A theory of skyrmion crystal formation

A generic theory about skyrmion crystal (SkX) formation in chiral magnetic thin films and its fascinating thermodynamic behaviours is presented. A chiral magnetic film can have many metastable states with an arbitrary skyrmion density up to a maximal value when the parameter kappa, which measures the relative Dzyaloshinskii-Moriya interaction (DMI) strength, is large enough. The lowest energy state of an infinite film is a long zig-zag ramified stripe skyrmion occupying the whole film in the absence of a magnetic field. Under an intermediate field perpendicular to the film, the lowest energy state has a finite skyrmion density. This is why a chiral magnetic film is often in a stripy state at a low field and a SkX only around an optimal field when kappa is above a critical value. The lowest energy state is still a stripy helical state no matter with or without a field when kappa is below the critical value. The multi-metastable states explain the thermodynamic path dependences of the various metastable states of a film. The decrease of the kappa value with the temperature explains why SkXs become metastable at low temperatures in many skyrmion systems. These findings open a new avenue for SkX manipulation and skyrmion-based applications.

Automated summary

A generic theory about the formation and thermodynamic behaviors of skyrmion crystal in chiral magnetic thin films is presented in this article. The existence of multiple metastable states in chiral magnetic films explains the path dependence of the various metastable states. The findings open up new possibilities for the manipulation and application of skyrmion crystals.