The thermal stability of magnetic skyrmions is a key issue for potential applications in spintronic devices. An Arrhenius law can be used to describe the skyrmion lifetime as a function of temperature, which requires knowledge of the energy barrier and the pre-exponential factor. While the energy barrier has already been addressed by several studies (e.g. [1]), the pre-exponential factor for the skyrmion collapse remains largely unexplored [2, 3]. Here, we obtain skyrmion lifetimes by calculating not only the energy barriers but also the pre-exponential factors for ultrathin films such as Pd/Fe bilayers on Ir(111) – a system which has been extensively studied from experiment [4]. We use an atomistic spin model based on parameters from first-principles via density functional theory [1]. In our approach, the minimum energy paths and thereby the energy barriers are calculated using the geodesic nudged elastic band method, while the pre-exponential factors are obtained using harmonic transition state theory [3]. We demonstrate that depending on the system the pre-exponential factor can change by orders of magnitude with magnetic field and thereby becomes crucial for skyrmion lifetimes. With our first-principles based approach we make predictions for the stability of skyrmions in other ultrathin film systems.

[1] von Malottki, S., et al. (2017) Enhanced skyrmion stability due to exchange frustration. Sci. Rep. 7, 12299.

[2] Wild, J., et al. (2017) Entropy-limited topological protection of skyrmions. Sci. Adv. 3, e1701704.

[3] Bessarab, P. F., et al. (2017) Lifetime of racetrack skyrmions. Sci. Rep. 8, 3433.

[4] Romming, N., et al. (2015) Field-Dependent Size and Shape of Single Magnetic Skyrmions. Phys. Rev. Lett. 114, 177203.