Transcript Magnetization dynamics with picosecond magnetic field pulses Christian Stamm Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center I.

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)
Why Magnetization Dynamics?
constant current
alignment parallel to field
pulsed current (5 ps)
precessional switching
Magnetic Field Pulse
8
FWHM = 5 ps
6
B [Tesla]
Relativistic electron
bunches from the
Stanford Linear
Accelerator are
focused to ~10 mm
4
2
peak field of ~7 Tesla
10 mm from center,
falling off with 1/R
0
-20
0
20
40
t [ps]
60
80
100
Dynamic equation for M
LandauLifshitzGilbert
1 dM
 - M H 
 dt
 M
change in angular Precession
momentum
torque
Direction of
torques
1 







M
dM
dt







Gilbert damping
torque
Motion of M for
constant H
After Magnetic Field Pulse
perpendicular magnetization
CoCrPt
granular media
Image of M:
Polar Kerr Microscopy
(size 150 mm)
50 mm
Multiple Field Pulses
1 pulse
3 pulses
5 pulses
7 pulses
50 mm
2 pulses
4 pulses
6 pulses
Transition Region
Observed: wide transition region
Calculated: sharp transitions
M [norm]
1
Model assuming
distribution of
initial direction
for M
0
exp. data
LLG calculation
distribution
-1
0
20
40
60
R [mm]
80
100
Initial Distributions of M
• Static:
angle of anisotropy axes
x-ray diffraction: q  ±4º
• Dynamic:
thermal motion, random fields
E  KUV sin2 q
q  10º
V=(6.5 nm)3
Look identical at one point in time
Differences appear with multiple pulses
2 Field Pulses
• static distribution is
deterministic
2 pulses should reverse
50 mm
not observed
• dynamic distribution is
stochastic
independent switching
probability for each pulse
Relative M
1
0
-1
0
20
40
60
R [mm]
80 100
YES
Stochastic Switching
1
M1(R)
1
2
3
4
5
Independent
stochastic events:
0
calculate switching
by successive
multiplication
-1
Relative Magnetization
1
0
-1
1
M2 = M1 · M 1
0
M3 = M2 · M 1
-1
1
7
6
:
0
Mn = (M1)n
-1
0
20
40
60
80
0
20
R[mm]
40
60
80
100
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
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
Epitaxial Fe layer
Fe / GaAs (001)
FMR characterization:
damping  = 0.004
and anisotropies
(G. Woltersdorf, B. Heinrich)
Au 10 layers
Fe 10 or 15 layers
GaAs (001)
Kerr hysteresis loop
HC = 12 Oe
Images of Fe / GaAs
SEMPA images of M
(SEM with Polarization Analysis)
one magnetic field pulse
10 ML Fe / GaAs (001)
M0
50 mm
50 mm
50 mm
Thermal Stability
Important aspect in recording media
Néel-Brown model (uniform rotation)
Probability that grain
has not switched:
with
   0e
K uV / kT
P(t )  e
and
for long-term stability:
t /
 0  10 s
10
  10 years
Comparison of Patterns
Observed (SEMPA)
Calculated (fit using LLG)
Anisitropies same as FMR
Damping  = 0.017
100 mm
4x larger than FMR
WHY?
Energy Dissipation
After field pulse:
Damping causes
dissipation of
energy during
precession
6
4
E/Ku
(energy barrier for
switching: KU)
2
10 ML Fe
15 ML Fe
0
0
1
2
3
Number of precessions
4
Enhanced Damping
Precessing spins in ferromagnet:
Tserkovnyak, Brataas, Bauer
Phys 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
sin 2 q
m   2
sin q  
(  0.01)
q  1º in FMR, but q  110º in our experiment
Effective Field H
3 components of H act on M
HE
externally applied field
HD
= -MS
demagnetizing field
M
HE
HK
= 2K/m0MS
crystalline anisotropy
HK
HD
Magnetic Field Strength
1010 electrons:
B*r=
50 Tesla * mm
duration of
magnetic field
pulse: 5 ps
Perpendicular Magnetization
Time evolution
perpendicular
anisotropy
0
M
Y
0
0
MX
MZ
M0=(0, 0, -MS)
5 ps field pulse
2.6 Tesla
precession and
relaxation
towards
(0, 0, +MS)
Granular CoCrPt Sample
TEM of magnetic grains
Size of grains  8.5 nm
Paramag. envelope  1 nm
1 bit  100 grains
Radial Dependence of M
Magnetization [a.u.]
Perpendicular magnetized sample (CoCrPt alloy)
1
1 Pulse
2 Pulses
3 Pulses
4 Pulses
5 Pulses
6 Pulses
7 Pulses
0
-1
0
20
40
60
80
Distance from Center [mm]
100
In-Plane Magnetization
MZ
Time evolution of M
after excitation by
5 ps field pulse
0.27 Tesla
(finished at *)
0
M0
0
M
X
switching by
precession around
demagnetizing
field
0
MY
(uniaxial in-plane)
Precessional Torque: MxH
in-plane magnetized sample: figure-8 pattern
M
circular in-plane
magnetic field H
lines of constant
(initial) torque
MxH
Magnetization Reversal
Magnetization is Angular Momentum
Applying torque changes its direction
immediate response to field
H
Fastest way to reverse
the magnetization:
initiate precession
around magnetic field
M0
M(t)
patented by IBM
Picosecond Field Pulse
Generated by electron bunch
from the
Stanford Linear Accelerator
data from: C.H. Back et al. Science 285, 864 (1999)
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
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