Effect of focal height on Faraday rotation distributions for magnetization analysis of magnetic holograms in Bi:RIG films using transmission scanning magneto-optical microscopy

We developed a scanning magneto-optical microscope to quantitatively evaluate three-dimensional magnetization distributions within magnetic holograms. By scanning a focused laser beam, we measured the focal-height-dependent Faraday rotation angle distributions of magnetic fringes in bismuth-substituted rare-earth iron garnet films. The rotation angle amplitude exhibited characteristic changes with focal height, governed by the magnetic fringe geometry and the laser's conical divergence. This divergence alters the effective probe region within the magnetic domains as the focal height varies. Numerical calculations based on a geometric model of the laser transmission successfully reproduced these experimental trends, including non-monotonic amplitude recoveries. This focal-height dependence provides a robust basis for estimating depth and spatial structure in embedded magnetic features, showing strong potential for reconstructing three-dimensional magnetization distributions.