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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:
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
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The ultrafasttechnology
gap
want to reliablyswitch smallmagnetic “bits”
The Technology Problem: Smaller and Faster
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186 years of “Oersted switching”….
How can we switch faster ?
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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
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Precessional or ballistic switching:
Discovery
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Creation of large, ultrafast magnetic fields
Conventional method
- too slow
Ultrafast pulse – use electron accelerator
C. H. Back et al., Science 285, 864 (1999)
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Torques on in-plane magnetization by beam field
Max. torque
Min. torque
Initial magnetization of sample
Fast switching occurs when H M┴
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Precessional or ballistic switching: 1999
Patent issued December 21, 2000: R. Allenspach, Ch. Back and H. C. Siegmann
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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)
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End of field pulse
M
The simplest case: perpendicular magnetic medium
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Pattern of perpendicular anisotropy sample CoCrPt perpendicular recording media (Seagate)
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Multiple shot switching of perpendicular sample
Dark areas mean M Light areas mean M
Tudosa et al., Nature 428, 831 (2004)
CoCrPt recording film
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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.
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Magnetization fracture under ultrafast field pulse excitation
Non-deterministic region partly due to fractured magnetization
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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)
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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)
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In-Plane Magnetization: Pattern development
• Magnetic field intensity is large
• Precisely known field size
540o
360o
180o
720o
Rotation angles:
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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)
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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
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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
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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 )