Confinement of skyrmion states in non-centrosymmetric magnets near ordering temperatures

Abstract

I consider skyrmionic states in non-centrosymmetric magnets near the ordering temperatures. I show that the interaction between the chiral skyrmions, being repulsive in a broad temperature range, changes into attraction at high temperatures. This leads to a remarkable confinement effect: near the ordering temperature skyrmions exist only as bound states. The confined skyrmions as discernible units may arrange in different mesophases. The confinement of multiple skyrmions is a consequence of the coupling between the magnitude and the angular part of the order parameter. Thus, near the ordering transitions, I consider the local magnetization not only as multiply twisted but also longitudinally modulated. From numerical investigations on 2D models of isotropic chiral ferromagnets, a staggered half-skyrmion square lattice at zero and low fields and a hexagonal skyrmion lattice at larger fields are found in overlapping regions of the phase diagram near the transition temperature. The creation of skyrmions as stable units and their condensation into different extended textures occurs simultaneously through a rare case of an instability-type nucleation transition1. As well, I introduce a new fundamental parameter, confinement temperature separating the peculiar region with "confined" chiral modulations from the main part of the phase diagram with regular helical and skyrmion states.