  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.
