Electronic structure of spin-orbit coupled vacancy ordered transition metal halides and formation of large magnetic anisotropy energy.

Vacancy-ordered antifluorite materials (A2BX6) are garnering renewed attention as novel magnetic states driven by spin-orbit coupling (SOC) can be realized in them. In this work, by pursuing density functional theory calculations and model studies, we analyze the ground state electronic and magnetic structure of face-centered cubic antifluorites K2ReCl6(KReC, 5d3), K2OsCl6(KOsC, 5d4), and K2IrCl6(KIrC, 5d5). We find that KReC stabilizes in the high-spinS = 3/2 state due to large exchange-splitting as compared to the SOC strength. The KOsC stabilizes inS = 1 simple Mott insulating state while KIrC stabilizes inJeff = 1/2 spin-orbit-assisted Mott insulating state. The presence of an isolated metal-chloride octahedron makes these antifluorites weakly coupled magnetic systems with the nearest and next-nearest-neighbor spin-exchange parameters (J1andJ2) are of the order of 1 meV. For KReC and KOsC, theJ1andJ2are estimated to be antiferromagnetic and ferromagnetic, which leads to a Type-I antiferromagnetic ground state, whereas for KIrC, bothJ1andJ2are antiferromagnetic, hence, it stabilizes with a Type-III antiferromagnetic state. Interestingly, in their equilibrium structure, these antifluorites possess large magnetic anisotropy energy (MAE) (0.6-4 meV/transition metal), which is at least one-to-two orders higher than traditional MAE materials like transition metals and multilayers formed out of them. Moreover, with epitaxial tensile/compressive strain, the MAE enhances by one order, becoming giant for KOsC (20-40 meV/Os).