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Switching with Ultrafast Magnetic Field Pulses

Switching with Ultrafast Magnetic Field Pulses. Ioan Tudosa. Outline. Motivation Experiments with in-plane samples Damping Anisotropy (induced by electric field) Future ideas Terahertz radiation switching Wish List. Motivation: Understanding Ultrafast Physics.

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Switching with Ultrafast Magnetic Field Pulses

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  1. Switching with Ultrafast Magnetic Field Pulses Ioan Tudosa

  2. Outline • Motivation • Experiments with in-plane samples • Damping • Anisotropy (induced by electric field) • Future ideas • Terahertz radiation switching • Wish List SLAC 2011

  3. Motivation: Understanding Ultrafast Physics Basic Question: Is there any new physics to be found in exploring the fundamental limits of fast magnetization dynamics? Current work : Extremely strong electromagnetic field SLAC 2011

  4. Experiment principle SLAC 2011 4

  5. Experimental Set Up Long and Short Pulse Field Strengths WE HAVE PEAK FIELD VALUES OF 60 TESLA AND 20 GV/m ! Short We use two pulse lengths: Long pulse τ = 2.3*10-12 sec Short pulse τ = 70*10-15 sec Long SLAC 2011

  6. Comparison of Field Magnitudes 60 T Magnetic Field! • Hard disk write head: 1-2 T • Superconducting magnet: 15-20 T •SLAC experiment: 60 T 20 GV/m Electric Field! • AlGaAs/GaAs quantum wells: 106 V/m • Vacuum breakdown (millitorr): 107 V/m •SLAC experiment: 2*1010 V/m SLAC 2011

  7. Beamline setup manipulator chamber Electron bunches SLAC 2011 SLAC 2011 7

  8. 1.Sample Holder Wire scanners Samples Sample holder 1cm SLAC 2011

  9. Precession Torques Sample is uniformly magnetized initially Maximum torque Lines of constant torque T ~ MxH ~ sin(M,H) Minimum Torque SLAC 2011

  10. Fe/GaAs Thin Films Au 10 layers Fe 10 or 15 layers GaAs • Grown using MBE • Uniaxial in-plane anisotropy • Imaged with SEMPA 15 ML Fe 10 ML Fe M0 100 mm SLAC 2011

  11. Precessional Magnetization Reversal 3 Step Process Final Alignment Field Pulse Kick Rotations Around Hdemag SLAC 2011

  12. Dynamics of Magnetization Damping dissipates the energy pumped into the system  circle widths SLAC 2011

  13. Damping of Magnetic Energy Experiment Calculation (LLG+magnons) FMR damping Inset: Reduction of magnetization due to magnon scattering SLAC 2011

  14. Experiment with ultrastrong fields Sample Composition: MgO/30nm Cr80Mo20/10nm Co70Fe30/1.5 nm Pt Imaging: SEM with Polarization Analysis Magnetism and topography electric field strength is up to 20 GV / m (2 V / Angstrom) SLAC 2011

  15. Magneto-electronic anisotropy is strong ~ E2 1000 times stronger B-field torque E-field torque SLAC 2011

  16. Manipulating Magnetic Anisotropy Method 1: Move Atoms ●New ATOMIC arrangement gives new axis ●True magnetocrystalline anisotropy alteration Method 2: Move Electrons ● New ELECTRONIC arrangement gives new axis ● New magneto-electric anisotropy alteration ● Need ~ fs to move electrons ● Uses spin-orbit coupling Gamble S. J. et al. - PRL, vol 102, 217201, (2009) SLAC 2011

  17. Experiment vs simulation Simulation • Takes into account: • Increased damping near the center • Additional E-field anisotropy SLAC 2011

  18. Topographic contrast data 2.3 Picosecond Exposure •Sample has heated to at least Tc = 1200 K in a 100 μm radius •Sample is visibly damaged at the point of beam impact 70 Femtosecond Exposure • Sample shows no evidence of heating or ablation SLAC 2011

  19. Energy loss transfer What do we know? • The pulse will excite the electron gas • The electron gas will equilibrate with the phonon system in ~1 picosecond This means some energy from the excited electron gas will reach the phonon system DURING the picosecond pulse! What do we need to know? • How does all the other energy get out? SLAC 2011

  20. Transition Radiation Transition Radiation  a charged particle crosses a boundary ε1| ε2 Coherent Transition Radiation is emitted for λ>lbunch Bunch Electric Field Sample Response SLAC 2011

  21. Half cycle terahertz radiation • Coherent transition radiation at λ bunch length. • Highly compressed bunches: λ 10 to 100 µm • Corresponding frequencies: 3 to 30 THz • Intense pulses with the time structure of the electron beam • When focused: • Electric fields > 1 GV/m = 0.1 V/Å • Magnetic fields > 3 T • Well above other THz sources SLAC 2011

  22. E-202 • Explore the electric field effect • Get the time scale of beam damage • Ferromagnetic and ferroelectric samples • Expose to THz radiation outside the e-beam • Use magnetic media with Hc > 9T SLAC 2011

  23. Wish List • Beamtime during day for better support • Predictable schedule • Better access for changing samples • Bunch length diagnostics • Less radiation background (for electronics) • Tighter focus • One shot or 0.1Hz mode SLAC 2011

  24. Conclusion • SLAC still a useful, powerful EM pulse source • Electric field influences magnetization dynamics • Potential to direct the EM pulse and focus it • Applications to magnetic recording?? SLAC 2011

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