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Femtosecond Heating as a Sufficient Stimulus for Magnetization Reversal

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## Femtosecond Heating as a Sufficient Stimulus for Magnetization Reversal

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**Theoretical/Modelling Contributions**T. Ostler, J. Barker, R. F. L. Evans and R. W. Chantrell Dept. of Physics, The University of York, York, United Kingdom. U. Atxitia and O. Chubykalo-Fesenko Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, Madrid, Spain.D. Afansiev and B. A. IvanovInstitute of Magnetism, NASU Kiev, Ukraine. Femtosecond Heating as a Sufficient Stimulus for Magnetization Reversal TMRC, San Jose, August 2012**Experimental Contributions**S. El Moussaoui, L. Le Guyader, E. Mengotti, L. J. Heyderman and F. NoltingPaul ScherrerInstitut, Villigen, SwitzerlandA. Tsukamoto and A. ItohCollege of Science and Technology, Nihon University, Funabashi, Chiba, Japan. A. M. Kalashnikova , K. Vahaplar, J. Mentink, A. Kirilyuk, Th. Rasing and A. V. KimelRadboud University Nijmegen, Institute for Molecules and Materials, Nijmegen, The Netherlands. Femtosecond Heating as a Sufficient Stimulus for Magnetization Reversal TMRC, San Jose, August 2012**Outline**• Model outline: atomistic LLG of GdFeCo and laser heating • Static properties of GdFeCo and comparison to experiment • Transient dynamics under laser heating • Deterministic switching using heat and experimental verification • Mechanism of reversal**Background**σ- • Inverse Faraday[1,2] effect relates E-field of light to generation of magnetization. • Can be treated as an effective field with the chirality determining the sign of the field. σ+ M(0) Inverse Faraday effect • Control of magnetization of ferrimagneticGdFeCo[3] • High powered laser systems generate a lot of heat. • What is the role of the heat and the effective field from the IFE? [1] Hertel, JMMM, 303, L1-L4 (2006). [2] Van derZielet al., Phys Rev Lett15, 5 (1965). [3] Stanciuet al. PRL, 99, 047601 (2007).**A model of laser heating**Laser input P(t) two temperature model energy transfers • Recall for circularly polarised light, magnetization induced is given by: • For linearly polarized light cross product is zero. Energy transferred as heat. • Two-temperature[1]model defines an electron and phonon temperature (Te and Tl) as a function of time. • Heat capacity of electrons is smaller than phonons so see rapid increase in electron temperature (ultrafast heating). Electrons Lattice Gel e- e- e- e- Two temperature model [1] Chen et al. International Journal of Heat and Mass Transfer.49, 307-316 (2006)**Model: Atomistic LLG**• We use a model based on the Landau-Lifshitz-Gilbert (LLG) equation for atomistic spins. • Time evolution of each spin described by a coupled LLG equation for spin i. • Hamiltonian contains only exchange and anisotropy. • Field then given by: • is a (stochastic) thermal term allowing temperature to be incorporated into the model. For more details on this model see Ostleret al. Phys. Rev. B.84, 024407 (2011)**Model: Exchange interactions/Structure**• GdFeCo is an amorphous ferrimagnet. • Assume regular lattice (fcc). • In the model we allocate Gd and FeCo spins randomly. Fe-Gd interactions are anti-ferromagnetic (J<0) Fe-Fe and Gd-Gd interactions are ferromagnetic (J>0) Fe Gd Atomic Level Sub-lattice magnetization For more details on this model see Ostleret al. Phys. Rev. B.84, 024407 (2011)**Bulk Properties**• Exchange values (J’s) based on experimental observations of sublattice magnetizations as a function of temperature. • Compensation point and TC determined by element resolved XMCD. • Variation of J’s to get correct temperature dependence. • Validation of model by reproducing experimental observations. compensation point Figure from Ostler et al. Phys. Rev. B.84, 024407 (2011)**Summary so far**Atomic level model of a ferrimagnet with time A way of describing heating effect of fs laser • We investigate dynamics of GdFeCo and show differential sublattice dynamics and a transient ferromagnetic state. • Then demonstrate heat driven reversal via the transient ferromagnetic state. • Outline explanation is given for reversal mechanism.**Transient Dynamics in GdFeCo by XMCD & Model**• Femtosecond heating in a magnetic field. • Gd and Fe sublattices exhibit different dynamics. • Even though they are strongly exchange coupled. Experiment Model results Figures from Raduet al.Nature 472, 205-208 (2011).**Timescale of Demagnetisation**Experiment • Characteristic demagnetisation time can be estimated as[1]: • GdFeCo has 2 sublattices with different moment (µ). • Even though they are strongly exchange coupled the sublattices demagnetise at different rates (with µ). Model results [1] Kazantsevaet al. EPL, 81, 27004 (2008). Figures from Raduet al.Nature 472, 205-208 (2011).**Transient Ferromagnetic-like State**Laser heating in applied magnetic field of 0.5 T • System gets into a transient ferromagnetic state at around 400 fs. • Transient state exists for around 1 ps. • As part of a systematic investigation we found that reversal occuredin the absence of an applied field. Figure from Raduet al.Nature 472, 205-208 (2011).**Numerical Results of Switching Without a Field**• Very unexpected result that the field plays no role. • Is this determinisitic? No magnetic field GdFeCo**Sequence of pulses**• Do we see the same effect experimentally?