Magnetic loss in the nickel-zinc-iron oxide system

Abstract

The formation of cubic inverted spinel structures in the nickel-zinc-iron oxide system produces ferrites with high initial magnetic permeability. However, the electrical conductivity of nickel-zinc ferrites, when prepared under oxidizing conditions, is extremely low. Consequently, the imaginary part of the dielectric constant is correspondingly low, and the resultant rf electric loss is small. Most loss is, therefore, associated with the imaginary part of the magnetic permeability, which declines inversely with frequency in the VHF-UHF frequency regime. In this work, dielectric constant and magnetic permeability were measured for a series of nickel oxide-zinc oxide-ferric oxide compositions in order to identify the region of the ternary diagram associated with higher values of magnetic loss. Ferrites were prepared under identical conditions by a reactive sintering process at 1200 °C. Compositions corresponded to stoichiometric spinels (NixZn1−xFe2O4), or to spinels with the presence of excess or insufficient ferric oxide. Toroids were machined for coaxial analysis. Variation of the imaginary part of permeability with ferric oxide concentration indicated that larger values of magnetic loss were obtained when a relatively large excess of Fe2O3 was absent. When values of the imaginary part of magnetic permeability were plotted versus the nickel-zinc atom ratio, the sensitivity was relatively weak near and above unity. Below a ratio of 0.75, magnetic loss decreased rapidly, disappearing below the ratio Ni/Zn≂0.2. Reasons for this behavior are discussed. The largest μ″ obtained with the stoichiometric iron oxide series was at Ni0.5Zn0.5Fe2O4.