Effect of changeable demagnetizing state of ferrite on the permeability of BaTiO₃/Ni0.5Zn0.5Fe₂O₄ composites

A BTO/NZFO ferroelectric/ferromagnetic ceramic composite was prepared by the traditional ceramic method and its permeability was analysed in detail. Results showed that as the BTO fraction increased, the permeability of the composite dropped much more rapidly than that predicted by Kirkpatrick's effective medium model, which was consistent with that usually observed in other ferromagnetic composites. A model was developed to interpret the permeability of ferrite particles with the usual multi-180°-domain structure in the composite by considering both the contributions of spin rotation and domain-wall motion. It was confirmed theoretically that the internal demagnetizing field on the ferrite particles, which was changeable with their shape and dispersion situation, and thus with their fraction, controlled the response of spin rotation and thus the permeability of ferrite particles dispersed in the composite. The rapid decrease in the effective permeability of the composite was ascribed to not only the universal compound effect, formation of defects by ion doping from the coexisting phases and the change in grain size with the ferrite fraction, but also the shielding of the spin rotation response of the ferrite, which depended on its demagnetizing state in the composite.