In this talk, we first investigate the mechanisms that govern the thermal stability of two technologically relevant systems: magnetic skyrmions, envisioned as information carriers in novel spintronics devices, and nanopillars, as used in magnetic tunnel junctions (MTJs) for data storage, and currently envisaged as stochastic p-bits for unconventional computing schemes. To do so, we combine the Kramers framework and a path sampling scheme in atomistic simulations.
In skyrmions, we find that their internal modes of deformation lead to a stabilizing entropic contribution to the Arrhenius prefactor [1]. On the other hand, in nanopillars, the domain-wall mediated magnetization reversal yields a large, destabilizing entropic contribution to the prefactor, which scales like an exponential of the activation energy [2]. Our simulations show good agreement with experimental measurements carried out on 50 nm diameter perpendicular MTJs, with mean dwell times down to the nanosecond [3]. We explain this observation with very low Arrhenius prefactors in the fs range. In both cases, we challenge the common assumption of a constant, 1 ns attempt time in these systems.

We then explore the Eigenmodes of periodic topological phases in ultrathin films [4], with potential applications in nanomagnonics and wave electronics. We compare the skyrmion lattice (skX) of the prototype system Pd/Fe/Ir(111), with Fe/Ir(111), host of the nanoskX. While we find higher order modes of deformation for skyrmions in Pd/Fe/Ir, we show how a distinct stabilization mechanism dominated by the 4-spin interaction in Fe/Ir leads to fundamentally different states.
Last, we look at the electric field control of skyrmions, a compelling low-power alternative to their manipulation via electric currents. We show how an electric field can induce a DMI-like Rashba contribution in metallic ultrathin films, and we exploit this effect in dynamics simulations combined with first principles calculations to demonstrate antiferromagnetic skyrmion nucleation at the 100 fs scale [5].

1. Desplat et al. PRB 98, 134407 (2018) ; Desplat et al. PRB 99, 174409 (2019) ; Desplat et al. PRB 101, 060403(R) (2020)
2. Desplat & Kim, PRL 125, 107201 (2020) ; Desplat & Kim, PR Appl. 14, 064064 (2020)
3. Soumah, Desplat et al. arXiv:2402.03452 (2024)
4. Desplat & Dupe, PRB 107, 144415 (2023) ; Yang, Desplat, et al, (2024), npj Quant. Mater. 9, 54 (2024)
5. Desplat et al., PRB 104 (6), L060409 (2021)
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More information :https://www.spintec.fr/seminar-atomistic-exploration-of-topological-states-in-ultrathin-magnetic-films-and-nanopillars/​

Videoconference ​: https://univ-grenoble-alpes-fr.zoom.us/j/98769867024?pwd=dXNnT3RMeThjYStybGVQSUN0TVdJdz09