Prediction of two-dimensional large-gap magnetic semiconductors in transition metal superhalogenides

Abstract

The coexistence of magnetism and semiconductivity in two-dimensional (2D) systems is of particular interest for both physics and application research. In recent years, several van der Waals (vdW) layered transition metal halides have been recognized as 2D magnetic semiconductors and received tremendous attention. However, the predicted candidates of 2D magnetic semiconductors are still very scarce, which hinders the synthesis and application of 2D magnetic semiconductors. Here, using first-principles calculations, we predict that the superhalogen, B(CN)₄, can act as a ligand and form strong chemical bonds with transition metal ions, leading to 2D transition metal superhalogenides that show a similar structural framework to transition metal halides (e.g. NiI₂). Most of these structures are magnetic semiconductors with large energy gaps and flat bands near the Fermi level. Among them, the Co[B(CN)₄]₂ monolayer is a stable ferromagnetic (FM) semiconductor exhibiting large out-of-plane magnetic anisotropy (∼0.3 meV per Co). Its Curie temperature is estimated to be ∼65 K. Moreover, a magnetic phase transition is observed under a small in-plane biaxial strain. These findings classify the transition metal superhalogenides as novel vdW 2D materials and expand the family of 2D magnetic semiconductors.