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

On the Ultimate Speed of Magnetic Switching

Joachim Stöhr

Stanford Synchrotron Radiation Laboratory

Collaborators:

A. Vaterlaus (ETH Zürich) A. Kashuba (Landau Inst. Moscow) ; A. Dobin (Seagate)

D. Weller, G. Ju, B.Lu (Seagate Technologies)G. Woltersdorf, B. Heinrich (S.F.U. Vancouver) samples

theorymagnetic imaging

The ultrafasttechnology

gap

want to reliablyswitch smallmagnetic “bits”

The Technology Problem: Smaller and Faster

186 years of “Oersted switching”….

How can we switch faster ?

Faster than 100 ps….Mechanisms of ultrafast transfer of energy and angular momentum

Optical pulse

Most direct way:Oersted switching

Precessional or ballistic switchingExchange switching (spin injection)

Shockwave

IR or THz pulseElectrons Phonons

Spin

τ ~ 1 ps

τ = ? τ ~ 100 ps

Precessional or ballistic switching:

Discovery

Creation of large, ultrafast magnetic fields

Conventional method

- too slow

Ultrafast pulse – use electron accelerator

C. H. Back et al., Science 285, 864 (1999)

Torques on in-plane magnetization by beam field

Max. torque

Min. torque

Initial magnetization of sample

Fast switching occurs when H M┴

Precessional or ballistic switching: 1999

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

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)

End of field pulse

M

The simplest case: perpendicular magnetic medium

Pattern of perpendicular anisotropy sample CoCrPt perpendicular recording media (Seagate)

Multiple shot switching of perpendicular sample

Dark areas mean M Light areas mean M

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

CoCrPt recording film

Landau-Lifshitz-Gilbert theory Experiment

1 = white

0 = gray

-1 = dark

Intensity profiles thru images

1 shot

2 shots

6 shots 7 shots

curve = M1

curve = (M1)3

curve = (M1)7

curve = (M1)5

curve = (M1)2

curve = (M1)4

curve = (M1)6

Data analysisLandau-Lifshitz-Gilbert theory Experiment

3 shots

5 shots4 shots

Multiplicative probabilities are signature of a random variable.Analysis reveals a memory-less process.

Magnetization fracture under ultrafast field pulse excitation

Non-deterministic region partly due to fractured magnetization

Magnetization fracture under ultrafast field pulse excitation

Macro-spin approximationuniform precession

Magnetization fracturemoment de-phasing

Breakdown of the macro-spin approximation

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

Precessional switching case 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

• Magnetic field intensity is large

• Precisely known field size

540o

360o

180o

720o

Rotation angles:

Origin of observed switching pattern

In macrospin approximation, line positions depend on:• angle γ = B τ• in-plane anisotropy Ku• out-of-plane anisotropy K┴• LLG damping parameter α

γ

from FMR data

H increases15 layers of Fe/GaAs(110)

Breakdown of the Macrospin Approximation H increases

With increasing field, deposited energy far exceeds macrospin approximationthis energy is due to increased dissipation or spin wave excitation

Breakdown of the macrospin approximation: why?

Experiments reveal breakdown for short pulse length τ and large B

peak power deposition ~ B2 / τ = α B / τ2

Breakdown appears to be caused by peak power induced fracture of magnetization – non-linear excitations of spin system

Conclusions

• The breakdown of the macrospin approximation forfast field pulses limits the reliability of magnetic switching

• Breakdown is believed to arise from energy & angular momentumtransfer 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. Stöhr and H. C. Siegmann Magnetism: From Fundamentals to Nanoscale Dynamics800+ page textbook ( Springer, to be published in Spring 2006 )