Variation of skyrmion forms and their stability in MnSi thin plates

By systematic real-space observations using Lorentz transmission electron microscopy, we report here various forms of skyrmions (and related spin textures) for a prototype of cubic helimagnet MnSi as well as their stability that depends on crystalline-orientation and crystal-plate thickness. Below a crossover thickness (t_c ∼ 75 nm) of a sample, the robust two-dimensional hexagonal skyrmion crystal (SkX) appears over a wide window of temperature and magnetic field, regardless of crystal orientation, while the SkX phase region shrinks to a small pocket near the helical transition temperature T_N in a (111) plate with the thickness above t_c, in contrast to the stable SkX in (110) and (001) plates. This observation indicates the importance of the magnetic anisotropy and thermal fluctuation effects for the skyrmion stability. Furthermore, the forms of skyrmions have been found to change from multidomain state of SkX to skyrmion glass structure via single domain state of SkX with increasing magnetic field.