Fabrication of Ferromagnetic Co–MgF2 Granular Film With High Transmittance and Large Faraday Effect for Optical Magnetic Field Sensor

In the magneto-optical devices and sensors using the Faraday effect, maintaining the transmitted light intensity of a Faraday element is a key issue. A granular film with ferromagnetic fine metals dispersed in an insulator matrix has been expected to have both high transmittance and large Faraday effect. In this paper, we fabricated the Co-MgF ₂ granular film prepared by co-evaporation and investigated the effect of substrate heating temperature during co-evaporation and post-annealing on the transmittance and the Faraday rotation of the granular film. The Co-MgF ₂ granular film deposited at high substrate temperature was exhibited like a ferromagnetic behavior and high transmittance. Also, we found that the figure of merit [deg./dB], which was defined as a ratio of a Faraday rotation angle and decay of the transmitted light, was improved by post-annealing.