Responsivity optimization in magneto-optic sensors based on ferromagnetic materials.

In an effort to optimize magnetic field detection sensitivities, the Faraday responsivity vector, which determines the relationship between the Faraday rotation angle and an externally applied magnetic field, was investigated in magneto-optic sensors based on bismuth-doped iron-garnet films. Under externally applied fields, Faraday rotation is produced principally by domain rotation and domain wall motion, whose relative contributions depend on the domain geometry and the direction of laser propagation. When optically probed along a principal magnetization axis, Faraday rotation is driven by a single magnetization mechanism, and the responsivity is linearized (reduced to an effective Verdet constant). When the films are probed along an oblique angle to the principal axes, the relationship between the Faraday rotation and the external field becomes tensorial and much more complex. Although this may lead to more complicated phenomena, the interplay of domain rotation and domain wall bowing can be exploited to improve responsivity or bandwidth. A generalized model for the magnitude and direction of the responsivity vector is formulated, which gives predictions that are consistent with the experimental data. Applications to arrayed sensors and three-axis field measurements are discussed.