Two-dimensional ferroelastic and ferromagnetic NiOX (X = Cl and Br) with half-metallicity and a high Curie temperature

Abstract

Two-dimensional (2D) multiferroic materials with distinctive properties, such as half-metallicity, high Curie temperature (T_C), and magnetoelastic coupling, hold potential applications in novel nanoscale spintronic devices, but they are rare. Using density functional theory (DFT) calculations and evolutionary algorithms, we identify new types of 2D NiOX (X = F, Cl and Br) monolayers that are stable in energy, dynamics, thermodynamics, and mechanics. Among them, NiOF is an indirect-gap antiferromagnetic (AFM) semiconductor, while NiOCl and NiOBr are half-metallic materials with ferromagnetic (FM) ordering with a T_C of 671 and 692 K and in-plane magnetic anisotropy energies (MAEs) of 541 and 609 μeV per Ni along the x-axis and y-axis, respectively. Notably, ferroelasticity is another important feature of NiOCl and NiOBr monolayers with energy barriers of 234.0 and 151.5 meV per atom, respectively. Moreover, the in-plane magnetic easy axis is strongly coupled to the lattice direction. The coexistence of high ferromagnetism, ferroelasticity, half-metallicity, and magnetoelastic coupling renders NiOCl and NiOBr monolayers great potential for future nanodevices.