View
47
Download
0
Category
Preview:
DESCRIPTION
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) - PowerPoint PPT Presentation
Citation preview
Christian Stamm
Stanford Synchrotron Radiation LaboratoryStanford 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)
Magnetization dynamics with picosecond
magnetic field pulses
Magnetization dynamics with picosecond
magnetic field pulses
Why Magnetization Dynamics?
constant current
alignment parallel to field
pulsed current (5 ps)
precessional switching
Magnetic Field Pulse
Relativistic electron bunches from the Stanford Linear Accelerator are focused to ~10 m
peak field of ~7 Tesla 10 m from center, falling off with 1/R
-20 0 20 40 60 80 100
0
2
4
6
8
B [T
esla
]
t [ps]
FWHM = 5 ps
dt
d
dt
d MM
M
HMM
1
- 1
Precession torque
Gilbert damping torque
change in angular momentum
Direction of torques
Motion of M for constant H
Dynamic equation for M
Landau-Lifshitz-Gilbert
CoCrPt
granular media
Image of M:
Polar Kerr Microscopy
(size 150 m)
After Magnetic Field Pulse
50 m
perpendicular magnetization
1 pulse 3 pulses 5 pulses
2 pulses
7 pulses
4 pulses 6 pulses
Multiple Field Pulses
50 m
Transition Region
Observed: wide transition region
Calculated: sharp transitions
Model assuming distribution of initial direction for M
0 20 40 60 80 100
-1
0
1
exp. data LLG calculation distribution
M [n
orm
]
R [m]
Initial Distributions of M
Look identical at one point in time
Differences appear with multiple pulses
• Static: angle of anisotropy axes x-ray diffraction: ±4º
• Dynamic:thermal motion, random fields
2sinVKE U 10ºV=(6.5 nm)3
2 Field Pulses
• static distribution isdeterministic2 pulses should reverse
not observed
• dynamic distribution is stochasticindependent switching probability for each pulse
YES
50 m
0 20 40 60 80 100
-1
0
1
Re
lativ
e M
R [m]
Stochastic Switching
Independent stochastic events:
calculate switching by successive multiplication
M2 = M1 · M1
M3 = M2 · M1
:
Mn = (M1)n
-1
0
1
-1
0
1
-1
0
1
0 20 40 60 80
-1
0
1
0 20 40 60 80 100
M1(R)
2 3
4
6 7
5
1
Rel
ativ
e M
agne
tizat
ion
R[m]
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 m
50 m
M0
GaAs (001)
Fe 10 or 15 layers
Au 10 layers
Epitaxial Fe layer
GaAs (001)
Fe 10 or 15 layers
Au 10 layers
Fe / GaAs (001)
FMR characterization:
damping = 0.004
and anisotropies
(G. Woltersdorf, B. Heinrich)
Kerr hysteresis loopHC = 12 Oe
Images of Fe / GaAs
SEMPA images of M(SEM with Polarization Analysis)one magnetic field pulse10 ML Fe / GaAs (001)
50 m
50 m50 m
M0
Thermal Stability
Important aspect in recording media
Néel-Brown model (uniform rotation)
Probability that grainhas not switched:
with and
for long-term stability:
/e)( ttP
kTVuK /
e0 s10 100
years10
Comparison of Patterns
Observed (SEMPA)
Calculated (fit using LLG)
Anisitropies same as FMR
Damping = 0.017
4x larger than FMR
WHY?100 m
0 1 2 3 40
2
4
6
E/K
u
Number of precessions
10 ML Fe 15 ML Fe
Energy Dissipation
After field pulse:
Damping causes dissipation of energy during precession
(energy barrier for switching: KU)
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
1º in FMR, but 110º in our experiment
)01.0(sin
sin2
2
Effective Field H
3 components of H act on M
HD = -MS
demagnetizing field
HK = 2K/0MS
crystalline anisotropy
HE
externally applied field
MHE
HD
HK
Magnetic Field Strength
1010 electrons:B * r =50 Tesla * m
duration of magnetic field pulse: 5 ps
Perpendicular Magnetization
perpendicular anisotropy
M0=(0, 0, -MS)
5 ps field pulse2.6 Tesla
precession and relaxation towards (0, 0, +MS)
00
0
Time evolution
Granular CoCrPt Sample
Size of grains 8.5 nm
Paramag. envelope 1 nm
1 bit 100 grains
TEM of magnetic grains
Radial Dependence of M
Perpendicular magnetized sample (CoCrPt alloy)
0 20 40 60 80 100
-1
0
1
1 Pulse 2 Pulses 3 Pulses 4 Pulses 5 Pulses 6 Pulses 7 Pulses
M
agne
tizat
ion
[a.u
.]
Distance from Center [m]
In-Plane Magnetization
switching by precession around demagnetizing field
after excitation by 5 ps field pulse0.27 Tesla(finished at *)
(uniaxial in-plane)
Time evolution of M
0
0
0
M0
Precessional Torque: MxH
in-plane magnetized sample: figure-8 pattern
circular in-plane magnetic field H
M
lines of constant (initial) torque
MxH
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)
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
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
Previously: Strong Coupling
Recommended