Magnetism in spin crossover systems: Short-range order and effects beyond the Heisenberg model

Kuz'min, V., I; Orlov, Yu S.; Zarubin, A. E.; Ovchinnikova, T. M.; Ovchinnikov, S. G. Source: PHYSICAL REVIEW B, 100 (14):10.1103/PhysRevB.100.144429 OCT 21 2019

To study non-Heisenberg effects in the vicinity of spin crossover in strongly correlated electron systems we derive an effective low-energy Hamiltonian for the two-band Kanamori model. It contains a Heisenberg high-spin term proportional to exchange constant as well as a low-spin term proportional to spin gap parameter ɛs. Using cluster mean field theory we obtain several non-Heisenberg effects. Near the critical value of spin gap ɛcs there is a magnetic phase transition of first order. In the vicinity of ɛcs in the paramagnetic phase we observe nontrivial behavior of the Curie constant in the paramagnetic susceptibility in the wide range of temperature. Reentrant temperature behavior of nearest-neighbor spin-spin correlations is observed at ɛs>ɛcs. Finally, the pressure-temperature magnetic phase diagram for ferroperriclase is obtained using the effective Hamiltonian.