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Magnetization dynamics with picosecond magnetic field pulses

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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

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

Dynamic equation for M

Landau-Lifshitz-Gilbert

change in angular momentum

Precession torque

Gilbert damping torque

Direction of torques

Motion of M for constant H

After Magnetic Field Pulse

perpendicularmagnetization

CoCrPt

granular media

Image of M:

Polar Kerr Microscopy

(size 150 mm)

50 mm

Transition Region

Observed: wide transition region

Calculated: sharp transitions

Model assuming distribution of initial direction for M

Initial Distributions of M

- Static:angle of anisotropy axes x-ray diffraction: q±4º
- Dynamic:thermal motion, random fields

q10º

V=(6.5 nm)3

Look identical at one point in time

Differences appear with multiple pulses

2 Field Pulses

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

50 mm

Stochastic Switching

Independent stochastic events:

calculate switching by successive multiplication

M2 = M1 · M1

M3 = M2 · M1

:

Mn = (M1)n

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

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

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 and

for long-term stability:

Comparison of Patterns

Observed (SEMPA)

Calculated (fit using LLG)

Anisitropies same as FMR

Damping a = 0.017

4x larger than FMR

WHY?

100 mm

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

q1º in FMR, but q 110º in our experiment

Effective Field H

3 components of H act on M

HEexternally applied field

HD = -MS

demagnetizing field

M

HE

HK = 2K/m0MS

crystalline anisotropy

HK

HD

Perpendicular Magnetization

Time evolution

perpendicular anisotropy

M0=(0, 0, -MS)

5 ps field pulse2.6 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

Perpendicular magnetized sample (CoCrPt alloy)

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)

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

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

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

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