**Experimental Verification: GdFeCo Microstructures**Initial state - two microstructures with opposite magnetisation - Seperated by distance larger than radius (no coupling) 2mm XMCD Experimental observation of magnetisation after each pulse.**Effect of a stabilising field**• What happens now if we apply a field to oppose the formation of the transient ferromagnetic state? • Is this a fragile effect? 10 T Bz=10,40,50 T 40 T 50 T GdFeCo • Suggests probable exchange origin of effect (more later).**Mechanism of Reversal**FMR Exchange • After heat pulse TM moments more disordered than RE (different demagnetisation rates). • On small (local) length scale TM and RE random angles between them. • The effect is averaged out over the system. • Exchange mode is excited when sublattices are not exactly anti-parallel.**Mechanism of Reversal**end of pulse RE • If we decrease the system size then we still see reversal via transient state. • For small systems a lot of precession is induced. • Frequency of precession associated with exchange mode. • For systems larger than 20nm3 there is no obvious precession induced (averaged out over system). • Further evidence of exchange driven effect. TM TM end of pulse TM sublattice**Summary**• Demonstrated numerically switching can occur using only a heat pulse without the need for magnetic field. • Switching is deterministic. • Verified the mechanism experimentally in microstructures (and thin films, see paper). Shown that stray fields play no role. • The magnetic moments are important for switching. • Demonstrated a possible explanation via a local excitation of exchange mode by decreasing system size and observing induced precession.**Acknowledgements**• Experiments performed at the SIM beamline of the Swiss Light Source, PSI. • Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), de Stichting voor Fundamenteel Onderzoek der Materie (FOM). • The Russian Foundation for Basic Research (RFBR). • European Community’s Seventh Framework Programme (FP7/2007-2013) Grants No. NMP3-SL-2008-214469 (UltraMagnetron) and No. 214810 (FANTOMAS), • Spanish MICINN project FIS2010-20979-C02-02 • European Research Council under the European Union’s Seventh Framework Programme (FP7/2007- 2013)/ ERC Grant agreement No 257280 (Femtomagnetism). • NASU grant numbers 228-11 and 227-11. • Thank you for listening.**Numerical Model**• Dynamics of each spin given by Landau-Lifshitz-Gilbert Langevin equation. • Effective field given by: • Moments defined through the fluctuation dissipation theorem as: • Energetics of system described by Hamiltonian:**The Effect of Compensation Point**• Previous studies have tried to switch using the changing dynamics at the compensation point. • Simulations show starting temperature not important (not important if we cross compensation point or not). • Supported by experiments on different compositions of GdFeCo support the numerical observation.**Experimental Verification: GdFeCo Thin Films**• After action of each pulse (σ+) the magnetization switches, independently of initial state. Fe Initially film magnetised “up” Gd MOKE • Similar results for film initially magnetised in “down” state. • Beyond regime of all-optical reversal, i.e. cannot be controlled by laser polarisation. Therefore it must be a heat effect.**What about the Inverse Faraday Effect?**• Orientation of magnetization governed by light polarisation. Stanciuet al. PRL, 99, 047601 (2007) Does not depend on chirality (high fluence) Depends on chirality (lower fluence)**Importance of moments**• If moments are equal the no reversal occurs μTM=μRE**Linear Reversal**• Usual reversal mechanism (in a field) below TC via precessional switching • At high temperatures, magnetisation responds quickly without perpendicular component (linear route[1]). • Laser heating results in linear demagnetisation[2].**The Effect of Heat**• Ordered ferromagnet • Uniaxial anisotropy E Heat E 50% 50% Cool M+ M- M+ M- M+ M- Cool below TC Heat (slowly) through TC System demagnetised Equal chance of M+/M-**Inverse Faraday Effect**• Magnetization direction governed by E-field of polarized light. • Opposite helicities lead to induced magnetization in opposite direction. • Acts as “effective field” depending on helicity (±). σ+ z σ- z http://en.wikipedia.org/wiki/Circular_polarization Hertel, JMMM, 303, L1-L4 (2006)**Outlook**• Currently developing a macro-spin model based on the Landau-Lifshitz-Bloch (LLB) formalism to further support reversal mechanism. • How can our mechanism be observed experimentally? Time/space/element resolved magnetisation observation → spin-spin correlation function/structure factor. • Once we understand more about the mechanism, can we find other materials that show the same effect?**Differential Demagnetization**• Atomistic model agrees qualitatively with experiments • Fe and Gd demagnetise in thermal field (scales with μ via correlator) Kazantsevaet al. EPL, 81, 27004 (2008). Gd slow, ~1ps Fe fast, loses magnetisation in around 300fs Raduet al.Nature 472, 205-208 (2011).**What’s going on?**0 ps - Ground state -T>TC Fe disorders more quickly (μ) 0.5 ps -T<TCprecessional switching (on atomic level) -Exchange mode between TM and RE - Transient state 1.2 ps 10’s ps time