Transitions from Ferromagnetism to Antiferromagnetism in Iron Aluminum Alloys

Abstract

From a study of magnetic isotherms over the range of compositions from pure Fe to 40 atomic percent Al in Fe and over temperatures from 4°K to 300°K, we have produced strong evidence for the existence of super-exchange interactions between iron atoms separated by aluminum atoms. The iron rich alloys show ferromagnetism in the usual way, but, for concentrations approaching 40 atomic percent Al, the alloys behave as antiferromagnetic with Néel points near liquid hydrogen temperatures. There is a transition range of composition in which the stable state just below the Curie temperature is ferromagnetic but at low temperatures is antiferromagnetic. The large changes in magnetic properties occur for the ordered structures and in composition ranges where the number of nearest neighbor iron-iron interactions is changing rapidly with composition, but where the number of next nearest neighbor iron-iron interactions remains practically constant. These considerations lead to a model where only the nearest neighbor direct positive exchange interactions between iron atoms, and indirect negative exchange interactions between iron atoms separated by an aluminum atom are taken into account. The treatment by a Bragg-Williams type calculation leads to a qualitative understanding of the observed magnetic effects in the iron aluminum system, including the differences between the disordered state and the two types of ordered states. The model also leads to a reinterpretation of neutron diffraction data.