Two-dimensional ferromagnetic semiconductors of monolayer BiXO 3 (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 ( T c ), and large magnetic anisotropy. Using first-principles calculations, we predict that two ferromagnetic monolayers, BiXO 3 (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 T c beyond 400 K. Interestingly, both BiXO 3 monolayers exhibit out-of-plane magnetic anisotropy, with magnetic anisotropy energy (MAE) of 1.07 meV per Ru for BiRuO 3 and 5.79 meV per Os for BiOsO 3 . The estimated MAE for the BiOsO 3 sheet is one order of magnitude larger than that for the CrI 3 monolayer (685 μeV per Cr). Based on the second-order perturbation theory, it is revealed that the large MAE of the monolayers BiRuO 3 and BiOsO 3 is mainly contributed by the matrix element differences between d xy and d x 2 - y 2 and d yz and d z 2 orbitals. Importantly, the ferromagnetism remains robust in 2D BiXO 3 under compressive strain, while undergoing a ferromagnetic to antiferromagnetic transition under tensile strain. The intriguing electronic and magnetic properties make BiXO 3 monolayers promising candidates for nanoscale electronics and spintronics.