Effects of annealing on the microwave properties of spin-spray Ni-Zn ferrites

We have studied the properties of a series of spinel ferrite thin films prepared from aqueous solutions at temperatures slightly below 100°C, for a range of compositions from pure Fe₃O₄ to about Ni_0.34Zn_0.18Fe_2.48O₄. As made, the films are semiconducting and have high dielectric losses, but have narrow ferromagnetic resonance linewidths and often display standing spinwave spectra. As a result of annealing in air at temperatures above 300°C, these ferrites undergo a transformation to a state having much higher resistivity and a reduced magnetic moment compared to as-grown films. This annealing decreases the microwave dielectric losses, but increases the ferromagnetic resonance linewidth. The changes in these properties with annealing are consistent with a partial transformation from spinel to maghemite, γ-Fe₂O ₃