3 - The structure of magnetic materials

Summary

Collective electron and localised moment models

Discussions about the magnetic structures of metals and alloys have traditionally involved consideration of two basic models which represent the extremes of electron localisation. The localised moment model assumes that the unpaired d or f electrons that give rise to a magnetic moment are confined to the atoms concerned. As such, Fe atoms in Cu should have the same integral moment atom as Fe atoms in Ni, for example. Below the spin ordering Curie temperature T_c the spins are spontaneously aligned, producing a net magnetic moment. Above T_c the thermal energy destroys the cooperative spin alignment resulting in zero net magnetic moment. On the other hand, the collective electron model (also known as the band or itinerant electron model) assumes that these electrons are completely delocalised and exist in narrow bands throughout the material, although the spin density need not be uniformly distributed. Below the Curie temperature the bands are exchange split, the resultant imbalance between the number of up and down spins leading to the observed magnetic moment. Above T_c there should be no splitting and so no moment.

The 3d electrons of metals of the first transition series definitely form narrow bands and their orbital angular momentum is effectively quenched by the crystal field giving a spin-only magnetic moment. These materials do not have integral moments and in fact usually display a moment per atom that varies smoothly with composition.