On the Ultimate Speed of Magnetic Switching
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On the Ultimate Speed of Magnetic Switching Joachim Stöhr Stanford Synchrotron Radiation Laboratory. Collaborators:. H. C. Siegmann, C. Stamm, I. Tudosa, Y. Acremann ( Stanford ). A. Vaterlaus (ETH Z. ü. rich). magnetic imaging. A. Kashuba (Landau Inst. Moscow) ; A. Dobin (Seagate).

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H c siegmann c stamm i tudosa y acremann stanford

On the Ultimate Speed of Magnetic Switching

Joachim Stöhr

Stanford Synchrotron Radiation Laboratory

Collaborators:

H. C. Siegmann, C. Stamm, I. Tudosa, Y. Acremann ( Stanford )

A. Vaterlaus (ETH Z

ü

rich)

magnetic imaging

A. Kashuba (Landau Inst. Moscow) ; A. Dobin (Seagate)

theory

D. Weller, G. Ju, B.Lu (Seagate Technologies)

samples

G. Woltersdorf, B. Heinrich (S.F.U. Vancouver)


H c siegmann c stamm i tudosa y acremann stanford

The Technology Problem: Smaller and Faster

The ultrafast

technology

gap

want to reliably

switch small

magnetic “bits”


H c siegmann c stamm i tudosa y acremann stanford

186 years of “Oersted switching”….

How can we switch faster ?


H c siegmann c stamm i tudosa y acremann stanford

Faster than 100 ps….

Mechanisms of ultrafast transfer of energy and angular momentum

Optical pulse

Shockwave

IR or THz pulse

t~ 1 ps

Electrons

Phonons

t= ?

t ~ 100 ps

Spin

Most direct way:

Oersted switching

Precessional or ballistic switching

Exchange switching (spin injection)



H c siegmann c stamm i tudosa y acremann stanford

Creation of large, ultrafast magnetic fields

Conventional method

- too slow

Ultrafast pulse – use electron accelerator

C. H. Back et al., Science285, 864 (1999)


H c siegmann c stamm i tudosa y acremann stanford

Torques on in-plane magnetization by beam field

Initial magnetization of sample

Max. torque

Min. torque

Fast switching occurs when HM


H c siegmann c stamm i tudosa y acremann stanford

Precessional or ballistic switching: 1999

Patent issued December 21, 2000: R. Allenspach, Ch. Back and H. C. Siegmann


H c siegmann c stamm i tudosa y acremann stanford

Precessional switching case 1:

Perpendicular anisotropy sample

I. Tudosa, C. Stamm, A.B. Kashuba, F. King, H.C. Siegmann,

J. Stöhr, G. Ju, B. Lu, and D. Weller

Nature 428, 831 (2004)


H c siegmann c stamm i tudosa y acremann stanford

M

End of field pulse

The simplest case: perpendicular magnetic medium


H c siegmann c stamm i tudosa y acremann stanford

Pattern of perpendicular anisotropy sample

CoCrPt perpendicular recording media (Seagate)


H c siegmann c stamm i tudosa y acremann stanford

Multiple shot switching of perpendicular sample

CoCrPt recording film

Light areas mean M

Dark areas mean M

Tudosa et al., Nature 428, 831 (2004)


H c siegmann c stamm i tudosa y acremann stanford

Data analysis

Intensity profiles thru images

Landau-Lifshitz-

Gilbert theory

Experiment

curve = M1

1 = white

1 shot

0 = gray

Landau-Lifshitz-

Gilbert theory

-1 = dark

Experiment

curve = (M1)2

curve = (M1)3

3 shots

2 shots

curve = (M1)4

curve = (M1)5

5 shots

4 shots

curve = (M1)6

curve = (M1)7

6 shots

7 shots

Multiplicative probabilities are signature of a random variable.

Analysis reveals a memory-less process.


Magnetization fracture under ultrafast field pulse excitation
Magnetization fracture under ultrafast field pulse excitation

Non-deterministic region partly due to fractured magnetization


Magnetization fracture under ultrafast field pulse excitation1
Magnetization fracture under ultrafast field pulse excitation

Macro-spin approximation

uniform precession

Magnetization fracture

moment de-phasing

Breakdown of the macro-spin approximation

Tudosa et al., Nature 428, 831 (2004)


H c siegmann c stamm i tudosa y acremann stanford

Precessional switching excitationcase 2:

In-plane anisotropy sample

C. Stamm, I. Tudosa, H.C. Siegmann, J. J. Stöhr, A. Yu. Dobin,

G. Woltersdorf, B. Heinrich and A. Vaterlaus

Phys. Rev. Lett. 94, 197603 (2005)


In plane magnetization pattern development
In-Plane Magnetization: Pattern development excitation

  • Magnetic field intensity is large

  • Precisely known field size

Rotation angles:

540o

180o

720o

360o


H c siegmann c stamm i tudosa y acremann stanford

Origin of observed switching pattern excitation

15 layers of Fe/GaAs(110)

H increases

g

  • In macrospin approximation, line positions depend on:

  • angle g = B t

  • in-plane anisotropy Ku

  • out-of-plane anisotropy K┴

  • LLG damping parameter a

from FMR data


H c siegmann c stamm i tudosa y acremann stanford

Breakdown of the Macrospin Approximation excitation

H increases

With increasing field, deposited energy far exceeds macrospin approximation

this energy is due to increased dissipation or spin wave excitation


H c siegmann c stamm i tudosa y acremann stanford

Breakdown of the macrospin approximation: why? excitation

Experiments reveal breakdown for short pulse lengthtand large B

peak power deposition ~ B2 / t = a B / t2

Breakdown appears to be caused by peak power induced

fracture of magnetization – non-linear excitations of spin system


H c siegmann c stamm i tudosa y acremann stanford

Conclusions excitation

  • The breakdown of the macrospin approximation for

  • fast field pulses limits the reliability of magnetic switching

  • Breakdown is believed to arise from energy & angular momentum

  • transfer within the spin system – excitation of higher spin wave modes

  • Details are not well understood….

For more, see:http://www-ssrl.slac.stanford.edu/stohr

and

J. StöhrandH. C. Siegmann

Magnetism: From Fundamentals to Nanoscale Dynamics

800+ page textbook ( Springer, to be published in Spring 2006 )