Predicted thermal dependence of barium ferrite dead layers and exchange parameters

Theoretical predictions are made for the thermal dependence of the macroscopic exchange parameter AM²_s and the magnetic dead layer thickness observed on the surface of barium ferrite particles. The calculations are performed within mean field theory and are based on atomic exchange integrals taken from bulk data. It is found that the exchange parameter is approximately 20% smaller along the c axis than within the basal plane. The dead layer calculations require knowledge of the surface structure: An assumption that the surface is near the middle of the spinel block leads to a layer thickness in an undoped sample at 300 K of 3 Å, in close agreement with experiment.