Transcript Magnetization switching without charge or spin currents J. Stöhr Sara Gamble and H.

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
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
Today’s Switching Process
190 years of “Oersted switching”….
switching time limited by field strength H and spin-lattice relaxation time ~100 ps
Fastest H (B) Field Switching = Ballistic Switching
Patent issued December 21, 2000: R. Allenspach, Ch. Back and H. C. Siegmann
M
end of
field pulse
Relaxation into easy axis is governed
by spin-lattice relaxation
- but process is deterministic !
Precise timing for a=180o reduces time
Beyond direct switching
by magnetic fields
---how about electric fields ?
Stöhr et al., Appl. Phys. Lett. 94, 072504 (2009)
Magnetic Field is a time-odd “axial vector”
Magnetic field has same symmetry properties as magnetization
- can switch magnetization -
Electric Field is a time-even “polar vector”
Electric field cannot directly switch magnetization
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
The concept of the magnetic anisotropy field
creates “direction”
HE =
2KE
M
after some time ……~ 100 ps M realigns along HE
but…rotation of M limited to < 90o
cos 
Cannot switch through rotation of M into HE
Cannot rotate past 90o – cannot “switch”
Use Concept of Ballistic Switching – pulsed fields
ballistic switching with H field pulse of length t < 100 ps
This concept works with E fields, too !
Comparison of H and E field ballistic switching
Imagine that E field can create H E fast
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 ( t < 100 ps) before M aligns with H E
Process is completely determined by “write pulse” length t
not by precession and damping time which may be slower
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
Magnetic writing with SLAC Linac beam
thin Co film on Si wafer
premagnetized
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
Experiment with ultrastrong fields
electric field strength is up to 20 GV / m (2 V / Angstrom)
Magnetic pattern is severely distorted
--- does not follow circular B-field symmetry
Calculation of pattern with Landau-Lifshitz-Gilbert theory
known magnetic properties of film, known length, strength, radial dependence of fields
B-field only
B-field and E-field
Magneto-electronic anisotropy is strong ~ E 2
352 or about 1000 times stronger than with previous 5 ps pulses
B-field torque
E-field torque
Use photon pulse instead of e-beam pulse
B field cancels, E 2 field does not cancel
E field only switching should be possible with THz photons
The End