ultrafast manipulation of the magnetization j. stöhr sara gamble and h. c. siegmann, slac, stanford...
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Ultrafast Manipulation of the Magnetization
J. Stöhr Sara Gamble and H. C. Siegmann,
SLAC, Stanford
A. Kashuba Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine
from “reading” to “writing” information
?
The big questions:
• What are possible switching methods?
• What are the physical processes (and intermediate states)?
• What limits the speed of switching?
“0” “1”
exchange orspin-orbit
Conceptual methods of magnetization switching
Optical pulse Lattice shock
Electrons Phonons
Spin
~ 1 ps
~ 100 ps
electrons move in femtoseconds
atoms move in picoseconds
?
field pulses or
spin currents into magnetic element
Creation of large, ultrafast magnetic fields
Conventional method
- t
oo slow
Use field pulse created by a moving electron bunch
Origin of the fast switching idea…What is the pattern written by a lightning bolt in magnetic rock?
100 kA in a flash of a few microseconds
Magnetization follows the field lines
thin Co film on Si wafer premagnetized
Magnetic writing with SLAC Linac beam
C. H. Back et al., Science 285, 864 (1999)
5 ps
1nC or 1010 electrons
In-Plane Magnetization: Pattern development
• Magnetic field intensity is large
• Precisely known field size
540o
360o
180o
720o
Rotation angles:
no circles around beam !
very different from lightningpattern
The pattern written by a picosecond beam field
Max. torque T = M x H
Min. torque = 0
initial magnetizationdirection of sample
Fast switching occurs when H M┴
beamdamage
M
end of field pulse
M
Ballistic Switching – From nano to picosecondsPatent issued December 21, 2000: R. Allenspach, Ch. Back and H. C. Siegmann
Relaxation into “down” direction governedby slow spin-lattice relaxation (100 ps)- but process is deterministic !
Precise timing for =180o reduces time
Toward femtosecond switching Experiments with sub-ps bunches
• reduce bunch length from FWHM5 ps → 140 fs
• keep beam energy and charge fixed (~1010 electrons or 1 nC)
• fields B ~ charge / and E = c B are increased by factor of 35
• our fields have unprecedented strength in materials science:
B-field: 60 Tesla
E-field: 20 GV/m or 2V / Angstrom
How does a relativistic e-beam interact with a material ?
note E and B fields are definedwithin and outside e-bunch
10 nm Co70Fe30
on MgO (110)
Magnetic pattern is severely distorted for short bunch
140 fs
5 ps
damaged area
15 layers Feon GaAs (110)
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
Consider effect of giant E-field of beam
Beam field E ~ 1010 V / m = 1 V / Å comparable to “bonding fields” leads to ultrafast distortion of valence charge - all electronic dynamic effect
all new “magnetoelectronic anisotropy” – ultrafast !
magnetocrystalline anisotropy caused by anisotropic atomic positions “bonding fields” distort valence charge – static effect
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
Practical Realization of E-field switching
• Giant accelerator is impractical
• Want to produce pure E-field effect – no B-field effect
• Field pulse needs to be fast
How about photons ? We know effect is ~E2 Linear B-field effect cancels over a full cycle
SLAC e-beam pulse corresponds to THz half-cycle pulse
red: SLAC pulse
black: THz half cycle pulsetrue “EM wave”
100 fs 10 THz
Can sample handle intense THz pulse ?
Heating of sample would be problem….
• Need strong THz radiation - not readily available
• Presently only produced by accelerators
• Laser generated THz about 100 times weaker at present
Pulse length: 4 ps
Pulse length: 140 fs
Compare beam impact region for different pulse lengths
Magnetic image Topological image by means of SEMPA microscopy
same sample: 10 nm Co70Fe30 on MgO (110)
beam damage
35 times shorter pulse & stronger fields cause
no heating, no damage !
If there is an E-field - why is there no heating?
Co bandwidth V ~ 3eV
a
E ~ 1010 V/m
a = 0.25 nm
V = e E a ~ 2.5 eV
potential gradient leads to breakup of conduction path no current flow due to field – no heating
Potential along E field direction
Offset of “bands” ~ bandwidth
strong E field should cause current flow - severe Joule heating
Potential of a regular linear lattice
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