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**2Instituto de Ciencia de Materiales de Madrid,**

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 1Department of Physics University of York 2Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid, Spain. Intermag, May 2014

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**How do we switch magnets?**

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

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**Ultrafast element resolved dynamics**

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

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**Atomistic Spin Dynamics**

Langevin Dynamics Landau-Lifshitz-Gilbert Equation thermal forces represented by a stochastic field term - ηi 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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**GdFeCo is an amorphous ferrimagnet.**

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 Confirmed experimentally with unpolarized laser light Two microstructures separated by a distance (no coupling) under the same laser spot.

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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**

So the AFM and FM modes are no longer pure but mixed! Linear Spin Wave Theory (Virtual Crystal Approximation) Barker et al. Sci. Rep. 2013

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**The transfer of energy between sublattices**

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

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**Excitation of one spin wave modes only causes demagnetization.**

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**

How is spin wave amplitude distributed in space? Below switching threshold Above switching threshold No significant change in the ISF Excited region during switching 1090K FeCo 975K Gd Add symmetry points. Talk about amplitude (normalisation). Spin system absorbs energy but we are interested in the distribution. X/2 X/2 M/2 M/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**

Demagnetization 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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