Far-infrared transmission spectra of dysprosium and terbium iron garnet were measured between 10 and 150 cm<sup>-1</sup> at temperatures less than 20 K by Fourier-transform spectroscopy. The temperature and magnetic-field dependence of the transmission spectrum of dysprosium iron garnet was also observed. For dysprosium iron garnet, absorption lines at 22.6, 59.5, 44.0, and 65 cm<sup>-1</sup> were identified as low-lying electronic levels. Magnetic resonances at 13.5, 17.0, and 28.0 cm<sup>-1</sup> and phonon absorption lines at 81, 107, 112, and 145 cm<sup>-1</sup> were found. For terbium iron garnet, two electronic lines at 36 and 68.5 cm<sup>-1</sup> and four phonon lines at 80.5, 108, 112, and 145 cm<sup>-1</sup> were found.
Junya Yamamoto, Bernard T. Smith, and Ely E. Bell, "Far-infrared spectra of dysprosium and terbium iron garnet," J. Opt. Soc. Am. 64, 880-883 (1974)