Skyrmion lattices are present in many different chiral magnetic systems and their existence derives from a number of competing interactions. In a recent work 1, we showed that Pt/FeCoB/Al₂O₃ multilayers can host skyrmion lattices over a range of applied perpendicular fields. A particular feature of this material system is the presence of a "high-frequency" mode (HFM) under transverse field pumping, in the range of 12-18 GHz, which involves the coherent precession of the skyrmion cores and results in the generation of spin waves that flow into the uniform background region between skyrmions. Here, we discuss the results of micromagnetics simulations with the MuMax3 code 2 where we examine the role of spin wave interactions with the skyrmion cores. As the applied field increases and skyrmions begin to annihilate, leaving vacancies in the hexagonal lattice, the excitation of the HFM can serve to "anneal" the lattice, resulting in a glassy state as shown in the Figure. The spin wave interactions are found to provide a dynamical repulsive force between the skyrmion cores, which result in a more homogeneous arrangement in terms of density, but at the expense of crystalline order. These results also shed new light on how dynamical excitations can influence phase transitions associated with the melting of skyrmion lattices 3.

References
1 T. Srivastava et al, arXiv:2111.11797 (2021). 2 A. Vansteenkiste et al, AIP Advances, vol. 4, 107133 (2014). 3 P. Huang et al, Nat. Nanotechnol., vol. 15, 761 (2020).

Simulated MFM images of skyrmion lattices under different applied fields, before and after the excitation of a high-frequency mode (HFM) that generates spin waves. The insets show discrete Fourier transforms of the spatial order.