Fig. 2

Thermodynamic stability diagram. a, Stability diagram of the quasi single domain (QSD), Néel skyrmion, and Néel stripe (represented by different line patterns) as a function of the CoFeB disk diameter and the interfacial DMI strength D. Typical spin structures are shown on the right. Areas of stable (i.e., ground-state) QSD, skyrmion, and stripe are colored blue, green, and yellow, respectively. Areas with overlapping line patterns indicate the existence of one or two metastable spin structures in addition to the stable spin structure. Color bar m_z denotes local normalized perpendicular magnetization. Dependence of the intrinsic magnetic free energy density f_intrin and the skyrmion diameter d_Sk on b, the interfacial DMI strength D, and c, the CoFeB disk diameter. The disk diameter is 220 nm for all cases in b, while the D is 0.75 mJ/m² for all cases in c. When D < 0.75 mJ/m² in b and when disk diameter < 140 nm in c (marked by blue crossed circles), the skyrmion exhibits a notably higher energy level than the other spin structures. These highly metastable skyrmions appear at equilibrium in simulations only when the in-plane size of a simulation cell is relatively small (i.e., 0.5 nm). The vertical line in b marks the critical interfacial DMI strength D_c ( ≈ 0.7 mJ/m²) for a 220-nm-diameter 1.1-nm-thick CoFeB disk