Skyrmion fractionalization and merons in chiral magnets with easy-plane anisotropy

TL;DR

This paper investigates how easy-plane anisotropy affects skyrmion lattices in chiral magnets, revealing a transition from skyrmion to vortex-antivortex phases and metastable meron states.

Contribution

It provides a detailed phase diagram showing the evolution of skyrmion lattices with anisotropy and uncovers metastable meron solutions during phase transitions.

Findings

Skyrmion number decreases continuously with increasing anisotropy.

Transition from skyrmion lattice to vortex-antivortex lattice is first order.

Metastable meron solutions appear near phase boundaries.

Abstract

We study the equilibrium phase diagram of ultrathin chiral magnets with easy-plane anisotropy $A$. The vast triangular skyrmion lattice phase that is stabilized by an external magnetic field evolves continuously as a function of increasing $A$ into a regime in which nearest-neighbor skyrmions start overlapping with each other. This overlap leads to a continuous reduction of the skyrmion number from its quantized value $Q=1$ and to the emergence of antivortices at the center of the triangles formed by nearest-neighbor skyrmions. The antivortices also carry a small "skyrmion number" $Q_A \ll 1$ that grows as a function of increasing $A$. The system undergoes a first order phase transition into a square vortex-antivortex lattice at a critical value of $A$. Finally, a canted ferromagnetic state becomes stable through another first order transition for a large enough anisotropy $A$. Interestingly enough, this first order transition is accompanied by {\it metastable} meron solutions.