Path sampling for lifetimes of metastable magnetic skyrmions and direct comparison with Kramers’ method

Our recent study on the lifetime of magnetic skyrmions, led by Louise Desplat, has just appeared as a Rapid Communication in Physical Review B. We studied the issue of thermal stability theoretically using two distinct methods. The first involves a generalisation of Kramer’s framework to multidimensional phase spaces due to Langer, while the second is a path sampling method, called forward flux sampling, which is useful for simulating rare events.

Despite the different assumptions that underpin these two approaches, we find overall quantitative agreement between the two, as shown in the figure above in which we plot the Arrhenius prefactor, f0, and the average lifetime, tau, as a function of applied magnetic field. This result provides confirmation of the validity of Langer’s approach (and consequently, similar techniques like harmonic transition state theory), and represents the first application of path sampling methods for studying skyrmion lifetimes.

Trochoidal dynamics and pair generation in skyrmion and antiskyrmion dynamics driven by spin-orbit torques

In a collaboration with colleagues at Johannes Gutenberg-Universität Mainz, Uppsala Universitet, and Christian-Albrechts-Universität zu Kiel, we have shown theoretically that skyrmions and antiskyrmions can behave very differently under spin-orbit torques. One key finding is that antiskyrmions can undergo trochoidal motion, which is analogous to Walker breakdown in domain wall dynamics. This threshold defines a speed limit for rectilinear displacement. Another is the onset of skrymion-antiskyrmion pair generation under large damping-like torques, which results in a gas with excess skyrmions. This imbalance in matter-antimatter production is a consequence of the asymmetric dynamics at high energies.

This work recently appeared in Nature Electronics.

Reference: U. Ritzmann, S. von Malottki, J.-V. Kim et al., Nat. Electron. 1, 451–457 (2018).

Current-driven skyrmion dynamics in disordered films

Reference: J.-V. Kim and M.-W. Yoo, Appl. Phys. Lett. 110, 132404 (2017).

We studied how current-driven skyrmion motion in ultrathin films is affected by disorder. We modelled the disorder by assuming a grain structure, where the local perpendicular anisotropy fluctuates from grain to grain. We find that the velocity versus current curves are reminiscent of behavior in driven elastic interfaces in disordered media, such as domain wall creep. Moreover, we identify an extrinsic contribution to the skyrmion Hall effect due to disorder scattering, which is drive dependent. This work has just appeared in Applied Physics Letters.