Metamagnetism of Methylamine Chrome Alum

Abstract

Methylamine chrome alum shows an antiferromagnetic ordering below 0.015°K–0.02°K and it is known that this ordering results from the magnetic dipole-dipole interaction among the chromic ions. The present paper investigates, on the basis of the molecular field approximation, the phenomenon of transition below the Néel point from the antiferromagnetic state to the paramagnetic state caused by an applied field, at constant temperature or at constant entropy. Depending on the direction of the applied field relative to the crystalline axes and also on the shape of sample, different behaviours of the transition are obtained. For a spherical sample three are studied, namely, the cases of H //[111], H //[100] and H //[10\bar1], and for an infinitely long sample two cases, i.e., the axis of the sample and H are both parallel to either [111] or [100]. In Appendix is given a summary of the previous theoretical study by Kanamori. Motizuki and Yosida of the same kind of antiferro-paramagnetic transition in two-sublattice antiferromagnet, in relation to the present investigation.