Thermal instabilities of cobalt containing MnZn ferrite cores for medium (500 kHz) frequency applications

Abstract

MnZn ferrites of the nominal composition (Mn0.81Zn0.19)Fe2.13O4, doped with cobalt, have been studied concerning the evolution of their medium (500 kHz) frequency power losses and initial magnetic permeabilities upon ageing at various temperatures for various times. Thermal ageing results to significant power loss reduction by about 50% at 400 kHz-50 mT (i.e. at 90 °C from initially 58–29 mW cm−3) and about 30% at 500 kHz–100 mT (i.e. at 90 °C from initially 690–480 mW cm−3), accompanied with a simultaneous decrease of the relative initial permeability from initially 1700–1200. The extent of power loss reduction depends on ageing time and temperature and can be described by a thermally activated process, during which Co2+ ions fall from a metastable high energy state to a stable lower energy state with an activation energy 0.33 eV/cation. Addition of TiO2 to an amount of 4000 ppm (wt.%) appears to retard the cobalt-induced ageing process by increasing the activation energy to 0.45 eV/cation, while equal amount additions of SnO2 did not seem to have any effect.