Stress sensitivity of inductance in NiMgCuZn ferrites and development of a stress insensitive ferrite composition for microinductors

Three series of NiMgCuZn ferrites were prepared by conventional sintering process. The formation of single phase in these ferrites was confirmed by x-ray diffraction. Initial permeability measurements on these samples were carried out in the temperature range of 30–400°C. The effect of the external applied stress on the open magnetic circuit type coil with these ferrites was studied by applying uniaxial compressive stress parallel to magnetizing direction and the change in the inductance was measured. The variation of ratio of inductance (ΔL/L)% increases upto certain applied compressive stress and there after it decreases, showing different stress sensitivities for different compositions of ferrites studied in the present work. With a view to develop stress insensitive NiMgCuZn ferrite, a low stress sensitivity composition among all the ferrites studied was chosen and different amounts of SiO₂ were added to it and a series of ferrite compositions were prepared. The variation of ratio of inductance (ΔL/L)% with external applied compressive stress was examined. These results show that, 0.05 wt% SiO₂ added Ni_0.3Mg_0.3Cu_0.1Zn_0.3Fe₂O₄ ferrite exhibited stress insensitivity. It was noticed that addition of SiO₂ was found to be effective in reducing the stress sensitivity. This was confirmed from the elastic behaviour studies at room temperature on these ferrite samples. These studies were carried out to develop a ferrite composition for its use as core material for microinductor applications.