Increase of Faraday Rotation Angle of Cerium-Substituted Yttrium Iron Garnet Film using Vacuum Annealing

Magnetooptical materials have been applied in various devices, such as optical isolators, spatial light modulators, and Q-switches. A magnetic garnet was used in these devices because of its large Faraday rotation angle and low optical loss. In particular, cerium-substituted yttrium iron garnet (Ce:YIG) shows good magnetooptical characteristics in the near-infrared region, used in optical isolators [1]. In Ref [2], the polycrystalline Ce:YIG was deposited onto silica substrates, showing the Faraday rotation (FR) angle of 1.64 degree/micron at a wavelength of 532 nm and the optical absorption <i>α</i> of 4.17 dB/micron. The figure of merit (FOM) defined as FR/<i>α</i> was 0.39 degree/dB. This study changed the annealing condition to increase this FOM, and magnetooptical properties were characterized.<br/>A 338 nm thick Ce₁Y₂Fe₅O₁₂ film was deposited on a 1-inch synthetic fused silica substrate using a radio-frequency ion beam sputtering (RF-IBS). The substrate temperature was held at 200°C during deposition. The sample was cut into 5×5 mm² and annealed using a heater in a vacuum glass chamber. The pressure was less than 2 Pa during the annealing.<br/>The annealing procedure comprised three parts: heating up, keeping, and cooling down. We changed the heating-up speed, the temperature of the keeping step, and the cooling-down speed. The heating-up speed was varied from 0.11 to 16 °C/s, and the temperature of the keeping step was varied from 600 to 900°C. The temperature was kept for 10 minutes. The FR loops and absorption of all samples prepared by the various annealing conditions were measured at the wavelength of 532 nm. The largest FR of 1.55 degree/micron, the smallest absorption of 3.24 dB/micron, and the largest FOM of 0.48 degree/dB were obtained with a heating-up speed of 1.33°C/s and a temperature of 800°C. The cooling-down rate varied from –0.097 to –0.39 °C/s, unaffecting the FR and absorption. Thus the fastest speed was used.<br/>The obtained FOM of 0.48 degree/dB was 1.23 times larger than the prior study's. The microscopic image of the sample surface became uniform as the heating-up speed decreased. Such a behavior may indicate the consistent growth of nanocrystalline grains. In the symposium, crystalline, electronic structures, and magnetic and optical properties will also be discussed.<br/><br/>[1] T. Goto, M. C. Onbasli, D. H. Kim, V. Singh, M. Inoue, L. C. Kimerling, and C. A. Ross, "A nonreciprocal racetrack resonator based on vacuum-annealed magnetooptical cerium-substituted yttrium iron garnet," Opt. Express 22, 19047-19054 (2014).<br/>[2] Y. Yoshihara, T. Sugita, P. B. Lim, Y. Tamba, H. Inoue, K. Ishiyama, M. Inoue, C. A. Ross, and T. Goto, "Thickness-dependent magnetooptical properties of ion beam sputtered polycrystalline Ce₁Y₂Fe₅O₁₂ films," Opt. Mater. 133, 112967 (2022).