On the Specific Heat of Fe(HCOO)₂·2H₂O

Abstract

A theoretical analysis of the magnetic specific heat of Fe(HCOO)₂·2H₂O is performed on the basis of a two site model, R_A=1⁄2 and S_B=2, where R_A is the fictitious spin for the lowest doublet. The contribution to the specific heat from A and B sites can be calculated separately and the effect of weak AB interaction is considered by the high-temperature series expansion technique. The experimental result turns out to be well explained by introducing a large S_z⁴-term for B site ions. As a microscopic origin of S_z⁴-term, the magnetostrictive energy at B site is shown to be favorable.