Faraday rotation has been measured in (Er2.2Y0.8)Fe5O12 crystals in the 11–300-K temperature range in a magnetic field along the three main crystallographic directions. A strong anisotropy is observed below the compensation temperature, which vanishes at the compensation. The origin of this anisotropy is discussed and it is shown that the magnetization cannot account for the whole Faraday rotation anisotropy and that the magneto-optical coefficient Ce of electric dipole type transitions is anisotropic. A comparison between (Er2.2Y0.8)IG and pure ErIG results is given. The strong anisotropy is tentatively related to the noncollinear magnetic structure observed below the compensation.

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In Ho3Fe5O12, canting of the rare-earth magnetic moments is evidenced by neutron diffraction. Assuming the garnet to be cubic, analytic expressions of the superstructure lines are given as simple combinations of both components (parallel and perpendicular to [111]) of the Ho3+ moments. New parameters of the ’’umbrella structure’’ are given.