Critical Field Phenomena in an Isotropic Paramagnetic Crystal

Measurements have been made of the variation of magnetic moment with applied field, at constant entropy, for a spherically-ground single crystal of chromic methylammonium alum. Steady fields up to 500 oersteds were applied along a cubic axis of the crystal and the differential susceptibility measured ballistically by reversing a field of 1.7 oersteds in the primary of a mutual inductance surrounding the specimen. The total magnetic moment was obtained by integration. At entropies below that of the "Curie point" the differential susceptibility showed a pronounced maximum in a field of about one hundred oersteds. This behavior is compared with that observed in earlier measurements on the same specimen in transverse fields. The magnetic moment at constant external magnetic field $H$, plotted as a function of entropy, shows a maximum for small values of $H$. The locus of these maxima is evidently a transition curve separating regions of paramagnetic and antiferromagnetic behavior.