magnetization switching without charge or spin currents j. stöhr sara gamble and h. c. siegmann,...
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![Page 1: Magnetization switching without charge or spin currents J. Stöhr Sara Gamble and H. C. Siegmann, SLAC, Stanford A. Kashuba Bogolyubov Institute for Theoretical](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649d0b5503460f949df6ab/html5/thumbnails/1.jpg)
Magnetization switching without charge or spin
currents
J. Stöhr Sara Gamble and H. C. Siegmann,
SLAC, Stanford
A. Kashuba Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine
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Switching with charge or spin currents
• Conventional H field pulses created by current flow through wires
• Time and amplitude limited by inductance laws and Joule heat in wires
• Switching with spin polarized currents has same problem
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switching time limited by field strength H and spin-lattice relaxation time ~100 ps
Today’s Switching Process 190 years of “Oersted switching”….
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end of field pulse
M
Fastest H (B) Field Switching = Ballistic SwitchingPatent issued December 21, 2000: R. Allenspach, Ch. Back and H. C. Siegmann
Relaxation into easy axis is governedby spin-lattice relaxation- but process is deterministic !
Precise timing for =180o reduces time
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Beyond direct switching
by magnetic fields
----
how about electric fields ?
Stöhr et al., Appl. Phys. Lett. 94, 072504 (2009)
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Magnetic field has same symmetry properties as magnetization - can switch magnetization -
Electric Field is a time-even “polar vector”
Magnetic Field is a time-odd “axial vector”
Electric field cannot directly switch magnetization
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E-fields can produce magnetic anisotropy axis
magnetocrystalline anisotropy caused by anisotropic atomic positions “bonding fields” distort valence charge, create axis
Ambiguity remains with respect to direction of M
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The concept of the magnetic anisotropy field creates “direction”
HE =2KE
Mcos
but…rotation of M limited to < 90o
after some time ……~ 100 ps M realigns along HE
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Cannot rotate past 90o – cannot “switch”
Cannot switch through rotation of M into HE
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Use Concept of Ballistic Switching – pulsed fields
ballistic switching with H field pulse of length100 ps
This concept works with E fields, too !
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Comparison of H and E field ballistic switching
Imagine that E field can create H E fast
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So what does it take to switch with E-field ?
• Strong enough E field to induce dominant anisotropy axis and field H E
• E field should be at angle ~ 45o to original easy axis
• Field pulse has to be fast ( < 100 ps) before M aligns with H E
Process is completely determined by “write pulse” lengthnot by precession and damping time which may be slower
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Two potential methods
1. Create new transient anisostropy axis in suitable multiferroic by E field pulse – optimum angle 45o
2. Use strong E field pulse to distort atomic valence charge in any material
E
second order Stark effect ~ E 2
Field strength needs to be > 1 Volt / nm comparable to valence potential
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thin Co film on Si wafer premagnetized
Magnetic writing with SLAC Linac beam
100 fs - 5 ps
1nC or 1010 electrons
J. Stöhr and H. C. Siegmann Magnetism: From Fundamentals to Nanoscale Dynamics Springer Series in Solid State Sciences 152
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Experiment with ultrastrong fields
electric field strength is up to 20 GV / m (2 V / Angstrom)
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Magnetic pattern is severely distorted --- does not follow circular B-field symmetry
Calculation of pattern with Landau-Lifshitz-Gilbert theoryknown magnetic properties of film, known length, strength, radial dependence of fields
B-field only
B-field and E-field
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B-field torque E-field torque
Magneto-electronic anisotropy is strong ~ E 2
352 or about 1000 times stronger than with previous 5 ps pulses
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B field cancels, E 2 field does not cancel
Use photon pulse instead of e-beam pulse
E field only switching should be possible with THz photons
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The End