Tailoring the temperature characteristics of the magnetic permeability of NiZn ferrites

The effect of cobalt addition on the temperature characteristics of the magnetic permeability of NiZn ferrites was studied and a comparison to the respective behaviour of cobalt in NiCuZn ferrites was examined. Cobalt-doped NiZn and NiCuZn ferrites were manufactured by the ceramic route and sintered under various atmosphere profiles. The chemical and morphological characteristics were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The evaluation of the magnetic performance of the sintered ferrites showed that N2-rich atmosphere profiles during the top temperature and cooling time of the sintering process favour the temperature stability of the permeability in the case of NiZn ferrites, while preserving the losses at low levels. Two mechanisms that take place at the same time are proposed: the change of the Fe2+/Fe3+ ratio due to the reduction-promoting atmosphere of N2 in combination with an increase in magnetocrystalline anisotropy and magnetostriction due to the presence of Co2+ suggest a useful method to tailor the temperature factor αF of NiZn ferrites. However, the method cannot be applied in NiCuZn ferrites, as the reduction Cu2+–Cu+ taking place under N2-rich atmospheres enhances secondary re-crystallization phenomena, causing a dramatic increase in losses.