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All of the ferrimagnetic garnets studied to date have been found to be transparent to visible light down to wavelengths of about 5200 A. Thin sections (e.g., 0.005 cm) have been examined to determine the absorption and the specific Faraday rotation through that part of the visible spectrum where they transmit light. These measurements have been made at temperatures down to 4.2°K. They are interpreted as giving us information on electronic energy levels. A magnetic birefringence and a circular dichroism have been observed. The Faraday rotation permits observation of the domain structure in these crystals at any temperature below the Curie point. Possible microwave-optical experiments are discussed briefly.
J. F. Dillon (1958) studied this question.