Presentation on theme: "2Instituto de Ciencia de Materiales de Madrid,"— Presentation transcript:
1 2Instituto de Ciencia de Materiales de Madrid, Two Magnon Bound State Causes the Ultrafast Thermally Induced SwitchingJ. Barker, U. Atxitia, T. A. Ostler, O. Hovorka, O. Chubykalo-Fesenko and R. W. Chantrell1Department of PhysicsUniversity of York2Instituto de Ciencia de Materiales de Madrid,CSIC, Madrid, Spain.Intermag, May 2014
2 How do we switch magnets? We apply a bias to break the symmetry of the system.External fields, Spin injection / Spin transfer torqueInitial stateApplying a biasSwitched stateQuestion 1: How do we do it?Question 2: Why does it occur?Initial StateFemtosecond laser heatingAfter cooling
3 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 stateReversal of the sublatticesDifferent demagnetization timesInitial State
4 Atomistic Spin Dynamics Langevin DynamicsLandau-Lifshitz-Gilbert Equationthermal forces representedby a stochastic field term - ηiFast laser heating is included with a two-temperature modelKaganov et al., JETP 173 (1957)Chen et al. International Journal of Heat and Mass Transfer 49, 307 (2006).
5 GdFeCo is an amorphous ferrimagnet. Modeling of GdFeCoGdFeCo 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.
6 Deterministic All Thermal Reversal Discovered in a systematic investigation using atomistic spin dynamicsT.A. Ostler et al., Nat. Commun. 3, 666 (2012)No applied fieldConfirmed experimentally with unpolarized laser lightTwo microstructures separated by a distance (no coupling) under the same laser spot.
7 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).
8 Spinwave modesFMAFMIn the disordered ferrimagnet the excitations are mixed
9 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
10 The transfer of energy between sublattices Barker et al. Sci. Rep. 2013Only a single band in the excited region.Non-linear energy transfer between bands.Large band gap precludes efficient energy transfer.
11 Excitation of one spin wave modes only causes demagnetization. Two spin wave modesExcitation 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.
12 Intermediate Structure Factor How is spin wave amplitude distributed in space?Below switching thresholdAbove switching thresholdNo significant change in the ISFExcited region during switching1090KFeCo975KGdAdd symmetry points. Talk about amplitude (normalisation). Spin system absorbs energy but we are interested in the distribution.X/2X/2M/2M/2Barker et al. Sci. Rep. 2013
13 ConlusionsMagnetization 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).
14 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