Far-infrared transmission spectra of dysprosium and terbium iron garnet were measured between 10 and 150 cm−1 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−1 were identified as low-lying electronic levels. Magnetic resonances at 13.5, 17.0, and 28.0 cm−1 and phonon absorption lines at 81, 107, 112, and 145 cm−1 were found. For terbium iron garnet, two electronic lines at 36 and 68.5 cm−1 and four phonon lines at 80.5, 108, 112, and 145 cm−1 were found.
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