Exploring Ultrafast Excitations in Solids with Pulsed e-beams

Transcription

1 Exploring Ultrafast Excitations in Solids with Pulsed e-beams Joachim Stöhr and Hans Siegmann Stanford Synchrotron Radiation Laboratory Collaborators: Y. Acremann, Sara Gamble, Mark Burkhardt ( SLAC/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 bits

3 Faster than 100 ps. Mechanisms of ultrafast transfer of energy and angular momentum Electric fields τ ~ 1 ps Shockwave Optical pulse Electron pulse Electrons Phonons IR or THz pulse τ =? τ ~ 100 ps Spin Magnetic fields

4 The simplest case: precessional magnetic switching M Conventional (Oersted) switching

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

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

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

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

9 Origin of observed switching pattern 15 layers of Fe/GaAs(110) H increases Beam center 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

10 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

11 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)

12 Results with ultrafast magnetic field pulses Compare 5ps pulse with 160fs bunch same charge 1.6x10 10 e - ~ 20 µm 160 fs 5 ps Magnetic pattern with 5 ps electron bunch Tudosa et al. Nature 428, 831 (2004)

13 New results with a 10 times shorter ( 167 fs ) pulse Magnetic pattern of 10 nm Fe film with 160 fs bunch length Pattern size 470 µm by 980 µm Pattern is severely asymmetric should follow circles No switching for certain magnetic field orientations! Violates conventional laws of angle-dependence of magnetic torque Puzzle is unresolved at present..

14 Magnetic flash images with different bunch lengths slow pulse fast pulse 5.5 ps, 1.6x10 10 e fs, 1.6x10 10 e - characteristic demagnetization domains ablation magnetic topographical

15 Surprising result: No beam damage at ultrafast time scales Tpographical image of beam impact area reveals no damage! Magnetic pattern indicates no heating! Maximum power density is 4 x W / cm 2 Sub-picosecond energy dissipation must exist (photons, electrons) Dissipation faster than electron-phonon relaxation time (ps) Results of importance for LCLS, as well.

16 From Ferro-magnetic to Ferro-electric Materials FM materials respond to magnetic field (axial vector) - broken time reversal symmetry of electronic system FE materials respond to electric field (polar vector) - broken inversion symmetry of lattice Example: PbTiO 3 Materials important as storage media but how fast do they switch?

17 Ferro-electric domains can be observed with x-rays E E PEEM Reverses with change in polarization direction. Dichroism

18 Use E-field of SLAC beam to switch In lab (Auston switch): ~70 ps with the E-field amplitude of 25 MV/m SLAC Linac: femtosecond pulses up to several GV/m

19 Pump-Probe Dynamics with SLAC-pulses

20 Summary The breakdown of the macrospin approximation for fast field pulses limits the reliability of magnetic switching At ultrafast speeds (< 1 ps) new ill-understood phenomena exist one approaches timescales of fundamental interactions between electrons, lattice and spin Future experiments to explore the details using both H and E fields In the future, e-beam pump / laser probe experiments are of interest, as well For more, see: and J. Stöhr and H. C. Siegmann Magnetism: From Fundamentals to Nanoscale Dynamics 800+ page textbook ( Springer, 2006 )