Robust metastable skyrmions and their triangular-square lattice-structural transition in a high-temperature chiral magnet

TL;DR

This study demonstrates the creation of robust, metastable skyrmion states in a room-temperature chiral magnet, which can transition between triangular and square lattice structures, broadening potential applications.

Contribution

It reveals that field-cooling can stabilize metastable skyrmions over wide conditions and uncovers reversible lattice transitions, advancing skyrmion control at room temperature.

Findings

Metastable skyrmions survive from zero temperature to critical magnetic field.

Reversible lattice transition between triangular and square skyrmion lattices.

Metastable skyrmions exhibit topological robustness.

Abstract

Skyrmions, topologically-protected nanometric spin vortices, are being investigated extensively in various magnets. Among them, many of structurally-chiral cubic magnets host the triangular-lattice skyrmion crystal (SkX) as the thermodynamic equilibrium state. However, this state exists only in a narrow temperature and magnetic-field region just below the magnetic transition temperature $T_\mathrm{c}$, while a helical or conical magnetic state prevails at lower temperatures. Here we describe that for a room-temperature skyrmion material, $\beta$-Mn-type Co$_8$Zn$_8$Mn$_4$, a field-cooling via the equilibrium SkX state can suppress the transition to the helical or conical state, instead realizing robust metastable SkX states that survive over a very wide temperature and magnetic-field region, including down to zero temperature and up to the critical magnetic field of the ferromagnetic transition. Furthermore, the lattice form of the metastable SkX is found to undergo reversible transitions between a conventional triangular lattice and a novel square lattice upon varying the temperature and magnetic field. These findings exemplify the topological robustness of the once-created skyrmions, and establish metastable skyrmion phases as a fertile ground for technological applications.