Magneto-optic observation of Bloch lines

Abstract

We report the first visual observation of Bloch lines in thin garnet films without the perturbing influence of magnetic particles in suspension (Bitter solution). A second-order magneto-optic effect reveals the Bloch line position and the signs and positions of the domain wall segments lying adjacent to it. The present study is limited to Neel walls that separate in-plane domains aligned along [110] "easy" magnetic axes in a (100) garnet film 0.8 μm thick with large in-plane uniaxial anisotropy (Ku≪0). Images are viewed using normally incident laser illumination and crossed polarizers in a polarizing microscope also having an adjustable optical phase compensator. The contrast image for 180° walls with incident light vector E ∥ polarized parallel to the wall is an adjacent pair of dark and light stripes. The usual first-order Faraday effect (magnetic circular birefringence) is excluded because all components of M are perpendicular to the beam. The quadratic dependence on M is verified, outside the wall at least, in domain contrast studies, e.g., by the lack of contrast between two opposing 180° domains. Within the wall magnetic linear birefringence would cause incident light E∥ to emerge as right- or left-handed elliptically polarized light according to the quadrants of the orientation of M wall in the E∥-E⊥ plane. Thus, the observed ordering of black/white or white/black stripes in the image reflects the sign of twist in M wall when the transmitted light is viewed through a compensator and analyser. The Bloch-line position is delineated by a discontinuous reversal in the black/white ordering.