Plasma-sprayed MnZn ferrites with insulated fine grains and increased resistivity for high-frequency applications

Plasma-sprayed MnZn ferrite thick films are built up by splats, which consist of columnar grains with diameter ∼200 nm and height ∼1 μm. The existence of the conductive wustite FeO in the as-sprayed films greatly reduces the dc resistivity. However, a useful structure can be developed in these ferrite films with fine equal-axis ferrite grains insulated by the high-resistivity hematite Fe₂O₃ because of the polygonization of the columnar grains and the oxidation of the wustite during an annealing process. The dc resistivity increases significantly after the annealing process, an effect ascribed to the growth of hematite Fe ₂O₃ on the basis of impedance analysis. The magnetic properties of these ferrite films improve concurrently. The high-frequency response of the annealed plasma-sprayed MnZn ferrites shows a permeability of ∼700 stabilized to above 10 MHz. The maximum Q factor at about 10 MHz increases from 5 to 20 as a result of the increase of the dc resistivity.