Ultrafast Laser-Induced Spin-Transfer Torque
Transcrição
Ultrafast Laser-Induced Spin-Transfer Torque
Ultrafast Laser-Induced Spin-Transfer Torque 0 1 Acknowledgements Eindhoven University of Technology 100 fs Sjors Schellekens Koen Kuiper Wouter Verhoeven, Ruud de Wit, Taco Vader Francesco Dalla Longa, Gregory Malinowski Bastiaan Bergman, Jeroen Rietjens, Carlos Bosco, Csaba Jozsa, Maarten Van Kampen, Harm Kicken Technische Universität Kaiserslautern Tobias Roth Mirco Cinchetti Martin Aeschlimann experimentalists of the week MPI Metalforschung Stuttgart Daniel Steiauf Manfred Fähnle Bert Koopmans Local dynamics vs. spin transport Outline 2 3 Introduction Local magnetization dynamics Spin transfer (super-diffusive) FM M Laser-induced spin transfer torque Why efficient Our recent experiments A first interpretation: Super-diffusive spin currents or Spin-Dependent Seebeck? Its importance for local magnetization dynamics Beaurepaire et al., Phys. Rev. Lett. 1996 What happens after fs laser excitation? Sub-ps loss of magnetization 5 4 S Beaurepaire et al., PRL 1996 Launching spin waves N S Van Kampen et al., PRL 2002 MO contrast Quenching magnetic moment N AF F phase transition S Ju et al., PRL 2004; Thiele et al. APL 2004 N 1.0 Ni thin film Switching by circularly polarized light Stanciu et al., PRL 2007 + S N N 0.5 S 0 “Toggle switching” ferrimagnets Radu et al., Nature 2011 linear S N N 50 fs pump/probe S Beaurepaire et al., Phys. Rev. Lett. 1996 Sub-ps loss of magnetization 5 10 15 ∆t (ps) Conservation of Angular Momentum 6 7 1,0 Temperature (K) MO contrast Angular Momentum Transfer M 500 E = magnetization 0,9 400 E 50 fs pump/probe 0 th 1 Delay (ps) 0.5 ps th < 0.1 ps electrons 0,8 lattice 300 electr. electr. 2 lattice Transfer from spin to orbit M < 0.2 ps Photons in or out spins From spin to the lattice J spins lattice Are photons and hot (highly excited) electrons relevant? Boeglin et et al., Nature 2010 Distinguishing orbital and spin moments 8 Ni 10 nm film low laser fluence 1,0 0,0 most demag. after thermalization M / M0 -0,5 M / M (%) 9 high laser fluence -1,0 -1,5 0,8 0,6 -2,0 0,0 0,5 1,0 0,4 1,5 delay (ps) 0 1 2 4 delay (ps) data: TU/e & U. Kaiserslautern The answer (?) Koopmans et al., PRL 2005 Nat. Mater. 2010 Minimalistic model 10 Like de Haas & Einstein Photons in or out Transfer from spin to orbit 11 Electrons: Constant DOS Phonons: Einstein model (+ Debye) Spins: mean-field S = ½ Weiss model e From spin to the lattice e e spin flip Figure of Assumption: τth merit ~0 e e e-p Highly-excited electrons / laser field e U e-e p asf TC a sf Leading to: μτat~ 0.4 ps E e-σ eσ DF ED, Dp TC, μat p Finite chance for spin-flip upon momentum scattering (Elliott-Yafet) What it can reproduce Open questions But which phonons, and how do they carry angular momentum? 12 Slow dynamics (Gd) vs. fast dynamics Do we treat magnetic excitations adequately? Stoner vs. Magnons Koopmans et al., Nature Mater. 2010 Temperature- and laser fluence dependence Roth et al., Phys. Rev. X 2012 Toggle switching of ferrimagnets (two spin sub-lattices) Schellekens et al., Phys. Rev. B Rapid 2012 Is it really possible to treat this highly nonequilibrium system thermodynamically? Three-particle interaction? (e-p + spin-flip) Combined with experiment: asf ~ 0.1 Roth et al. Phys. Rev. X 2012 Carva et al. Phys. Rev. Lett. 2011 13 Isn’t our picture of the rare earth dynamics too crude? Note: very much like atomistic LLG and LLB Kazantseva et al. PRB 2008, etc., Mentink et al., PRL 2012 Or is it something completely else? Or: Claims that spin transfer may explain all Fs spin-transfer Malinowski et al. Nature Physics 2008 15 Pt Co NiO Pt Co Ru Normalized (arb.u.) 14 NiO Ru 0,0 0,5 1 Delay (ps) Majority spins travel further 2 3 And even more exciting / surprising… The approach and the surprize 16 Can this highenergy state live that long? ? 