Collective effects in spin-crossover chains with exchange interaction

TL;DR

This paper investigates the collective magnetic and thermal properties of one-dimensional spin-crossover chains with elastic and exchange interactions, revealing phase diagrams and temperature-dependent behaviors relevant for molecular memory applications.

Contribution

It provides an exact transfer-matrix analysis of spin-crossover chains, mapping their phase diagrams and exploring effects of exchange interactions on their collective properties.

Findings

Mapped ground-state phase diagram of spin-crossover chains

Derived Curie-Weiss law for susceptibility and calculated Curie temperature

Identified sharp crossovers between low- and high-temperature regimes

Abstract

The collective properties of spin-crossover chains are studied. Spin-crossover compounds contain ions with a low-spin ground state and low lying high-spin excited states and are of interest for molecular memory applications. Some of them naturally form one-dimensional chains. Elastic interaction and Ising exchange interaction are taken into account. The transfer-matrix approach is used to calculate the partition function, the fraction of ions in the high-spin state, the magnetization, susceptibility, etc., exactly. The high-spin-low-spin degree of freedom leads to collective effects not present in simple spin chains. The ground-state phase diagram is mapped out and compared to the case with Heisenberg exchange interaction. The various phases give rise to characteristic behavior at nonzero temperatures, including sharp crossovers between low- and high-temperature regimes. A Curie-Weiss law for the susceptibility is derived and the paramagnetic Curie temperature is calculated. Possible experiments to determine the exchange coupling are discussed.