Superparamagnetism in the ZnFe₂O₄–NiFe₂O₄ System

Abstract

The magnetic properties of the ZnFe₂O₄–NiFe₂O₄ system were examined at low temperatures by means of a Cioffi-type automatic fiuxmeter. It was found that 90ZnFe₂O₄–10NiFe₂O₄ shows superparamagnetic behavior above 20°K. By analyzing the superparamagnetic magnetization curves of this specimen, the size distribution of the superparaniagnetic clusters, as well as the temperature dependence of magnetization inside the cluster, were determined. The superparamagnetic cluster was found to be a ferrimagnetic ordered region separated magnetically from the matrix because all tetrahedral sites surrounding the cluster are occupied by nonmagnetic Zn²⁺ ions. It is suggested that the deviation of the saturation magnetic moment of this system from the theoretical value is not due to a canted alignment of spins, but due to the existence of paramagnetic and superparamagnetic spins in the crystal. This suggestion was confirmed by a Mössbauer study carried out at room temperature for 60ZnFe₂O₄–40NiFe₂O₄.