FeS2 monolayer: A high-valence and high-TC Ising ferromagnet

Two-dimensional (2D) magnetic materials are currently of great interest for their promising applications in spintronics. Strong magnetic coupling and anisotropy are both highly desirable for the achievement of hightemperature magnetic order. Here we propose the unusual high -valence FeS2 hexagonal monolayer as such candidate for a strong Ising 2D ferromagnet by spin -orbital state analyses, first -principles calculations, and the renormalized spin -wave theory (RSWT). We find that, very importantly, the high -valence Fe4+ ion is in the low -spin state (t42g, S = 1) with degenerate t2g orbitals rather than the high -spin state (t32ge1g, S = 2). It is the low -spin state that allows us to carry a large perpendicular orbital moment and then produces a huge single -ion anisotropy (SIA) of 25 meV/Fe. Moreover, the negative charge -transfer character associated with the unusual high valence, strong Fe 3d -S 3p hybridization, wide bands, and a small band gap all help to establish a strong superexchange. Indeed, our first -principles calculations confirm the strong ferromagnetic superexchange and the huge perpendicular SIA, both of which are further enhanced by a compressive strain. Then, our RSWT calculations predict that the ferromagnet TC is 261 K for the pristine FeS2 monolayer and could be increased to 409 K under -5% compressive strain. The high TC is also reproduced by our Monte Carlo simulations. Therefore, it is worth exploring high-TC Ising ferromagnets in high -valence 2D magnetic materials with degenerate orbitals.