Transcript PPT - SLAC
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 The Technology Problem: Smaller and Faster The ultrafast technology gap want to reliably switch small “bits” Faster than 100 ps…. Mechanisms of ultrafast transfer of energy and angular momentum Electric fields t ~ 1 ps Electrons Shockwave Phonons Optical pulse Electron pulse t= ? t ~ 100 ps Spin Magnetic fields IR or THz pulse The simplest case: precessional magnetic switching M Conventional (Oersted) switching Creation of large, ultrafast magnetic fields Ultrafast pulse – use electron accelerator C. H. Back et al., Science 285, 864 (1999) Torques on in-plane magnetization by beam field Initial magnetization of sample Max. torque Min. torque Fast switching occurs when H ┴ M Precessional or ballistic switching: 1999 Patent issued December 21, 2000: R. Allenspach, Ch. Back and H. C. Siegmann In-Plane Magnetization: Pattern development • Magnetic field intensity is large • Precisely known field size 540o Rotation angles: 720o 180o 360o Origin of observed switching pattern 15 layers of Fe/GaAs(110) H increases Beam center g In macrospin approximation, line positions depend on: • angle g = B t • in-plane anisotropy Ku from FMR data • out-of-plane anisotropy K┴ • LLG damping parameter a 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 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) Results with ultrafast magnetic field pulses Compare 5ps pulse with 160fs bunch – same charge 1.6x1010 e- ~ 20 mm 160 fs – 5 ps Magnetic pattern with 5 ps electron bunch Tudosa et al. Nature 428, 831 (2004) 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….. Magnetic flash images with different bunch lengths slow pulse 5.5 ps, 1.6x1010 e- fast pulse 160 fs, 1.6x1010 e- characteristic demagnetization domains ablation magnetic topographical 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 1019 W / cm2 • Sub-picosecond energy dissipation must exist (photons, electrons) • Dissipation faster than electron-phonon relaxation time (ps) • Results of importance for LCLS, as well. 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: PbTiO3 Materials important as storage media – but how fast do they switch? Ferro-electric domains can be observed with x-rays E E Reverses with change in polarization direction. Dichroism PEEM 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 Pump-Probe Dynamics with SLAC-pulses 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: http://www-ssrl.slac.stanford.edu/stohr and J. Stöhr and H. C. Siegmann Magnetism: From Fundamentals to Nanoscale Dynamics 800+ page textbook ( Springer, 2006 )