Two-Dimensional Intrinsic Ferromagnetic Materials of Janus 2H-Nbxy (X, Y = S, Se and Te, X ≠ Y) Monolayers with High Curie Temperature and Large Magnetic Anisotropy

Two-dimensional (2D) intrinsic ferromagnetic materials with large magnetic anisotropy (MA) and high Curie temperature (TC) are desirable for low-dimensional spintronic applications. In this work, a highly stable Janus NbXY (X, Y = S, Se and Te, X not equal Y) monolayers with intrinsic ferromagnetism is investigated by using first-principles calculations. The results demonstrate that the MAE values of the NbSSe, NbSTe and NbSeTe MLs are as high as -2.185 meV, -4.013 meV and -4.495 meV per unit cell, respectively, which is higher than the previously reported 2D intrinsic ferromagnetic materials such as FeBrI, NiI2, and VSSe MLs etc. And they are still large enough for practical applications. The Curie temperature (Tc) is estimated to be 160 K for NbSSe monolayer, 260 K for NbSTe monolayer, and 200 K for NbSeTe monolayer based on Monte Carlo simulation. The MAE and Tc could be effectively controlled and enhanced by the biaxial strains. The MAE of NbSSe, NbSTe and NbSeTe MLs can be increased by 8.7%, 24.9% and 14.1%, and reach up to -2.375, -5.015 and -5.131 meV at 5% compressive strain, respectively. Remarkably, at 5% tensile strain, the room temperature T c of 300 K can be reached for NbSTe monolayer, which is rather promising for its practical application. The large magnetic anisotropy energy and controllable Curie temperature make the NbXY monolayers a promising candidate for applications in spintronic devices at the nanoscale and in high-density data storage.