Infrared Faraday Rotation in Ferrites

The absorption and Faraday rotation of crystals of the spinel ferrites Li0.5Fe2.5O4, NiFe2O4, and MgFe2O4 and of the hexagonal ferrites BaFe12O19 and Ba2Zn2Fe12O22 have been measured at 300°K between 1 and 8 μ. The rotation consists of a nearly wavelength-independent term θM arising from the magnetic resonance and of a strongly dispersive term θd originating mainly from electronic transitions of Fe3+. As in YIG, the Fe3+ ions in tetrahedral and octahedral coordination contribute differently to θd. The experimental curves of the Faraday rotation of YIG, Li0.5Fe2.5O4 and MgFe2O4 can be described rather accurately with the same constants for the rotary powers of Fe3+ in tetrahedra and of Fe3+ in octahedra. Also the contribution of Ni2+ to θd in NiFe2O4 at 80°, 300°, and 373°K has been determined.