h. c. siegmann, c. stamm, i. tudosa, y. acremann ( stanford ) on the ultimate speed of magnetic...
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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
theory
magnetic imaging
The ultrafast technology gap
want to reliably switch small
magnetic “bits”
The Technology Problem: Smaller and 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
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)
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 )