Radboud Universeit Nijmegen

Transcription

1 Magnetism and Light Theo Rasing Radboud Universeit Nijmegen

2 Challenges Magnetism and on the Opportunities timescale of fth the exchange interaction ti Theo Rasing Radboud University Nijmegen JFe-Fe=1.4x10-19 J ~ 40 fs

3 Magnetism and Light 40fs

4 Magnetism and Light

5 Time-scales in magnetism 1 ns 100 ps 1ps Spin-orbit (LS) interaction Spin excitation 10 ps L S ~ 1 ps - 1 ns 100 fs 10 fs Exchange interaction ~ 0.1 ps-1 ps ~ 001ps ps fs femtosecond!

6 Length-scales in magnetism l= ~nm Tb+THz THz nm+fs!

7 1-2eV,10-100fs New JFe-Fe=1.4x10 Mechanisms? -19 J ~ 40 fs

8 1996 Beaurepaire et al: 60fs Magnetization is changed within a picosecond!!!

9 Ultrafast excitation of spins in DyFeO 3 via the Inverse Faraday effect n (deg) 0.2 Referee A: + But, unfortunately, the observed signal 0.1 H H is so small that it seems impractical to utilize 0.0 DyFeO3 the inverse Faraday T = 95 K effect for the Time delay (ps) purpose of ultrafast control A. Kimeletal et al, Nature (2005). of magnetization in metallic materials. Farad day rotatio

10 Ultrafast all optical recording Femtosecond Magnetic Recording in GdFeCo! C.D. Stanciu et al., patent #P77323PC00, PRL 99, (2007)

11 GdFeCo: multi-sublattice ferrimagnet! Femtosecond Magnetic Recording in GdFeCo!

12 Ultrafast t laser induced d reversal in multi-sublattice magnets Gd Fe 40 fs Fe Gd 12

13 Gd Fe Fe Gd Angular momentum transfer? Role of temperature? Role of sublattices?..

14 Beyond Landau-Lifshitz dynamics =0.28 Ghz/T Typical llb laboratory fields 1T Reversal time 1 ns! Magnetism on the timescale of exchange: LONGITUDINAL spin dynamics?

15 Onsager s relations for spin dynamics Baryakhtar, JETP 1984 Magnetic free energy Generalized force Symmetry of kinetic coefficients Generalized position angular momentum Anti symmetric precession Symmetric longitudinal

16 More general: multiple sublattices Rate of change free energy Longitudinal dynamics to lowest order relativistic exchange J.H. Mentink et al., Phys. Rev. Lett. (2012)

17 More general: multiple sublattices formation sublattice spin moment mutual alignment sublattices Temperature dominated: T>>T C, t 100fs

18 Temperature dominated regime Relativistic relaxation dominates electrons Gd Fe Gd Fe Bloch relaxation Distinct dynamics Gd and Fe Brown Phys. Rev Kubo et al. Prog. Theor. Phys. Suppl. 1970

19 Exchange dominated: T<T C, t 1ps Conservation total angular momentum S 1 What if S2=0? S 2 Nonequilibrium Equilibrium value Ground state AFM, transient FM!

20 Exchange dominated regime M Gd M M Gd M M Fe M Fe spin-spin interactions conserve total magnetization M Gd Fe Gd Fe Transfer of magnetization between sublattices! Baryakhtar Fiz. Nizk. Temp. 1985

21 Laser-induced spin dynamics Ultrafast heating of electrons fs laser pulse J ex 25 mev 40 fs Access to both temperature and exchange dominated d regime!

22 Element specific view? Laser pump X-ray probe BESSY X-rays ev ps (FWHM) 50 fs laser pulses 1.5 ev, 5-50 mj/cm 2 3 khz rep. rate

23 Fe and Gd sub-lattices switch simultaneously within 50 ps 1.8 mj/cm 190 K Gd 29 Fe 62.1 Co 8.9

24 Can t we look faster?

25 FEMTO-SLICING! X-rays ev 100 fs (FWHM)

26 Element-specific magnetization reversal: 100 fs time resolution Fe: 100±23 fs Gd: 427±102 fs Different magnetization switching dynamics at Fe and Gd sites!!! Mentink:

27 Switching via a novel non-equilibrium state H Transient FERROMAGNETIC ALIGNMENT for ~1.2 ps The system evolves against the exchange interaction!!! JFe-Gd=4.77x10-20 J ~ 140 fs

28 Ultrafast magnetism in multi-sublattice magnets Gd Fe 40 fs Fe Bloch relaxation Exchange relaxation Gd What about multi-sublattice ferromagnets?

29 Prediction: ultrafast demagnetization FM coupling positive/negative Couplingmakes onesublatticefaster, otherslower AFM coupling always positive Both sublattices faster than in uncoupled case! J.H. Mentink et al., Phys. Rev. Lett. (2012)

30 FeNi JFe-Ni=1.4x10-19 J ~ 40 fs 1-2eV,10-100fs

31 simulations experiment Fe slower than Ni

32 Nanolocalization of all-optical switching Smallest switching obtained: ~200 nm

33 Perspectives emergent phenomena

34 Perspectives Skyrmions?

35 Perspectives THz

36 Perspectives J= J(t) H(t) Xrays+XMCD+nm+fs!

37 To conclude: With light Coherent optical control of magnetism! All-optical ultrafast magnetic recording Novel (linear) ultrafast reversal path! Novel transient ferromagnetic state Future challenges Femto magnetism! Combine chemical, magnetic spatial and time resolution: X(Z)FEL!

38 with many thanks to: IMM A.V. Kimel A.Kirilyuk F. Hansteen D.Stanciu A.v.Etteger K.Vahaplar A. Kalashnikova J. Mentink University of York R. Chantrell T.A. Ostler JB J.Barker R.Evans Ioffe Institute Prof. R.V. Pisarev P. Usachev Nihon University Prof. A. Itoh A. Tsukamoto University of Konstanz Denise Hinzke Ulrich Nowak Bessy I. Radu C. Stamm T.Kachel N.Pontius Herman Durr (Stanford) STW, NWO, EU,NanoNed, UltraMagnetron, FANTOMAS

39 Spectroscopy of Solids and Interfaces Several PhD / postdoc positions available! 2011