Electrical Resistivity of Nickel Ferrite

Abstract

UP to the present the only systematic investigation published on the semiconducting behaviour of ferrites is a study of cobalt ferrite, CoFe₂O₄, by Jonker¹. In the case of other ferrites insufficient information of the basic structure of the material has impeded an understanding of the electrical properties. In the case of nickel ferrite it is known that these properties are closely related to a small deficiency, δ, of oxygen (a few tenths of 1 per cent) which occurs when the ferrite is formed in the sintering process. This may be represented by NiO + Fe₂O₃ → Ni⁺⁺Fe⁺⁺⁺ _2-2δFe⁺⁺ _2δO⁻⁻ _4-δ + ½δ(O₂). The creation of iron ions with two valencies in the crystal lattice is known to lead to a marked lowering of activation energy of conduction electrons², and the resistivity of nickel ferrites therefore depends on the temperature and atmosphere of the sintering process and on the rate of subsequent cooling. To produce high-resistivity nickel ferrite it has been found desirable to sinter at relatively low temperatures (below 1,300° C.) in an oxygen atmosphere^3,4; but it is very doubtful if a nickel ferrite with electrical properties representative of the stoichiometric material has so far been produced.