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

A room-temperature metastable skyrmion phase, which undergoes reversible transitions between a triangular and square lattice upon varying the temperature and magnetic field, is found in β-Mn-type Co 8 Zn 8 Mn 4 . Skyrmions, topologically protected nanometric spin vortices, are being investigated extensively in various magnets 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 . Among them, many 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 c , while a helical or conical magnetic state prevails at lower temperatures. Here we describe that for a room-temperature skyrmion material 12 , β-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. 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.