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Two Magnon Bound State Causes the Ultrafast Thermally Induced Switching J. Barker, U. Atxitia, T. A. Ostler, O. Hovorka, O. Chubykalo-Fesenko and R. W. Chantrell 1 Department of Physics University of York 2 Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid, Spain. Intermag, May 2014

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How do we switch magnets? External fields, Spin injection / Spin transfer torque We apply a bias to break the symmetry of the system. Initial state Applying a bias Switched state Initial State After cooling Femtosecond laser heating Question 1: How do we do it? Question 2: Why does it occur? Question 1: How do we do it? Question 2: Why does it occur?

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Element-resolved dynamics of the Fe and Gd magnetic moments measured by time- resolved XMCD with femtosecond time- resolution. Ultrafast element resolved dynamics Initial State Different demagnetization times Transient ferromagnetic-like state Reversal of the sublattices Radu et al., Nature 472, 205 (2011).

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Atomistic Spin Dynamics Landau-Lifshitz-Gilbert Equation Langevin Dynamics thermal forces represented by a stochastic field term - η i Fast laser heating is included with a two-temperature model Kaganov et al., JETP 173 (1957) Chen et al. International Journal of Heat and Mass Transfer 49, 307 (2006).

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Modeling of GdFeCo GdFeCo is an amorphous ferrimagnet. We approximate this as a random lattice of TM (FeCo) and RE (Gd) spins. Statistical probability of Gd clusters: they are correlated over some length- scale.

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Deterministic All Thermal Reversal Discovered in a systematic investigation using atomistic spin dynamics T.A. Ostler et al., Nat. Commun. 3, 666 (2012) No applied field Two microstructures separated by a distance (no coupling) under the same laser spot. Confirmed experimentally with unpolarized laser light

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Deterministic All Thermal Reversal Question 1: How do we do it? ✓ Question 2: Why does it occur? No obvious symmetry breaking, to answer that we have to consider microscopic interactions (spinwaves).

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Spinwave modes FM AFM In the disordered ferrimagnet the excitations are mixed

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The dynamic structure factor Linear Spin Wave Theory (Virtual Crystal Approximation) Barker et al. Sci. Rep So the AFM and FM modes are no longer pure but mixed!

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The transfer of energy between sublattices Non-linear energy transfer between bands. Only a single band in the excited region. Large band gap precludes efficient energy transfer. Barker et al. Sci. Rep. 2013

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Two spin wave modes Excitation of one spin wave modes only causes demagnetization. Excitation of two spin wave modes causes transient ferromagnetic like state. The switching is caused when both branches of the FM and AFM are excited! The part of the BZ that is significant is determined by clustering.

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Intermediate Structure Factor Below switching threshold No significant change in the ISF Above switching threshold How is spin wave amplitude distributed in space? Excited region during switching 975K 1090K FeCo Gd M/2 X/2 M/2 X/2 Barker et al. Sci. Rep. 2013

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Conlusions Magnetization can be switched deterministically with heat alone. This is due to the excitation of a two spin wave mode. In GdFeCo the two spin wave mode is determined by the length scale of Gd clusters. For full details see Nature Scientific Reports, 3, 3262 (2014).

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Acknowledgements/references ReferencesDemagnetization times: Atxitia et al. arXiv: (2013).Transient ferromagnetic-like state: Radu et al. Nature 472, (2011).Atomistic model of GdFeCo: T. Ostler et al., Phys. Rev. B 84, (2011).Thermally induced switching: Nat. Commun. 3, 666 (2012).Switching in heterostructures: R. Evans et al. arXiv: (2013).Switching mechanism: J. Barker et al. Nat. Sci. Rep. (in press) arXiv: Thank you for your attention

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