In this talk, 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 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. The applicability of this general mechanism extends beyond the perovskite crystal structures, and oxides, as demonstrated for the pyrochlore oxide, Tl2Mn2O7 and the square-net chalcogenides KMnX2 (X = S, Se, Te). 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 computationally predicted double perovskites Sr(Ca)2FeRhO6. This opens up a vast canvas to explore.

Based on the topical review, Tanusri Saha-Dasgupta and Koushik Pradhan, Tanusri Saha-Dasgupta and Koushik Pradhan 2025 J. Phys.: Condens. Matter 37 023001 (DOI 10.1088/1361-648X/ad841a)