Orbital and magnetic ordering in single-layer FePS3: A DFT+U study

Among the numerous two-dimensional (2D) systems that can be prepared via exfoliation, iron phosphorus trisulfide (FePS3) attracted a lot of attention recently due to its broad-range photoresponse, its unusual Ising-type magnetic order and possible applications in spintronic nanodevices. Despite various experimental and theoretical-computational reports, there are still uncertainties in identifying its magnetic ground state. In this paper, we investigate the structural and magnetic properties of single-layer FePS3 by using density functional theory. Our findings show that orbital ordering leads to a variation in distance between pairs of iron atoms by 0.14 angstrom. These lattice distortions, albeit small, trigger different (ferromagnetic and antiferromagnetic) exchange couplings so that the ground state consists of ferromagnetically aligned zigzag chains along the long Fe-Fe bonds which couple antiferromagnetically along the shorter Fe-Fe bonds. Within the DFT + U framework, we parametrize a spin Hamiltonian including Heisenberg, single-ion anisotropy, Dzyaloshinskii-Moriya, and biquadratic interactions. Using U = 2.22 eV gives a consistent description of both the electronic band gap and the Neel temperature in 2D FePS3.