effect, size,
sign and dynamics
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Negative Damping by Spin Injection
Minority spin injection
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Nano-structure for Spin Injection
2 nm 1 1000 nm 30 nm 40 nm
Focused Ion Beam Holes
NiFe
Cu
Si3N4
Co
1 µm
Si3N4

• Current flows through Co to become spin polarized
• Spin polarized current enters the NiFe layer
  changes the domain structure

PEEM microscopy
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Oersted Switching by Current through Contact Holes
Fe 0.8 nm Cu 10 nm Si3N4 30 nm Cu 160
nm
Current is not polarized, switching due to
Oersted-like field
100 contact holes diameter 40 nmj ? 1012 A/m2
initial after 180 mA
10 mm
10-100 mm
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Measuring Precession by Pump/Probe Technique
Current pulse

• Pump induce spin polarization by current pulse
• Probe Image with delayed photon pulse
• Vary the delay between current and photon pulse
• Vary the strength of the current pulse

50 ps
330 ns
Photon pulse
?
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Part 3 The Future X-Ray Free Electron Lasers
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• SASE gives 106 intensity gain
• over spontaneous emission
• FELs can produce ultrafast
• pulses (of order 100 fs)

l
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LINAC COHERENT LIGHT SOURCE
0 Km
2 Km
3 Km
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Concepts of the LCLS

• Based on single pass free electron laser (FEL)
• Uses high energy linac (15 GeV) to provide
  compressed electron beam to long undulator(s)
  (120 m)
• Based on SASE physics to produce 800-8,000eV (up
  to 24KeV in 3rd harmonic) radiation
• Analogous in concept to XFEL of TESLA project at
  DESY

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Proposed Schedule and Budget

• FY2003-2004
• Prepare preliminary designs
• FY2005
• Procure undulator
• Construct injector
• FY2006-2007
• Convert linac,install undulator, begin FEL
  commissioning
• FY2008
• Complete civil construction, characterize photon
  beam

• Estimated Total Project Cost M 221 M 47
  M 268

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Example Nanoscale Magnetism
Reciprocal Space Imaging Speckle
Sample is non-periodic no Bragg peaks
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Nanoscale Magnetism PEEM versus SPECKLE
PEEM X-ray Absorption
Speckle Coherent X-ray Scattering

• Photon-in / electron-out
• Spatial resolution set by electron
• optics
• No strong external magnetic fields
• Equilibrium dynamics gt 1msec
• Pump-probe no single shot space charge
  limit

• Photon-in / photon-out
• Spatial resolution set by x-ray
• wavelength ? (775 eV) ? 16 Å)
• Magnetic and electric fields
• Equilibrium dynamics gt 1msec (now)
• 
  fsec (in future)
• Pump-probe
• ultrafast and single shot

Technique of choice for dynamics, future X-FELs
Comparison to STXM No zone plate optics -gt less
beam damage in FEL
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Incoherent vs. Coherent X-Ray Scattering
Small Angle Scattering Coherence length larger
than domains, but smaller than illuminated area
information about domain statistics
Speckle Coherence length larger than illuminated
area
true information about domain structure
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Imaging by Coherent X-Ray Diffraction
Phase problem can be solved by oversampling
speckle image
? 5 ?m (different areas)
S. Eisebitt, M. Lörgen, J. Lüning, J. Stöhr, W.
Eberhardt, E. Fullerton (unpublished)
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Spin Block Fluctuations around Critical
Temperature
Tc
Magnetization
Temperature
t (T-Tc) / Tc
T lt Tc
T ? Tc
T gt Tc
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Collaborators
Stanford Synchrotron Radiation Laboratory Hendrik
Ohldag Christian Stamm Hans Christoph
Siegmann Yves Acremann
Advanced Light Source Andreas Scholl Frithjof
Nolting (now SLS) Simone Anders (now IBM)
Stanford University Scott Andrews Bruce Clemens
BESSY Stefan Eisebitt Marcus Lörgen Wolfgang
Eberhardt
IBM Almaden Erik Fullerton Charles
Rettner Jan-Ulrich Thiele
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Summary
X-FELs will deliver unprecedented brightness and
femtosecond pulses Understanding of laser
physics and technology well founded FELs
promise to be extraordinary scientific
tools Applications in many areas chemistry,
biology, plasma physics, atomic physics,
condensed matter physics
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The End
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Spin Injection
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Fast Magnetization Dynamics is governed by
Landau-Lifschitz-Gilbert equation
Angular momentum change
Precession torque
Gilbert damping torque
Typically a ltlt 1, 100 ps
1 Tesla field 90o rotation in 10 ps
We want to understand a on atomic level a
controls switching time, a1 optimal
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Stoner Excitations Changing the Magnetization by
Electron Scattering
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X-FEL Radiation Electric and Magnetic Fields
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Part 3 Ultrafast Magnetization Dynamics
Switching with Oersted Fields
ca. 200 BC
1995
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Experimental Principle

• High field pulses up to 5 T
• High beam precision allows multiple shots on the
  same spot

C. H. Back, R. Allenspach, W. Weber, S. S. P.
Parkin, D. Weller, E. L. Garwin, H. C. Siegmann,
Science 285, 864-867 (1999)
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Precession Torques
Maximum torque
Minimum Torque
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Simulation Results
Small damping, many precessions
Medium damping, fewer precessions
Large damping, magnetization creeps into field

• Results show high sensitivity to damping a(
  optimal 1)

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Experiment

• State-of-art media (courtesy of Komag)

Pattern read by PEEM

• Comparison with simulation shows a must be
  larger than 1
• Larger than expected from all previous results
• But media have distribution of nanostructured
  regions
• Need to control nanostructure
• to understand why a is so large

Photoelectron emission microscopy image
Same image drawn to simulation scale
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Resonant Magnetic Soft X-ray Scattering
Fe
charge
magnetic -XMCD
f

e
'

e
-
i
(
e
'

e
)

M
F
(
1
)
F
(
0
)
n
n
n
n
where Fn(i) are complex
f1 i f2
Note at resonance f1 0
Kortright and Kim, Phys. Rev. B 62, 12216 (2000)
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Resonant Magnetic Scattering Cross-Section
Charge
magnetic
magnetic
Circular dichroism XMCD
Linear Dichroism XMLD
J. P. Hannon, G. T. Trammell, M. Blume, D. Gibbs,
Phys. Rev. Lett 61, 1245 (1988)
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Motivation

• Present methods of writing are unfavorable
• present recording time 1 ns
• unfavorable torque and dependent on thermal
  activation
• Understand switching of soft and hard materials
  on sub-nanosecond scale
• faster switching
• avoid configuration with small torque on
  magnetization

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Magnetization and Spin Dynamics
Magnetism ruled by four fundamental
interactions Exchange interaction gt produces
magnetic order on atomic scale, magnetic
stiffness, TC , TN, spin-spin scattering,
coherence time of spin excitations
Spin-orbit interaction gt produces
magneto-crystalline anisotropy,
spin-phonon
(thermal) excitations, friction (Gilbert
damping) Zeeman interaction gt produces
macroscopic spin alignment, torque
(Landau-Lifshitz), magnetic switching Dipolar
interaction gt produces shape anisotropy,
magnetic domain structure and motion
Energy/atom time scale length
scale Exchange eV fs atomic Spin-orbit meV-
meV ps - ns nano (nm) Zeeman lt meV ps - ns gt
nano Dipolar lt meV ps - ms gt nano
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