Magnetic aging in TiO2-doped Mn-Zn ferrites

Abstract

The soft magnetic properties of the sintered Mn-Zn ferrites can be stabilized versus changing temperature by addition of CoO in suitable proportion. The material benefits in this case of anisotropy compensation brought about by the Co2+ cations. However, prolonged exposure of magnetic cores to high temperatures, as likely to occur in automotive applications, can be associated with local phenomena of induced anisotropy, resulting in magnetic viscosity and aging. We show in this paper that proper addition of TiO2 in optimally CoO-doped ferrites can restrain aging and the ensuing detrimental effects on loss and permeability. We find, in particular, that on passing from conventional doping with 1000 ppm to 5000 ppm of TiO2, the deterioration of the soft magnetic properties of the ferrite following an aging treatment of 100 h at 200 °C is in good part reduced below a few hundred kHz. It is concluded that this effect chiefly relates to a correspondingly reduced value of the magnetic anisotropy induced by directional ordering at 200 °C. This beneficial effect appears to descend from hindered diffusion of the Co2+ cations by the dissolved Ti4+ cations. The induced anisotropy, however, while leading to increased hysteresis and excess losses, brings about a decrease of the total energy loss at intermediate frequencies, by modifying the distribution of the resonance frequencies and the associated damping of the rotational processes. Little to negligible effects are observed beyond a few MHz.