Ultrafast Manipulation of the Magnetization J. Stöhr Sara Gamble and H. C. Siegmann, SLAC, Stanford A. Kashuba Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine

The ultrafast technology gap Drivers of Modern Magnetism Research: Smaller and Faster

“for the discovery of giant magnetoresistance” GMR “reads” the magnetization state “pinned” “0”“1”

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 or spin-orbit Conceptual methods of magnetization switching Optical pulse Lattice shock ElectronsPhonons 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 - too 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

The world’s biggest lightning bolt Stanford Linear Accelerator Center - SLAC 3km, 30GeV

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 electrons

In-Plane Magnetization: Pattern development Magnetic field intensity is large Precisely known field size 540 o 360 o 180 o 720 o Rotation angles: no circles around beam ! very different from lightning pattern

The pattern written by a picosecond beam field Max. torque T = M x H Min. torque = 0 initial magnetization direction of sample Fast switching occurs when H M ┴ beam damage M

end of field pulse M Ballistic Switching – From nano to picoseconds Patent issued December 21, 2000: R. Allenspach, Ch. Back and H. C. Siegmann Relaxation into “down” direction governed by slow spin-lattice relaxation (100 ps) - but process is deterministic ! Precise timing for  =180 o reduces time

Toward femtosecond switching Experiments with sub-ps bunches reduce bunch length from FWHM  5 ps → 140 fs keep beam energy and charge fixed (~10 10 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 defined within and outside e-bunch

10 nm Co 70 Fe 30 on MgO (110) Magnetic pattern is severely distorted for short bunch 140 fs 5 ps damaged area 15 layers Fe on GaAs (110)

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

Consider effect of giant E-field of beam Beam field E ~ 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 torqueE-field torque Magneto-electronic anisotropy is strong ~ E 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 ~E 2 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 pulse true “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 imageTopological imageby means of SEMPA microscopy same sample: 10 nm Co 70 Fe 30 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 ~ 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

Unusual E and B field effects No apparent heating or damage by beam Extreme THz science just starting…. Summary material behave very strange in extreme fields !