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Femtosecond opto-magnetism Alexey Kimel

IMM, Radboud Universiteit Nijmegen, Nijmegen, The Netherlands h h

N S S N

Radboud University Nijmegen

Spin, ensemble of spins, spin excitation

Spin

L S

Ensemble of spins

N S exchange interaction

Excitation of spins

Goals

Ultrafast magnetization reorientation Discovery of new fundamental limits on magnetic switching

?

Ultrafast frontier in science and technology

MRAM Hard disks

Thermodynamics

1 ns

100 ps 10 ps

1 ps

100 fs 10 fs

UNEXPLORED

The ultrafast technology frontier

Stimulus

1 fs

Laser control of magnetism

Is it possible?

100 fs

Femtosecond laser excitation

Magnetization reversal

Femtosecond laser control of magnetism

Demagnetization (shape anisotropy) E. Beaurepaire et al, Phys. Rev. Lett. (1996). J. Hohlfeld et al, Phys. Rev. Lett. (1997). A. Scholl et al, Phys. Rev. Lett. (1997). B. Koopmans et al, Phys. Rev. Lett. (2000). Magneto-crystalline anisotropy A. V. Kimel et al Nature (2004). M. Vomir et al, Phys. Rev. Lett. (2005). F. Hansteen et al, Phys. Rev. Lett. (2005). Exchange integral G. Ju et al, Phys. Rev. Lett. (2004). J. U. Thiele et al, Appl. Phys. Lett. (2004).

Heating!

Magneto-optics/ Opto-magnetism

Spins Photons

???

Magneto-optics

F ~Mz

Opto-magnetism

E E Mz Faraday effect

(-)

M M

?

(+)

Inverse Faraday effect

Ultrafast excitation of spins in DyFeO3

Faraday rotation (deg)

0.2

+

L

0.1

H H

0.1 ps

photons

0.0

DyFeO3 T = 95 K

0 15 30 45 60

L

0.1 ps

Time delay (ps)

as strong as 1 T!

A. Kimel et al, Nature 435 655 (2005).

How can light magnetize a medium?

W

0

E ( )E ( )

Light-wave energy M ­ magnetization Heff - magnetic field

H eff (0)

(+)

1

0

W M ( 0)

H H

W M

E( )E* ( ) 0

M

(-)

ij jj

M

(0) jj

M 3 ... M

2

M

4

...

0 0

L. Onsager (1931)

Light acts as a magnetic field (inverse Faraday effect)

H eff (0)

E( )E* ( )

L. P . Pitaevskii, Sov. Phys. JETP 12, 1008 (1961). J. P. van der Ziel et al, Phys. Rev. Lett. 15,190 (1965).

Can X-ray pump magnetize a medium?

Possible NLS-project: X-ray Inverse Faraday effect

(+)

H H

(-)

Dream: Element specific ultrafast magnetic excitations at the nanoscale

t< 10 fs

H eff (0)

0 0

E( )E* ( )

Can X-ray pump magnetize a medium?

Ultrafast frontier in magnetism

THERMODYNAMICS 1 ns

100 ps 10 ps

Possible NLS-project: X-ray Inverse Faraday effect

Macrospin approximation Landau-Lifshitz equation

Dream: Element specific ultrafast magnetic excitations at the nanoscale

t< 10 fs

1 ps

100 fs 10 fs

?

Stimulus

1 fs

Can light reverse M?

10THz

100 fs

Opto-Magnetic Recording

NO MAGNETIC FIELD!

Is all optical magnetization reversal feasible?

"The amount of the photons involved in the experiments is far not enough to contribute a significant angular momentum."

see also: Ju et al., PRB 57, R700 (1998); Wilks et al., JAP 95, 7441 (2004).

Is subpicosecond magnetic recording feasible?

2.3 ps, several T pulses

"No matter how short and strong the magnetic-field pulse, magnetic recording cannot be made ever faster."

Experiment on laser-induced magnetization reversal

Static magneto-optical imaging + pulsed laser

Linear polarized laser pulses Amplified Ti:Sapphire 1 KHz 40 fs and 800 nm Circularly polarized laser pulses Quarter wave plate Polarized light source Microscope objective Sample Analyzer

20 nm GdFeCo film on glass (from A. Tsukamoto & A. Itoh, Nihon University, Japan)

CCD camera

Laser-induced magnetization reversal

40 fs pulses, 1 kHz

GdFeCo

Hext = 0

L

C.D. Stanciu et al., Phys. Rev. Lett. 99 047601 (2007).

Magnetization reversal induced by a single 40 fs laser pulse

Sweeping the pulsed laser beam at high speed across the sample

Each domain is written with a single 40 fs laser pulse Magnetization reversal must occur in 1 ps ?!

C.D. Stanciu et al., Phys. Rev. Lett. 99 047601 (2007).

Can X-ray-probe reveal the mechanisms of ultrafast magnetism?

Possible NLS-project: Ultrafast single-shot magnetic imaging at the nanoscale Dreams: Discovery of new fundamental limits on speed of magnetic switching THz magnetic recording

100 fs

10 THz

Opto-Magnetic Recording

NO MAGNETIC FIELD!

Summary

Femtosecond opto-magnetism is feasible 100 fs laser pulse is equivalent to 100 fs magnetic field pulse of 1 T All-optical magnetic recording is feasible. The mechanism is unknown Opto-magnetism could be a key for investigation of ultrafast magnetism

Femtosecond Magnetic Recording Is a Reality!

Patent, ref. #P77323PC00

Possible NLS-projects

Ultrafast single-shot magnetic imaging at the nanoscale X-ray Inverse Faraday effect

Possible NLS-dreams

Element specific ultrafast magnetic excitations at the nanoscale Discovery of new fundamental limits on speed of magnetic switching THz magnetic recording

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