Kinetic energy driven two-sublattice double-exchange: a general mechanism of magnetic exchange in transition metal compounds.

One of the most important phenomena in magnetism is the exchange interaction between magnetic centres. In this topical review, we focus on the exchange mechanism in transition-metal compounds and establish kinetic-energy-driven two-sublattice double-exchange as a general mechanism of exchange, in addition to well-known mechanisms like superexchange and double exchange. This mechanism, which was first proposed (Sarmaet al2000Phys. Rev. Lett.852549), in the context of Sr₂FeMoO₆, a double-perovskite compound, later found to describe a large number of 3d and 4d or 5d transition metal-based double perovskites. The magnetism in multi-sublattice magnetic systems like double-double and quadrupolar perovskites involving 3d and 4d or 5d transition-metal ions have also been found to be governed by this as a primary mechanism of exchange. For example, the numerical solution of a two-sublatice double exchange with additional superexchange couplings for the FeRe-based double double and quadrupolar perovskites are found to reproduce the experimentally observed magnetic ground state as well as the high transition temperature of above 500 K. The applicability of this general mechanism extends beyond the perovskite crystal structures, and oxides, as demonstrated for the pyrochlore oxide, Tl₂Mn₂O₇and the square-net chalcogenides KMnX₂(X = S, Se, Te). The counter-intuitive doping dependence and pressure effect of magnetic transition temperature in Tl₂Mn₂O₇is explained, while KMnX₂(X = S, Se, Te) compounds are established as half-metallic Chern metals guided by two sublattice double exchange. While the kinetic energy-driven two-site double-exchange mechanism was originally proposed to explain ferromagnetism, a filling-dependent transition can lead to a rare situation of the antiferromagnetic metallic ground state, as found in La-doped Sr₂FeMoO₆, and proposed for computer predicted double perovskites Sr(Ca)₂FeRhO₆. This opens up a vast canvas to explore.