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

theory

magnetic imaging

The ultrafast technology gap

want to reliably switch small

magnetic “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

~ 1 ps

= ? ~ 100 ps

Precessional or ballistic switching:

Discovery

Creation of large, ultrafast magnetic fields

Conventional method

- t

oo 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 theoryExperiment

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 analysis

Landau-Lifshitz-Gilbert theoryExperiment

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 approximation uniform precession

Magnetization fracture moment 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 = B • in-plane anisotropy Ku

• out-of-plane anisotropy K┴

• LLG damping parameter

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 approximation this energy is due to increased dissipation or spin wave excitation

Breakdown of the macrospin approximation: why?

Experiments reveal breakdown for short pulse length and large B

peak power deposition ~ B2 / = B / 2

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 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. Stöhr and H. C. Siegmann Magnetism: From Fundamentals to Nanoscale Dynamics 800+ page textbook ( Springer, to be published in Spring 2006 )