Kinetics of Magnetic Annealing in Cobalt-Substituted Magnetite

Abstract

The time dependence of the magnetic annealing effect in single crystals of magnetite containing various amounts of substituted cobalt has been investigated by a technique which permits observation of the effect at the annealing temperature. The annealing kinetics in a particular crystallographic direction are determined by measuring the decay of the torque in the (001) plane following a preparatory anneal in a direction 45° removed. The directions chosen for study are the [100] and [110] directions, which are nodes of the cubic torque curve. The absence of a cubic torque in the directions of measurement allows precise observation of the anneal-induced uniaxial torque. The annealing kinetics observed in the two directions are different, and the nature of the torque decay in both directions depends upon the cobalt concentration of the sample. The torque decay in the [100] direction is attributed to the redistribution of single cobalt ions and certain pairs of adjacent cobalt ions over the octahedral cation sites, whereas the decay in the [110] direction is attributed to the redistribution of cobalt ion pairs only. The results of a theoretical analysis of the annealing kinetics that would result from such ionic redistribution are in good agreement with the experimental observations. Comparison of the annealing behavior of samples in different states of oxidation indicates that ionic redistribution occurs by a vacancy diffusion mechanism, and the activation energy observed for this process is 1.05±0.03 ev. In addition to the measurements performed at the annealing temperature, the torque in the [110] direction was determined as a function of temperature in a sample which had been quenched at the end of its preparatory anneal. These two measurements are used to show that there exists a repulsive interaction energy of 0.093±0.04 ev between two adjacent cobalt ions.