Two-dimensional ferromagnetic semiconductors of monolayer BiXO3 (X = Ru, Os) with direct band gaps, high Curie temperatures, and large magnetic anisotropy

Two-dimensional (2D) ferromagnetic semiconductors are highly promising candidates for spintronics, but are rarely reported with direct band gaps, high Curie temperatures (Tc), and large magnetic anisotropy. Using first-principles calculations, we predict that two ferromagnetic monolayers, BiXO3 (X = Ru, Os), are such materials with a direct band gap of 2.64 and 1.69 eV, respectively. Monte Carlo simulations reveal that the monolayers show high Tc beyond 400 K. Interestingly, both BiXO3 monolayers exhibit out-of-plane magnetic anisotropy, with magnetic anisotropy energy (MAE) of 1.07 meV per Ru for BiRuO3 and 5.79 meV per Os for BiOsO3. The estimated MAE for the BiOsO3 sheet is one order of magnitude larger than that for the CrI3 monolayer (685 μeV per Cr). Based on the second-order perturbation theory, it is revealed that the large MAE of the monolayers BiRuO3 and BiOsO3 is mainly contributed by the matrix element differences between dxy and dx2-y2 and dyz and dz2 orbitals. Importantly, the ferromagnetism remains robust in 2D BiXO3 under compressive strain, while undergoing a ferromagnetic to antiferromagnetic transition under tensile strain. The intriguing electronic and magnetic properties make BiXO3 monolayers promising candidates for nanoscale electronics and spintronics.