Magnetization dynamics with picosecond magnetic field pulses

1
Magnetization dynamics with picosecond magnetic
field pulses
Christian Stamm Stanford Synchrotron Radiation
Laboratory Stanford Linear Accelerator Center
I. Tudosa, H.-C. Siegmann, J. Stöhr
(SLAC/SSRL) A. Vaterlaus (ETH Zürich) A. Kashuba
(Landau Inst. Moscow) D. Weller, G. Ju (Seagate
Technologies) G. Woltersdorf, B. Heinrich (S.F.U.
Vancouver)
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Why Magnetization Dynamics?
constant current alignment parallel to
field pulsed current (5 ps) precessional
switching
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Magnetic Field Pulse
Relativistic electron bunches from the Stanford
Linear Accelerator are focused to 10 mm peak
field of 7 Tesla 10 mm from center, falling off
with 1/R
FWHM 5 ps
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Dynamic equation for M
Landau-Lifshitz-Gilbert
Precession torque
Gilbert damping torque
change in angular momentum
Direction of torques
Motion of M for constant H
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After Magnetic Field Pulse
perpendicular magnetization
CoCrPt granular media Image of M Polar Kerr
Microscopy (size 150 mm)
50 mm
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Multiple Field Pulses
1 pulse
3 pulses
5 pulses
7 pulses
50 mm
2 pulses
4 pulses
6 pulses
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Transition Region
Observed wide transition region Calculated shar
p transitions
Model assuming distribution of initial direction
for M
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Initial Distributions of M

• Static angle of anisotropy axes x-ray
  diffraction q ? 4º
• Dynamicthermal motion, random fields

q ? 10º V(6.5 nm)3
Look identical at one point in time Differences
appear with multiple pulses
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2 Field Pulses

• static distribution isdeterministic2 pulses
  should reverse
• not observed
• dynamic distribution is stochasticindependent
  switching probability for each pulse
• YES

50 mm
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Stochastic Switching
Independent stochastic events calculate
switching by successive multiplication M2 M1
M1 M3 M2 M1 Mn (M1)n
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Conclusions

• A picosecond fast magnetic field pulse causes the
  magnetization to precess and - if strong enough -
  switch its direction
• In granular perpendicular magnetic media,
  switching on the ps time scale is influenced by
  stochastic processes
• Possible cause is the excitation of the spin
  system due to inhomogeneous precession in the
  large applied field

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Epitaxial Fe / GaAs
SEMPA images of M (SEM with Polarization
Analysis) one magnetic field pulse
50 mm
M0
Au 10 layers
Fe 10 or 15 layers
GaAs (001)
50 mm
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Epitaxial Fe layer
Au 10 layers
Fe 10 or 15 layers
Fe / GaAs (001) FMR characterization damping a
0.004 and anisotropies (G. Woltersdorf, B.
Heinrich)
GaAs (001)
Kerr hysteresis loop HC 12 Oe
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Images of Fe / GaAs
SEMPA images of M (SEM with Polarization
Analysis) one magnetic field pulse 10 ML Fe /
GaAs (001)
50 mm
M0
50 mm
50 mm
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Thermal Stability
Important aspect in recording media Néel-Brown
model (uniform rotation) Probability that
grain has not switched with and for
long-term stability
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Comparison of Patterns
Observed (SEMPA) Calculated (fit using
LLG) Anisitropies same as FMR Damping a
0.017 4x larger than FMR WHY?
100 mm
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Energy Dissipation
After field pulse Damping causes dissipation of
energy during precession (energy barrier for
switching KU)
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Enhanced Damping

• Precessing spins in ferromagnet
• Tserkovnyak, Brataas, BauerPhys Rev Lett 88,
  117601 (2002)Phys Rev B 66, 060404 (2002)
• source of spin current
• pumped across interface into paramagnet
• causes additional damping
• spin accumulation
• q ? 1º in FMR, but q ? 110º in our experiment

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Effective Field H
3 components of H act on M
HD -MS demagnetizing field
HEexternally applied field
M
HE
HK 2K/m0MS crystalline anisotropy
HK
HD
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Magnetic Field Strength
1010 electrons B r 50 Tesla mm duration
of magnetic field pulse 5 ps
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Perpendicular Magnetization
Time evolution
perpendicular anisotropy M0(0, 0, -MS) 5 ps
field pulse2.6 Tesla precession and relaxation
towards (0, 0, MS)
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Granular CoCrPt Sample
TEM of magnetic grains
Size of grains ? 8.5 nm Paramag. envelope ? 1
nm 1 bit ? 100 grains
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Radial Dependence of M
Perpendicular magnetized sample (CoCrPt alloy)
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In-Plane Magnetization
Time evolution of M
switching by precession around demagnetizing
field after excitation by 5 ps field pulse0.27
Tesla(finished at ) (uniaxial in-plane)
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Precessional Torque MxH
in-plane magnetized sample figure-8 pattern
M
circular in-plane magnetic field H
lines of constant (initial) torque MxH
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Magnetization Reversal

• Magnetization is Angular Momentum
• Applying torque changes its direction
• immediate response to field
• Fastest way to reversethe magnetization
• initiate precession around magnetic field
• patented by IBM

H
M0
M(t)
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Picosecond Field Pulse
Generated by electron bunch from the Stanford
Linear Accelerator
data from C.H. Back et al. Science 285, 864
(1999)
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Outline

• Magnetization Dynamics What is precessional
  switching?
• How do we generate a picosecond magnetic field
  pulse?
• Magnetization reversal in granular perpendicular
  media
• Enhanced Gilbert damping in epitaxial Fe / GaAs
  films

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Previously Strong Coupling
Co/Pt multilayer magnetized perpendicular Domain
pattern after field pulse from C.H. Back et
al.,PRL 81, 3251 (1998) MOKE line scan
through center switching at 2.6 Tesla