Ultrafast Heating as a Sufficient Stimulus for Magnetisation Reversal in a Ferrimagnet (reversal of a bistable magnetic system with heat alone!) MMM, Scottsdale, AZ Oct/Nov 2011 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. S. El Moussaoui, L. Le Guyader, E. Mengotti, L. J. Heyderman and F. Nolting Paul Scherrer Institut, Villigen, Switzerland A. Tsukamoto and A. Itoh College of Science and Technology, Nihon University, Funabashi, Chiba, Japan. D. Afansiev and B. A. Ivanov Institute of Magnetism, NASU Kiev, Ukraine. A. M. Kalashnikova, K. Vahaplar, J. Mentink, A. Kirilyuk, Th. Rasing and A. V. Kimel Radboud University Nijmegen, Institute for Molecules and Materials, Nijmegen, The Netherlands.

Motivation anti-parallel ground state. Radu et al. Nature 472, (2011). Recently we showed recently that using linearly polarised laser pulses, in the presence of a magnetic field, that we can induce switching in ferrimagnetic (GdFeCo). Showed that reversal occurs via a so-called transient ferromagnetic-like state. For more on this see (ED-07). What is the role of the magnetic field? application of laser pulse H align parallel Gd Fe H Initially sublattices align anti- parallel. Fe demagnetis es very quickly and reverses to align with Gd. Gd then reverses. Back to ground state. What is the role of the magnetic field? Can we see reversal without it? ?

Overview (Brief) details of numerical model. Results of numerical model. Experimental confirmation of switching in thin films of GdFeCo, independent on initial state. Switching in microstructures using linearly polarised laser pulse. Mechanism for switching?

GdFeCo is amorphous. In numerical model we allocate Gd and Fe spins randomly on closed packed lattice to required composition. Exchange parameters paremeterised on experimental observations J Fe-Fe >0 (ferromagnetic) J Fe-Gd 0 (ferromagnetic) Model features local moment variation  Fe <  Gd, important for reversal. We can use the model to observe the dynamics of individual spins with time. Fe Gd Atomic LevelMacrospin Numerical Model For more details on this model see Ostler et al. Phys. Rev. B. 84, (2011).

Numerical Results Starting temperature is 300K. Sequence of 50fs (FWHM) gaussian heat pulses. Increases electronic temperature (TTM[1]) to which the spin system in coupled. Heat dissipates on 100ps time-scale. Reversal occurs each time a pulse is applied. No applied field throughout simulation. [1] - Chen et al. International Journal of Heat and Mass Transfer, 49, (2006).

Switching in GdFeCo Thin Films We have experimentally verified the switching mechanism by studying the response of ferrimagnetic Gd 24 Fe 66.5 Co 9.5 to the action of 100fs (FWHM) right circularly polarised laser pulses. After action of each pulse the magnetization switches, independently of initial state. Initially film magnetised “up” 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. Gd Fe MOKE Stanciu et al. Phys. Rev. Lett. 99, (2007). 20  m

Reversal in Microstructures Reversal seen in 2  m microstructures of Gd 25 Fe 65.6 Co 9.4. Large enough distance apart to eliminate dipolar coupling effect. Magnetisation direction measured using a PEEM employing the XMCD effect (measuring Fe edge). Switching occurs every time, even with just linearly polarised light. XMCD 2m2m

Mechanism of Reversal What breaks the symmetry? Numerical simulations suggest that the fact that the sublattices are non-equivalent in longitudinal relaxation time is key for reversal. anti-parallel ground state. Fe demagnetises faster than Gd. ~1ps~2ps Fe spins reverse and begin to form “stable” sublattice. AFM exchange field drives Gd to opposite state. ~3ps 0ps time

Summary/Outlook Demonstrated numerically switching can occur using only a heat pulse without the need for magnetic field. Shown that reversal with polarised light on thin films can occur independently on polarisation and initial state. Microstructures show switching under the action of linearly polarised laser. Have shown that stray fields do not play an effect in the mechanism. The importance of the non-equivalence in longitudinal relaxation times of the sublattices. This switching mechanism is a feature of this type of ferrimagnetic material and only requires heat!

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/ ) Grants No. NMP3-SL (UltraMagnetron) and No (FANTOMAS), Spanish MICINN project FIS C02-02 European Research Council under the European Union’s Seventh Framework Programme (FP7/ )/ ERC Grant agreement No (Femtomagnetism). NASU grant numbers and Thank you for listening.

Numerical Model Energetics of system described by Hamiltonian: Dynamics of each spin given by Landau-Lifshitz-Gilbert Langevin equation. Moments defined through the fluctuation dissipation theorem as: Effective field given by:

Helicity Independent Switching Previously it was shown that all-optical reversal controllable using circular polarisation of light[1]. Beyond a certain pump fluence we have shown that this control is not possible and the system reverses independently on polarisation. Below threshold fluence pump fluence see control of magnetisation. High Fluence Low Fluence [1] - Stanciu et al. Phys. Rev. Lett. 99, (2011).

The Effect of Compensation Previous studies have tried to switch using the changing dynamics at the compensation point[ref]. Simulations show starting temperature not important. Supported by experiments on different compositions of GdFeCo support the numerical observation.

Effect of Field So far all results show reversal in no field, with numerical model showing the mechanism is mediated by the transient ferromagnetic-like state. What happens now if we apply a field to oppose the formation of this state? Numerical model shows that in certain conditions the field required to prevent the formation of this state can be 40T! Field required to prevent formation of this state depends on measurement time as system will begin to precess back.