Weak ferromagnetism in a band model: Application to Sc₃In

We study the properties of a band model recently proposed to describe metallic ferromagnetism in the regime where the saturation magnetization at zero temperature is a small fraction of a Bohr magneton. The model contains, in addition to the usual Stoner exchange energy, a band-narrowing parameter that arises from an off-diagonal matrix element of the Coulomb interaction between electrons. Parameters in the model are chosen to describe the weak ferromagnet Sc₃In and several observables are calculated and compared with experiment. The model overcomes the major difficulties of the Stoner model to describe weak ferromagnets, and it is argued, contrary to other recent work on this subject, that explicit inclusion of spin fluctuations is unnecessary for a description of this class of materials.