Superexchange Interaction in Magnetic Insulators with Spin Crossover

Gavrichkov, V. A.; Polukeev, S. I.; Ovchinnikov, S. G. Source: JOURNAL OF EXPERIMENTAL AND THEORETICAL PHYSICS, 127 (4):713-720; 10.1134/S1063776118100023 OCT 2018

We present the derivation of a microscopic superexchange Hamiltonian for undoped magnetic insulators with an arbitrary spin. It is established that the sign of the (ferromagnetic or antiferromagnetic) superexchange between magnetic ions in the dn configuration depends on the spin nature of virtual multielectron states dn± 1, viz., low-spin or high-spin partners with S ± 1/2 relative to ground state of the dn configuration with spin S. A macroscopic substantiation is given for the Goodenough–Kanamori rules and simple mean-field estimates connecting the magnetic ordering temperature with the exchange constant. The conventional Anderson superexchange for magnetic materials with spin S = 1/2 and the P/T magnetic phase diagram for ferroborate FeBO3 with spin crossover S = (5/2 ↔ 1/2) at the Fe3+ ion under a high pressure are also reproduced as a test.