Magnetic Aftereffects in Ferrimagnetic Oxidic Spinels

The typical aftereffect spectra of ferrites and related oxides could usually be classified as due to either ionic (and vacancy) or electron processes. The present status of the study of these phenomena is outlined especially from the point of view of the underlying microscopic mechanisms. As an illustration, the results obtained with Mn–Fe and Mn–Cu–Fe oxidic spinels are presented. The magnetic-loss spectra were studied in a broad temperature range from 2° to 360°K and some additional measurements of induced anisotropy and permeability disaccommodation were performed. The role played in magnetic aftereffect by various valency states of the ions present is compared with electrical conductivity measurements. Three distinct relaxation bands of basically electronic origin were found in Mn–Fe spinels. The corresponding tanδ maxima for 105 Hz frequency range were situated at ≲2°K, 10°−50°K, and 90°−350°K, respectively, the exact temperature and related activation energies depending on the composition. These relaxation processes were ascribed successively with descending temperature to reorientations of Jahn-Teller distortions induced by Mn3+ ions stabilized by means of trapped electrons, to ordering of Fe-ion valencies by means of electron transfer, and to reorientation of unstabilized Jahn-Teller distortions of Mn3+ occupied octahedra.