Switching with Ultrafast Magnetic Field Pulses Ioan Tudosa

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

SLAC 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 Motivation: Understanding Ultrafast Physics

SLAC Experiment principle 4

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

SLAC Comparison of Field Magnitudes 60 T Magnetic Field! Hard disk write head: 1-2 T Superconducting magnet: T SLAC experiment: 60 T 20 GV/m Electric Field! AlGaAs/GaAs quantum wells : 10 6 V/m Vacuum breakdown (millitorr): 10 7 V/m SLAC experiment: 2*10 10 V/m

SLAC Beamline setup SLAC manipulator chamber Electron bunches

SLAC Sample Holder 1cm Wire scannersSamples Sample holder

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

SLAC Fe/GaAs Thin Films 100 m M0M0 GaAs Fe 10 or 15 layers Au 10 layers 15 ML Fe10 ML Fe  Grown using MBE  Uniaxial in-plane anisotropy  Imaged with SEMPA

SLAC Precessional Magnetization Reversal 3 Step Process Field Pulse KickRotations Around H demag Final Alignment

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

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

SLAC Experiment with ultrastrong fields electric field strength is up to 20 GV / m (2 V / Angstrom) Sample Composition: MgO/30nm Cr 80 Mo 20 /10nm Co 70 Fe 30 /1.5 nm Pt Imaging: SEM with Polarization Analysis Magnetism and topography

SLAC Magneto-electronic anisotropy is strong ~ E 2 B-field torqueE-field torque 1000 times stronger

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

SLAC Experiment vs simulation Simulation Takes into account:  Increased damping near the center  Additional E-field anisotropy

SLAC Topographic contrast data 2.3 Picosecond Exposure Sample has heated to at least T c = 1200 K in a 100 μ m radiusSample is visibly damaged at the point of beam impact 70 Femtosecond Exposure Sample shows no evidence of heating or ablation

SLAC 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? Energy loss transfer

SLAC Transition Radiation  a charged particle crosses a boundary ε 1 | ε 2 Coherent Transition Radiation  is emitted for λ>l bunch Bunch Electric FieldSample Response Transition Radiation

SLAC 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 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 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 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??