Complex permeability variations of iron excess CoNiZn ferrites with annealing temperature

Induced anisotropy due to Co
²⁺
ions in excess iron ferrites is known to increase the magnetic Q, and the technologically important μQ product, by increasing the restoring force on domain walls. This anisotropy can be induced by slow cooling from below the Curie temperature. This paper studies the further increases in the magnetic Q that occur for slow cooling from above the Curie temperature. We show from measurements of the complex permeability spectrum between 1 and 1000 MHz that such improvements are due to a decrease in domain wall damping. Measurements of d.c. resistivity, an Auger spectroscopy analysis of grain boundary impurities and a thermogravimetric analysis indicate that oxidation of Fe
²⁺
ions in both grain boundaries and interiors is the origin of the decrease in the domain wall damping.