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