Hu Xu-Chong, Wu Hai-Tao, Wang X R
Physics Department, The Hong Kong University of Science and Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong.
HKUST Shenzhen Research Institute, Shenzhen 518057, China.
Nanoscale. 2022 May 26;14(20):7516-7529. doi: 10.1039/d2nr01300b.
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 , 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 is above a critical value. The lowest energy state is still a stripy helical state no matter with or without a field when 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 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.
本文提出了一种关于手性磁性薄膜中斯格明子晶体（SkX）形成及其迷人热力学行为的通用理论。当测量相对Dzyaloshinskii-Moriya相互作用（DMI）强度的参数足够大时，手性磁性薄膜可以具有许多亚稳态，其斯格明子密度可任意达到最大值。在没有磁场的情况下，无限薄膜的最低能量状态是占据整个薄膜的长之字形分支条纹斯格明子。在垂直于薄膜的中间场作用下，最低能量状态具有有限的斯格明子密度。这就是为什么当参数高于临界值时，手性磁性薄膜在低场时通常处于条纹状态，而仅在最佳场附近形成SkX。当参数低于临界值时，无论有无磁场，最低能量状态仍然是条纹螺旋状态。多种亚稳态解释了薄膜各种亚稳态的热力学路径依赖性。参数值随温度的降低解释了为什么在许多斯格明子系统中，SkX在低温下会变成亚稳态。这些发现为SkX的操控和基于斯格明子的应用开辟了一条新途径。
