Dielectric Properties of and Conductivity in Ferrites

The dielectric properties of ferrites are dependent upon their methods of preparation. Low resistivity ferrites in which the individual grains are separated by air gaps or are coated with films that have high resistivities behave as inhomogeneous dielectric materials. Such low resistivity grains covered by high resistivity films are formed upon cooling slightly reduced ceramic ferrites in an oxygen containing atmosphere. Apparent low-frequency dielectric constants as high as 105are found as a consequence of this. Ferrites with dc resistivities of 109 Ω cm or higher, in general, have dielectric losses which are low enough to meet the requirements of microwave applications. Resistivities of this order can be obtained in modified magnesium and nickel ferrites. The effects on conduction of varying composition and temperature are discussed in some detail.