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PGI-1 Talk: Louise Desplat

Thermal stability of metastable magnetic skyrmions

begin
29 May 2019 11:30
venue
Seminar room 142

Univ. Paris-Sud, France

Abstract:

Magnetic skyrmions are localized, particlelike magnetic spin textures carrying an integer topological charge. Isolated magnetic skyrmions live on the ferromagnetic background as metastable excitations, but will eventually collapse back to the uniformly magnetized state. The rate of such thermally activated processes across an energy barrier can be described by the Arrhenius law. In recent year, magnetic skyrmions have attracted interest for spintronics applications as racetrack memories and logic gates. For such applications, a 10-year retention rate against thermal fluctuations is a technical requirement, and being able to estimate the stability of skyrmions is therefore of crucial importance.

We are thus faced with the problem of calculating rate constants of rare events. For structures with lifetimes well above the nanosecond range, direct Langevin simulations fail due to the exponential increase in CPU hours required to obtain reliable statistics. One possible approach is a form of transition state theory (TST), which allows for a direct computation of the rate prefactor once the first order saddle point has been precisely identified along the transition path. We firstly follow this approach in the form of Langer's theory in conjunction with the GNEB scheme to obtain lifetimes of skyrmions. While this is undoubtedly a computationally optimal solution, the use of TST is based on many assumptions which may not always hold. For this reason, we propose an alternative method: forward flux sampling (FFS), which enables an exploration of phase space free from assumptions. We vary the destabilizing applied magnetic field and obtain a good agreement between FFS and Langer calculations of a skyrmion's collapse rate. Within both methods, we report variations of the Arrhenius prefactor by several orders of magnitude when varying the applied field, which translates a strong entropic contribution to skyrmions' stability linked with their internal eigenmodes.

Contact

Prof. Dr. Stefan Blügel
Phone: +49 2461 61-4249
Fax: +49 2461 61-2850
email: s.bluegel@fz-juelich.de

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