Characterization of grain boundary conductivity of spin-sprayed ferrites using scanning microwave microscope

Grain boundary electrical conductivity of ferrite materials has been characterized using scanning microwave microscope. Structural, electrical, and magnetic properties of Fe₃O₄ spin-sprayed thin films onto glass substrates for different length of growth times were investigated using a scanning microwave microscope, an atomic force microscope, a four-point probe measurement, and a made in house transmission line based magnetic permeameter. The real part of the magnetic permeability shows almost constant between 10 and 300MHz. As the Fe₃O₄ film thickness increases, the grain size becomes larger, leading to a higher DC conductivity. However, the loss in the Fe₃O₄ films at high frequency does not increase correspondingly. By measuring the reflection coefficient s₁₁ from the scanning microwave microscope, it turns out that the grain boundaries of the Fe₃O₄ films exhibit higher electric conductivity than the grains, which contributes loss at radio frequencies. This result will provide guidance for further improvement of low loss ferrite materials for high frequency applications.