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

switching time limited by field strength H and spin-lattice relaxation time ~100 ps

Today’s Switching Process 190 years of “Oersted switching”….

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

Beyond direct switching

by magnetic fields

----

how about electric fields ?

Stöhr et al., Appl. Phys. Lett. 94, 072504 (2009)

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

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

Mcos

but…rotation of M limited to < 90o

after some time ……~ 100 ps M realigns along HE

Cannot rotate past 90o – cannot “switch”

Cannot switch through rotation of M into HE

Use Concept of Ballistic Switching – pulsed fields

ballistic switching with H field pulse of length100 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 ( < 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

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

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

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 theoryknown magnetic properties of film, known length, strength, radial dependence of fields

B-field only

B-field and E-field

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

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

The End