Inter-Cation Charge Transfer Mediated Antiferromagnetism in Co${}_{1+x}$Ir${}_{2-x}$S${}_4$

Abstract

The antiferromagnetism in transition metal compounds is mostly mediated by the bridging anions through a so-called superexchange mechanism. However, in materials like normal spinels $A{B}_2{X}_4$ with local moments only at the $A$ site, such an anion-mediated superexchange needs to be modified. Here we report a new spinel compound Co${}_{1+x}$Ir${}_{2-x}$S${}_4$ ($x$ = 0.3). The physical property measurements strongly suggest an antiferromagnetic-like transition at 292 K in the Co($A$) diamond sublattice. The first-principle calculations reveal that the nearest-neighbor Co($A$) spins align antiferromagnetically with an ordered magnetic moment of 1.67 $\hat{I}{¼}_{\mathrm{B}}$, smaller than the expected $S=3/2$ for Co${}^{2+}$. In the antiferromagnetic state, there exists an inter-cation charge-transfer gap between the non-bonding Ir-$t_{2g}$ orbitals at the valence band maximum and the Co-S antibonding molecular orbitals at the conduction band minimum. The small charge transfer energy significantly enhances the virtual hopping between these two states, facilitating a robust long-range superexchange interaction between two neighboring CoS${}_4$ complexes, which accounts for the high Néel temperature in Co${}_{1+x}$Ir${}_{2-x}$S${}_4$. This inter-cation charge transfer mediated magnetic interaction expands the traditional superexchange theory, which could be applicable in complex magnetic materials with multiple cations.

10 pages, 7 figures