D. Chaves-O’Flynn1, Daniel L. Stein2 and P. Kuświk1
1 Institute of Molecular Physics, Polish Academy of Sciences, Poznań, Poland
2 Ctr. for Quantum Phenomena and Dept. of Physics, NYU, New York, USA
We present String Method calculations of Minimum Energy Paths for thermally activated transitions in ferromagnetic nanodisks with perpendicular anisotropy and varying strength (D) of the interfacial Dzyaloshinskii-Moriya interaction. We show that motion of topological singularities in the disk provide a good description for thermally activated switching between metastable micromagnetic configurations.
We have identified three regimes as D increases from slightly below Dc (uniform phase) to several times Dc (cycloid phase). For low D, isolated skyrmions drift to the edge of the nanodisk where they are expelled; in intermediate D, the expulsion of individual merons allow for the existence of metastable states with half integer difference in topological number; for large D there exist multiple energy minima which can be described as networks of disclinations connected by tubular structures of zero topological number. In this regime, additional transition states are related to merging of two separate –p disclinations (Fig 2e) into a single –2p (Fig 2f) disclination. This work highlights the importance of the dynamic of topological singularities in thermally activated processes in micromagnetic systems.
This research was supported in part by the US National Science Foundation Grant No. DMR 1610416, and the National Science Centre Poland under OPUS funding grant No. 2019/33/B/ST5/02013 Polish National Science Center, project no. UMO-2018/30/Q/ST3/00416.
Fig. 1 Left: MEP obtained from String Method Calculation. Right: (a) Helical Phase and typical (b,c,e,f) disclinations. (d) Skyrmion (g) meron embedded in cycloidal background.