Ferromagnetic TM₂BC (TM = Cr, Mn) monolayers for spintronic devices with high Curie temperature

Transition metal boro-carbide (TM2BC) structures crystallize in the layered orthorhombic structure in their bulk phases. In this study, however, we find that TM2BC (TM = Cr, Mn) prefer a tetragonal (t) crystal structure in their monolayer phases due to the occurrence of strong sp2 bonds between the metal and B/C atoms instead of sp3 + sp2 bonds which exist in the orthorhombic phase. The calculated energy difference between the orthorhombic and the tetragonal structures based on density functional theory (DFT) is more than 1 eV per unit cell. In addition, t-Cr2BC and t-Mn2BC monolayers are dynamically and thermally stable with their magnetic metal electronic structures. For further investigations, we combine our DFT calculations with the Monte Carlo simulations and find that both t-TM2BC monolayers show ferromagnetic properties. The calculated Curie temperatures are 846 K and 128 K for t-Cr2BC and t-MnBC sheets, respectively. In addition, we examine the magnetic anisotropy energies (MAE) of t-TM2BC monolayers and find that both structures prefer out-of-plane as the easy axis magnetization direction and the applied electric field can easily modulate the MAE of the monolayers. Our theoretical calculations reveal that t-TM2BC (TM = Cr, Mn) sheets have great potential for the future design of controllable spintronic devices with their tunable MAE properties.