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 switching Exchange switching (spin injection) Shockwave IR or THz pulse ElectronsPhonons Spin  ~ 1 ps  = ?  ~ 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 = M 1 curve = (M 1 ) 3 curve = (M 1 ) 7 curve = (M 1 ) 5 curve = (M 1 ) 2 curve = (M 1 ) 4 curve = (M 1 ) 6 Data analysis Landau-Lifshitz- Gilbert theory Experiment 3 shots 5 shots 4 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, (2005)

In-Plane Magnetization: Pattern development Magnetic field intensity is large Precisely known field size 540 o 360 o 180 o 720 o Rotation angles:

Origin of observed switching pattern In macrospin approximation, line positions depend on: angle  = B  in-plane anisotropy K u out-of-plane anisotropy K ┴ LLG damping parameter   from FMR data H increases 15 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 ~ B 2 /  =  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: and J. Stöhr and H. C. Siegmann Magnetism: From Fundamentals to Nanoscale Dynamics 800+ page textbook ( Springer, to be published in Spring 2006 )