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