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The Curie point, lattice constants, sublattice magnetization, spontaneous magnetic moment, susceptibility, and the nonlinear susceptibility have been measured on the compounds RFeO3 with R yttrium or a rare earth. The main conclusions from these measurements are: (1) The rare-earth ions contribution to the magnetic properties of these materials at relatively high temperature is mainly through the geometrical effect of ionic radii. (2) The ratio of iron sublattice to the weak ferromagnetic moment shows that the canting angle is temperature independent. (3) Between liquid-air temperature and the Curie point, the rare-earth ions behave like a paramagnet in an effective magnetic field of the iron ions. This field is of the order of a few thousand gauss of either sign. (4) From measurements of the spontaneous moment, the susceptibility and the nonlinear dependence of the magnetization on applied fields it appears that the antisymmetric exchange interaction is responsible for the weak ferromagnetism of these materials. (5) It is found that near the Curie temperature there exists an appreciable deviation of the iron sublattice magnetization from the theoretical S = 52 curve.

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The optical properties of magnetic materials are studied using symmetry considerations. It is found that in addition to birefringence, Faraday rotation, and optical activity there should exist another effect, namely gyrotropic birefringence. This birefringence is of opposite sign for magnetic domains related to each other by time reversal, and can, therefore, in principle be used for observation of antiferromagnetic domains so related. The dependence of the optical properties on external agents such as electric and magnetic fields and stress is also considered.

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