Ni-Zn ferrites were prepared in air by using the conventional ceramic powder methodology. The compositions analysed belong to the type NixZn1-xFe2O4 ferrites, with x ranging from 0.3-0.4. Copper wire was coiled round the specimens, previously pressed to a toroidal shape, to characterize their magnetic properties as a function of the frequency of the applied electric field. Powder X-ray diffraction, scanning electron microscopy and microanalysis, and the magnetic susceptibility of the ferrites, were studied, indicating that the highest permeability is achieved at the composition x=0.3, a result which is correlated to the microstructural characterization. The Curie temperature was also determined to range from 60-130 degrees C, depending on the specific composition.