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The Faraday effect in the thick epitaxial films of magnetic garnets of the type used in magneto‐optic isolators can be used as the basis for a fiber‐optic magnetic field sensor. These films have uniaxial anisotropy perpendicular to the surface and they contain bismuth to enhance the Faraday rotation. The typical magnetic domain pattern of meandering stripes changes in response to an applied field perpendicular to the film and this changes the polarization of infrared light propagating perpendicular to the film. Theory and experiment show that the speed of operation is limited by relaxation or resonance effects to upper frequencies between 106 and 109 Hz. Maximum sensitivity requires low magnetic moment and large thickness, in conflict with the requirements for high speed.