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

Summary form only given. The Faraday-effect has been used for the measurement of magnetic field induced by electric current. Actually, an optical fiber sensor using the Faraday-effect has been developed as an alternative method of the current-transformer in the substation of the AC electric power system. However, in the optical fiber electric current sensor, it is difficult to minimize the sensor head because the optical fiber must be wound to a conductor wire. Recently, we proposed an optical magnetic field sensor with a small head [1] for the switching current measurement for the SiC/GaN power-devices, which consisted of a Faraday-element using a magnetic thin film. Such a magnetic thin film for the Faraday-element must have both higher transmittance for the light (transparency) and large Faraday-effect. Rare earth substituted yttrium iron garnet film (R:YIG) with high transmittance and large Faraday-effect has been widely studied, and a cerium substituted film (Ce:YIG) with high performance has recently been reported [2]. However, Ce:YIG film has a temperature-dependent magnetization owing to the low Curie temperature around or below 300 deg C. In this study, we focus on a granular film with ferromagnetic fine metal particles dispersed in an insulator matrix to obtain high transmittance and large Faraday-effect. Actually, N. Kobayashi et al. [3] reported a FeCo-AlF granular film with high transmittance. J. L. Dorman et al. [4] also reported a Fe-Al₂O₃ granular film with large Faraday rotation angle of 3000 to 4000 deg./cm at 1550 nm wavelength. In our study, to develop the Faraday-element for the optical magnetic field sensor, the granular film was fabricated by co-evaporation method using cobalt (Co) and magnesium fluoride (MgF₂). The evaporation rate ratio was kept to Co: MgF₂ = 1: 2 (Co volume fraction of 0.33). The substrate- temperature during co-evaporation was R.T. to 450 deg C. The magnetic and magneto-optical properties of the film were characterized. The substrate-temperature dependences of the cobalt particle diameter and the extinction coefficient k in the Co-MgF₂ granular film deposited by co-evaporation are shown in Fig. 1. The Co particle diameter became large proportional to the substrate-temperature during the deposition. At low substrate-temperature during the deposition, average Co-granule diameter was about 3 nm and the granular film exhibited a superparamagnetism. On the other hand, not shown here in detail, at the substrate-temperature during the deposition of 350 deg C or above, the granular film exhibited a magnetization curve with a hysteresis as an evidence of the ferromagnetic behaviour. O. Kitakami et al. [5] reported a superparamagnetic critical diameter of hcp-Co, it was estimated to be about 7 nm, in our study, such the critical Co diameter was estimated to be about 4 nm. The cobalt particle size became large and the k of the film became low with increasing the substrate-temperature during the deposition. The minimum k of the film was 0.028 at the substrate-temperature of 450 deg C, which was much smaller than those of previous studies. Even in constant concentration of Co or MgF₂ in the film, the k changed by substrate-temperature during the deposition. We considered that the distance between the adjacent cobalt particles became wider when the cobalt particle became larger, and such a relationship between cobalt particle size and the distance between the adjacent particles was considered to be strongly related to the low k of the Co-MgF₂ granular film. Fig. 2 shows the magnetization curve of Co-MgF₂ granular film deposited at 350 deg C measured in the film plane and perpendicular to the film plane, and the Faraday-rotation angle versus applied magnetic field. From Fig. 2, the Co-MgF₂ granular film had the in-plane aligned magnetization, and the perpendicular magnetization process was considered to be due to the magnetization rotation by the perpendicular demagnetizing effect. Although not shown here, even at ambient temperature of 350 deg C, the decrease of saturation magnetization M_s of the Co-MgF₂ granular film deposited at 350 deg C was a little 10%. Therefore, the Co-MgF₂ granular film was considered to have a Curie temperature much higher than 350 deg C. In Fig. 2, the Faraday-rotation loop was corresponding to the magnetization curve well. The Faraday-rotation angle was linearly proportional to the applied magnetic field within ±2 kOe. The upper limit of magnetic field 2 kOe of the linear relation was considered to be due to the perpendicular demagnetizing magnetic field H_d, where H_d was estimated to be about 4 kOe from the saturation magnetization M_s; of 4 kG. Since the H_d was considered to be dependent only on the M with the curie temperature much higher than 350 deg C. Therefore, the Co-MgF₂ granular film will be useful for the Faraday-element of the optical magnetic field sensor used in high ambient temperature.