Crystallite size dependence of natural resonance frequency for spin-sprayed Fe3O4∕γ–Fe2O3 films highly permeable up to gigahertz range

We successfully prepared Fe3O4∕γ–Fe2O3 polycrystalline films having a wide range of crystallite sizes by changing the plating conditions at 90°C. The films had lattice constants (determined by x-ray diffraction) independent of the crystallite size D within our experimental error. Natural resonance frequency fr for the film increased from 0.8to1.9GHz as the crystallite size D decreased from 63to30nm. In our polycrystalline films, magnetic anisotropy field Hk is induced mainly by crystallite boundary/surface effect so that a larger crystallite will cause a weaker Hk. Those Fe3O4∕γ–Fe2O3 films are also promising to be used in conducted noise suppressors operated in a gigahertz range.