On the Ultimate Speed of Magnetic Switching

Transcription

1 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

2 The Technology Problem: Smaller and Faster The ultrafast technology gap want to reliably switch small magnetic bits

3 186 years of Oersted switching. How can we switch faster?

4 Faster than 100 ps. Mechanisms of ultrafast transfer of energy and angular momentum Optical pulse Electrons τ ~ 1 ps Phonons Shockwave IR or THz pulse τ =? τ ~ 100 ps Spin Most direct way: Oersted switching Precessional or ballistic switching Exchange switching (spin injection)

5 Precessional or ballistic switching: Discovery

6 Creation of large, ultrafast magnetic fields Conventional method - too slow Ultrafast pulse use electron accelerator C. H. Back et al., Science 285, 864 (1999)

7 Torques on in-plane magnetization by beam field Initial magnetization of sample Max. torque Min. torque Fast switching occurs when H M

8 Precessional or ballistic switching: 1999 Patent issued December 21, 2000: R. Allenspach, Ch. Back and H. C. Siegmann

9 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)

10 The simplest case: perpendicular magnetic medium M End of field pulse

11 Pattern of perpendicular anisotropy sample CoCrPt perpendicular recording media (Seagate)

12 Multiple shot switching of perpendicular sample CoCrPt recording film Light areas mean M Dark areas mean M Tudosa et al., Nature 428, 831 (2004)

13 Data analysis Intensity profiles thru images curve = M 1 1 = white Landau-Lifshitz- Gilbert theory Experiment 0 = gray 1 shot Landau-Lifshitz- Gilbert theory Experiment -1 = dark curve = (M 1 ) 2 curve = (M 1 ) 3 2 shots 3 shots curve = (M 1 ) 4 curve = (M 1 ) 5 4 shots 5 shots curve = (M 1 ) 6 curve = (M 1 ) 7 6 shots 7 shots Multiplicative probabilities are signature of a random variable. Analysis reveals a memory-less process.

14 Magnetization fracture under ultrafast field pulse excitation Non-deterministic region partly due to fractured magnetization

15 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)

16 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)

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

18 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 K u out-of-plane anisotropy K LLG damping parameter α from FMR data

19 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

20 Breakdown of the macrospin approximation: why? Experiments reveal breakdown for short pulse length τ and large B peak power deposition ~ B 2 / τ = α B / τ 2 Breakdown appears to be caused by peak power induced fracture of magnetization non-linear excitations of spin system

21 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 )