CrI3, noteworthy as one of the few insulating ferromagnets, is one of a series of presumably isomorphous compounds (CrCl3, CrBr3, and CrI3) which show similar or closely related magnetic ordering. CrI3 samples containing some single crystals (plates a few square millimeters in area) were made by reacting chromium metal and iodine in a sealed quartz tube. Magnetization measurements of the powder were made by Williams and Sherwood at several fields up to 15 300 Oe over the range 1.5° to 300°K. In addition, M (H) was plotted at 4.2°K up to 70 000 Oe. The ferromagnetic Curie temperature is 68°K, and the paramagnetic Curie temperature is 70°K. The low-temperature saturation magnetization is 3.10 μB/Cr+ + +, corresponding to 4πM = 2690 G. The field for ferromagnetic resonance was measured at 86, 91, and 99 Gc/sec. The spectroscopic splitting factor g = 2.07, and the anisotropy field 2K/M = 28.6 kOe at 1.5°K. Optical transmission measurements on CrI3 single crystals showed a very strong absorption band down to a band edge near 10 000 cm−1. The specific magnetic rotations associated with the band edge are almost as large as those observed in the tribromide. Evidence of domain diffraction was seen.

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Ferrimagnetic-resonance experiments have been performed using disk-and rod-shaped samples cut from single crystals of a manganese ferrite and of yttrium iron garnet. A multiplicity of absorption maxima were observed which can be identified as magnetostatic modes. The excitation of different sets of modes by rf magnetic field configurations of varying symmetries aids in assigning indices to the modes observed. By far the narrowest lines were observed in thin disks with the steady field normal to the plane of the disk.

The spectra of eight crystals containing trivalent chromium have been measured, and the energy levels fitted to a four-parameter crystal-field theory. The values of the parameters derived from the spectra are discussed, and some mechanisms responsible for absorption intensities are evaluated.

Light transmitted by a magnetic crystal interacts with the magnetization. In particular, the axis of linear polarization undergoes a rotation proportional to the fractional projection of M on the direction of propagation. This magneto-optical rotation, the Faraday rotation in magnetic materials, has been studied for many years. The advent of insulating magnetic materials has led to renewed interest of two sorts: (1) The rotational dispersion coupled with that of the absorption coefficient in some cases makes possible the spectroscopic assignment of optical transitions which show magnetic effects. Thus we may use the rotation to extract information on the magnetic behavior of energy levels well above the ground state. (2) Any experimental or technological use of the rotation makes it necessary to take into account the attendant absorption of light. Some insulating magnetic materials have very high values of the rotation per unit attenuation. The magneto-optical rotation may then be used to ``see'' both dc (domain structure) and rf (magnetostatic spin waves) magnetization distributions within the crystal. Furthermore, the low rotation attenuation is crucial in a number of magneto-optical devices: rotators, wide-band modulators, phase shifters, memories, the YIG laser, etc.