Saturation Magnetization of Nickel Ferrites in Relation to Lattice Strain

Abstract

Oxalate of Ni²⁺ and Fe²⁺ was co-precipitated by titrating an aqueous solution containing Ni²⁺ and Fe²⁺ (Ni/Fe=0.5) to an oxalic acid solution. The oxalate changed to nickel ferrite on firing at various temperatures from 500° to 1000°C in air. The nickel ferrite thus obtained was an Fe-deficient spinal-type phase, of which chemical composition can be written as Ni_1.09Fe_1.91O_3.95 from the results of chemical analysis and density measurements. The analysis of X-ray diffraction line-broadening showed that the sample fired at fairly low temperatures have many crystal imperfections such as lattice strain (Δd/d) and lattice defects, which decreased with increasing firing temperature and vanished above 800°C. The nickel ferrite having such crystal imperfections has a low saturation magnetization, which increased with increasing firing temperature. In order to clarify the relation between saturation magnetization and lattice imperfections, another sample of nickel ferrite, which was prepared by solid state reaction of NiO and Fe₂O₃, was mechanically ground for an extended tune period to introduce lattice strain. The saturation magnetization of the nickel ferrite thus obtained also decreased linearly with increasing lattice strain, as in the sample prepared by co-precipitation method. These facts suggest that the lattice strain, regardless of its origin, lowers the saturation magnetization.