The antiferromagnetic state in the chrome alums

Summary

This paper is a report on a calculation made by Miss Mary O’Brien which she has presented briefly at the Yale meeting of the American Physical Society, and which she plans to publish more fully in thePhys. Rev. The most striking result of Miss O’Brien’s work is that purely dipolar interaction can lead to an antiferromagnetic Curie (Néel) point even though the paramagnetic ions are cubically spaced. It is usually assumed that cubic symmetry precludes this result, but caculations which are an extension of those previously made byDaniels show that when different atoms in the unit cell have different directions for their crystalline fields, antiferromagnetism can ensue at sufficiently low temperatures. The predicted Néel point for methylamine chrome alum agrees with experiment ((.015 ÷ .020) °K) practically within the limits of error of the latter. The calculated isentropic curves of moment versus field strength also agree quite well with experiment if a small amount of exchange is included.