Y. Acremann, Sara Gamble, Mark Burkhardt ( SLAC/Stanford )

Exploring Ultrafast Excitations in Solids withPulsed e-Beams

Joachim Stöhr and Hans Siegmann

Stanford Synchrotron Radiation Laboratory

Collaborators:

A. Vaterlaus (ETH Zürich)

A. Kashuba (Landau Inst. Moscow) ; A. Dobin (Seagate)

D. Weller, G. Ju, B.Lu (Seagate Technologies)

G. Woltersdorf, B. Heinrich (S.F.U. Vancouver) samples

theory

magnetic imaging

The ultrafast technology gap

want to reliably

switch small “bits”

The Technology Problem: Smaller and Faster

Faster than 100 ps….Mechanisms of ultrafast transfer of energy and angular momentum

Electric fields

Optical pulseElectron pulse

Magnetic fields

Shockwave

IR or THz pulseElectrons Phonons

Spin

~ 1 ps

= ? ~ 100 ps

The simplest case: precessional magnetic switching

M

Conventional (Oersted) switching

Creation of large, ultrafast magnetic fields

Conventional method

- t

oo slow

Ultrafast pulse – use electron accelerator

C. H. Back et al., Science 285, 864 (1999)

Torques on in-plane magnetization by beam field

Max. torque

Min. torque

Initial magnetization of sample

Fast switching occurs when H M┴

Precessional or ballistic switching: 1999

Patent issued December 21, 2000: R. Allenspach, Ch. Back and H. C. Siegmann

In-Plane Magnetization: Pattern development

• Magnetic field intensity is large

• Precisely known field size

540o

360o

180o

720o

Rotation angles:

Origin of observed switching pattern

In macrospin approximation, line positions depend on:• angle = B • in-plane anisotropy Ku

• out-of-plane anisotropy K┴

• LLG damping parameter

from FMR data

H increases15 layers of Fe/GaAs(110)

Beamcenter

Breakdown of the Macrospin Approximation H increases

With increasing field, deposited energy far exceeds macrospin approximation this energy is due to increased dissipation or spin wave excitation

Magnetization fracture under ultrafast field pulse excitation

Macro-spin approximation uniform precession

Magnetization fracture moment de-phasing

Breakdown of the macro-spin approximation

Tudosa et al., Nature 428, 831 (2004)

Results with ultrafast magnetic field pulsesCompare 5ps pulse with 160fs bunch – same charge 1.6x1010 e-

Tudosa et al. Nature 428, 831 (2004)

160 fs – 5 ps

Magnetic pattern with 5 ps electron bunch

~ 20 m

Magnetic pattern of 10 nm Fe film with

160 fs bunch length

Pattern size 470 µm by 980 µm

New results with a 10 times shorter ( 167 fs ) pulse

Pattern is severely asymmetric – should follow circles

No switching for certain magnetic field orientations !

Violates conventional laws of angle-dependence of magnetic torque

Puzzle is unresolved at present…..

5.5 ps, 1.6x1010 e- 160 fs, 1.6x1010 e-

ablation

Magnetic flash images with different bunch lengths

characteristicdemagnetization domains

slow pulse fast pulse

magnetic topographical

Surprising result: No beam damage at ultrafast time scales

• Tpographical image of beam impact area reveals no damage !

• Magnetic pattern indicates no heating !

• Maximum power density is 4 x 1019 W / cm2

• Sub-picosecond energy dissipation must exist (photons, electrons)

• Dissipation faster than electron-phonon relaxation time (ps)

• Results of importance for LCLS, as well.

Example: PbTiO3

From Ferro-magnetic to Ferro-electric Materials

• FM materials respond to magnetic field (axial vector) - broken time reversal symmetry of electronic system

• FE materials respond to electric field (polar vector) - broken inversion symmetry of lattice

Materials important as storage media – but how fast do they switch?

Ferro-electric domains can be observed with x-rays

E EPEEM

Reverses with change in polarization direction.

Dichroism

Use E-field of SLAC beam to switch

In lab (Auston switch): ~70 ps with the E-field amplitude of 25 MV/mSLAC Linac: femtosecond pulses up to several GV/m

Pump-Probe Dynamics with SLAC-pulses

Summary

• The breakdown of the macrospin approximation for fast field pulses limits the reliability of magnetic switching

• At ultrafast speeds (< 1 ps) new ill-understood phenomena exist one approaches timescales of fundamental interactions between electrons, lattice and spin

• Future experiments to explore the details using both H and E fields

• In the future, e-beam “pump”/ laser “probe” experiments are of interest, as well

For more, see: http://www-ssrl.slac.stanford.edu/stohr andJ. Stöhr and H. C. Siegmann Magnetism: From Fundamentals to Nanoscale Dynamics 800+ page textbook ( Springer, 2006 )