The Magnetic Properties of the Iron Group Anhydrous Chlorides at Low Temperatures. II. Theoretical

Abstract

A theoretical study is made of the experimental results reported in Part I. The predictions of paramagnetic theory are compared with the room-temperature results, and an attempt is made to explain some of the departures from theory in terms of the crystalline field. The low temperature experimental anomalies require the development of a theory of metamagnetism. The assumption is made that at temperatures below the Curie point neighboring atoms behave as groups, which are spontaneously formed at the Curie point. The atomic magnetic moments within each group are assumed to be either parallel or antiparallel to each other, and the antiparallel arrangement is assumed to have the lowest energy. The theory developed on these assumptions satisfactorily explains the general behavior of the metamagnetic chlorides (Cr${\mathrm{Cl}}_3$, Fe${\mathrm{Cl}}_2$, Co${\mathrm{Cl}}_2$, and Ni${\mathrm{Cl}}_2$), and also gives a qualitative explanation of the behavior of the other anhydrous chlorides which exhibit a minimum in the reciprocal susceptibility $\mathrm{vs}$. temperature curve but do not show any field dependence of the susceptibility.