The antiferromagnetic-ferromagnetic transition in iron-rhodium alloys

The critical field required to induce this transition in Fe-Rh alloys close to the equiatomic composition has been measured using pulsed magnetic fields up to 280 kOe. Measurements of the thermal expansion and high field magnetization have also been carried out. It is found that application of Kittel's theory of exchange inversion to the results leads to serious numerical discrepancies. A four sublattice model is developed which indicates the importance of the excess entropy of the ferromagnetic over the antiferromagnetic phase in exciting the antiferromagnetic-ferromagnetic transition. The exchange iteration β between iron sublattices is small and negative and, although its lattice parameter dependence is large and positive, there is no suggestion of exchange inversion corresponding to a change in sign of β at the antiferromagnetic-ferromagnetic transition. Thus the ferromagnetic state is stabilized by the excess entropy even in the presence of antiferromagnetic interactions and this stability is maintained right up to the Curie temperature