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Chromium tribromide is one of the few ionic ferromagnetic crystals known. It has a hexagonal layer structure, and its Curie point is at about 36°K. Thin sections are transparent from the near infrared up to a strong sharp band edge at about 20 000 cm−1. Below this band edge a number of broad bands and sharp lines are observed. Linearly polarized light passing through a magnetically saturated crystal experiences a rotation of the axis of polarization which varies with wavelength and is proportional to the magnetization component along the line of sight. Exceedingly large magnetic rotations are associated with the band edge near 20 000 cm−1 where values of over 500 000° / cm were measured. The detailed structure of the 2T2 absorption at about 19 000 cm−1 seems to involve the ferromagnetic exchange coupling. An assignment is given for the band edge. Microwave-optical experiments have been performed in which ferromagnetic resonance has been observed visually and light has been amplitude modulated at 23 and 46 kMc.
Dillon et al. (1963) studied this question.