FeS2 monolayer: a high valence and high-T_ C Ising ferromagnet

Two-dimensional (2D) magnetic materials are of current great interest for their promising applications in spintronics. Strong magnetic coupling and anisotropy are both highly desirable for the achievement of a high temperature magnetic order. Here we propose the unusual high valent FeS_2 hexagonal monolayer as such a candidate for a strong Ising 2D ferromagnet (FM), by spin-orbital state analyses, first-principles calculations, and the renormalized spin-wave theory (RSWT). We find that very importantly, the high valent Fe^4+ ion is in the low-spin state (t_2g^4, S=1) with degenerate t_2g orbitals rather than the high-spin state (t_2g^3e_g^1, S=2). It is the low-spin state that allows 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 FM superexchange and the huge perpendicular SIA, both of which are further enhanced by a compressive strain. Then, our RSWT calculations predict that the FM T_ C is 261 K for the pristine FeS_2 monolayer and could be increased to 409 K under the compressive –5% strain. The high T_ C is also reproduced by our Monte Carlo (MC) simulations. Therefore, it is worth exploring the high-T_ C Ising FMs in the high valent 2D magnetic materials with degenerate orbitals.