Microstructural Phenomena Controlling Losses in NiCuZn-Ferrites as Studied by Transmission Electron Microscopy

The magnetic losses of NiCuZn-ferrites with the chemical formula (Ni_0.37−δZn_0.63Cu_δ)Fe_1.93O₄ (0<δ<0.22) are found to exhibit a minimum as a function of the copper content. This minimum corresponds to a certain sintering temperature, which is also a function of the copper content. At sintering temperatures lower than the optimum, insufficient densification is considered to be the reason for increased losses. At sintering temperatures higher than the optimum, the increased losses are caused by the presence of copper segregation to the grain boundaries and by some identified microstructural defects such as subgrain boundaries and compositional modulations. Internal stress relaxation is believed to be the reason for those chemical and morphological imperfections. In contrast to the power losses, the magnetic permeability does not show the previous dependency and increases monotonically with increased density or grain size.