Magnetization switching without charge or spin currents - PowerPoint PPT Presentation

Magnetization switching

without charge or spin currents

J. Stöhr

Sara Gamble andH. 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

Relaxation into easy axis is governed

by spin-lattice relaxation

- but process is deterministic !

Precise timing for a=180oreduces time

end of

field pulse

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”

2KE

cos

HE =

M

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

but…rotation ofMlimited to < 90o

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 lengtht < 100 ps

This concept works with E fields, too !

Comparison of Hand Efield ballistic switching

Imagine that E field can create HE fast

So what does it take to switch with E-field ?

• Strong enough E field to induce dominant anisotropy axis and field HE
• Efield should be at angle ~ 45o to original easy axis
• Field pulse has to be fast ( t < 100 ps) before M aligns with HE

Process is completely determined by “write pulse” length t

not by precession and damping time which may be slower

Two potential methods

• Create new transient anisostropy axis in suitable multiferroic
• by E field pulse – optimum angle 45o
• Use strong E field pulse to distort atomic valence charge
• in any material

E

second order Stark effect ~ E2

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. StöhrandH. 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 ~ E2

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, E2 field does not cancel

E field only switching should be possible with THz photons

The End