Cooperative Phenomena in Spin Crossover Systems

TL;DR

This paper develops an effective Hamiltonian to study spin crossover phenomena in magnetic insulators, revealing how cooperativity influences phase transitions and metastable states under pressure and temperature changes.

Contribution

It introduces a realistic multi-band p-d-model to analyze the effects of exchange interactions and cooperativity on spin crossover and phase transitions.

Findings

Quantum phase transition at T=0 without cooperativity.

Cooperativity converts quantum transition into a first-order transition.

Pressure-temperature phase diagram describes magnetization and high-spin states.

Abstract

Within the framework of a realistic multi-band p-d-model, we derived an effective Hamiltonian to describe the exchange interaction effects near the spin crossover in magnetic Mott-Hubbard insulators under pressure. It is shown that single-ion mechanism of spin crossover under change of the crystal field does not lead to a thermodynamic phase transition, however, at T = 0 a quantum phase transition appears. It has been found that the cooperativity leads to a modification of the quantum phase transition to a first-order phase transition and the appearance of metastable states of the system. The pressure - temperature phase diagram has been obtained to describe the magnetization and high-spin population near the spin crossover of the Motts dielectrics with d6-ions.