17 Fe Ni Fe Ni Outline All previous results: collinear systems 18 Introduction Local magnetization dynamics Spin transfer (super-diffusive) 19 If spin transfer assists thermodynamically stable final state: OK Dynamics just speeds up (Malinowski) But if spin transfer creates strong non-equilibrium? Laser-induced spin transfer torque Why efficient Our recent experiments A first interpretation: Super-diffusive spin currents or Spin-Dependent Seebeck? Its importance for local magnetization dynamics Either local dissipation of angular momentum (100 fs) Or compensating spin transport (also just femtoseconds) (?) What if non-collinear spin transfer? Motivation 20 21 Ideal method for quantifying laser-induced spin transport Addressing role of super-diffusive spin transport to ultrafast demagnetization Device options? All-optical switching? Measuring thermal STT without lithography? Quick dissipation of spin momentum (mixing conductance or precession) Efficient absorption of angular momentum Causing laser-induced STT Final state just rotation of quantization axis So final state in thermal equilibrium Recent experiments @ TU/e Sample properties 22 Polar MOKE measuring Mz Probe pulse after delay ∆t Longitudinal MOKE Sample stack Field @ 45o with respect to sample plane B in-plane in-plane Co HK Cu Pt 4 nm HK [Co/Ni]n PMA Pt 1 nm Co 3 nm Cu 0 - 20 [Co / Ni ]x4 0.2/0.6 nm PMA Four remnant configurations magnetic bilayer! Schellekens et al. (TU/e), In preparation Indeed two precessions! Cu vs Pt spacer layer 24 Two oscillations - with proper symmetry B Can be assigned to top and bottom Oscillation top in-plane layer suppressed: STT! Precession bottom layer different origin “∆K” Schellekens et al. (TU/e), In preparation Suppression of STT on top layer Phase of the STT-induced precessions angular momentum transfer (%) 26 27 3 2 0.06 deg Cu z 1 y y Pt 0 0 z 2 4 0.12o x 0.6o 6 t (nm) 8 10 12 All consistent with fs Spin-Transfer Torque pulse M H eff M depending on x maj. / min. sin(t ) = 0 or = ±/2 The ∆K artefact Overview phases 28 29 sin(t ) M STT short HK Heff HK M z y STT long ∆K short ∆K long Maj. Min. Maj. Min. dec. inc. dec. inc. IP π π π 0 π π π OOP π π 0 π π π π Heff majority minority decrease M Ha Ha experiment: x Analysis Analysis Field dependence Angle dependence = -/2, = -/2, STT ∆K STT ∆K STT STT ∆K Top layer Frequency Amplitude Phase Top layer Bottom layer STT K Frequency Phase Amplitude STT K Frequency Amplitude Phase STT Anisotropy Precessions top layer consistent with USTT! ∆K inefficient due to large Tc Precessions bottom layer consistent with ∆K, USTT not visible (poor sensitivity bottom layer + overwhelmed by ∆K) Possible origin of laser-induced STT Calculated transient temperature profile 32 33 7 K / nm Optical absorption Laser-induced torques: Transient T profiles t = 0.1 ps t = 0.5 ps sd T sc K t = 500 ps t = -0.5 ps Super-diffusive spin currents (+ screening currents) Spin-Dependent Seebeck effect due to large T gradients 7 K / nm Calculated spin currents due to SDS The alternative: Super-diffusive Transient T profiles M Continuity equations: up/down spins absorbing up/down spins within 1 nm Spin dependent current with T gradient: Drift diffusion 34 35 Experiment (top IP layer): Amplitude of precession: Spin transfer corresponds to 2% of demagnetization OOP layer Phase of precession: Majority spin flow from bottom OOP layer Both consistent with super-diffusive spin current ( + screening spin polarized charge current) M Canting IP layer = 3 mdeg experiment: 60 mdeg Similar results for Co / spacer / Pt/Co/Pt Outline 36 But more difficult to interpret… (combination of ∆K and STT) 37 Introduction Local magnetization dynamics Spin transfer (super-diffusive) Laser-induced spin transfer torque Why efficient Our recent experiments A first interpretation: Super-diffusive spin currents or Spin-Dependent Seebeck? Also positive results and rich data for Co/Pt/Au/Co Co Au Co Its importance for local magnetization dynamics Pt A decisive experiment? Schellekens, Verhoeven et al. APL 2013 … nothing … 38 Schellekens, Verhoeven et al. APL 2013 39 Front pump Back pump Conclusions Questions Acknowledgements 40 41 Thermo-dynamical model using Elliott-Yafet process seems to explain all local magnetization dynamics Did we prove minor rol of spin transport? And what about using the effect? Or are we too naïve? What about all the questions I posed? Laser-induced spin transfer can play a role (?) And can exert a STT (?) Likely of super-diffusive (and not SDS) origin (?) Are there pitfalls in our interpretation? Is our optical / thermal / SDS correct? How to model super-diffusive STT more accurately? What about back flow / screening charge? Etc. etc. The ∆K artefact Is it magnetism at all in Ni? 42 Koopmans et al. PRL 2000 Regensburger et al. PRB 2000 Guidoni et al. PRL 2002 MO rotation MO ellipticity Ha M Haff z y x Ha HK demag. field shape anisotropy M HK ind. MO response (%) ~ = + i 0 -5 0 1 2 delay (ps) 43