Magnetic susceptibility of soft magnetic composite materials

A mathematical model is developed for the susceptibility of a composite of soft magnetic material, held together by a magnetic binder. The model ignores the volume average of the stray field in the binder, and allows for different volume fractions of soft magnetic particles, with different shape factors, yielding results directly analogous to those of Bottcher for dielectrics in Effective Medium Theory. A high and a low susceptibility is identified, with the composite susceptibility almost always equal to the low susceptibility term. The electrical analogy is that susceptibility corresponds to a non-dimensional electrical capacitor, with the high and low capacitors in series. The high susceptibility term results from weighted particle susceptibilities in series. The low susceptibility term is usually independent of particle susceptibility. This model is validated against both finite element calculations, and published data obtained from composites of titanomagnetite ironsand, industrial ferrite and nano-sized magnetite particles.