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 Zürich)
magnetic imaging
A. Kashuba (Landau Inst. Moscow) ; A. Dobin (Seagate) theory
D. Weller, G. Ju, B.Lu (Seagate Technologies)
G. Woltersdorf, B. Heinrich (S.F.U. Vancouver)
samples
The Technology Problem: Smaller and Faster
The ultrafast
technology
gap
want to reliably
switch small
magnetic “bits”
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
τ ~ 1 ps
Electrons
Shockwave
Phonons
τ= ?
τ ~ 100 ps
Spin
Most direct way:
Oersted switching
Precessional or ballistic switching
Exchange switching (spin injection)
IR or THz pulse
Precessional or ballistic switching:
Discovery
Creation of large, ultrafast magnetic fields
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Ultrafast pulse – use electron accelerator
C. H. Back et al., Science 285, 864 (1999)
Torques on in-plane magnetization by
beam field
Initial magnetization of sample
Max. torque
Min. torque
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)
The simplest case: perpendicular magnetic medium
M
End of
field pulse
Pattern of perpendicular anisotropy sample
CoCrPt perpendicular recording media (Seagate)
Multiple shot switching of perpendicular sample
CoCrPt recording film
Light areas mean M
Dark areas mean M
Tudosa et al., Nature 428, 831 (2004)
Data analysis
Intensity profiles thru images Landau-LifshitzExperiment
curve = M1
Gilbert
theory
1 = white
1 shot
0 = gray
Landau-LifshitzGilbert theory Experiment
-1 = dark
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
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
Rotation angles:
720o
180o
360o
Origin of observed switching pattern
15 layers of Fe/GaAs(110)
H increases
γ
In macrospin approximation, line positions depend on:
• angle γ = B τ
• in-plane anisotropy Ku
from FMR data
• out-of-plane anisotropy K┴
• LLG damping parameter α
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 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 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 )