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We have developed a new technique1 for direct optical observation of domain wall (DW) polarity (direction of midwall magnetization M) in order to determine the location of vertical Bloch lines (VBL) in LPE bismuth based garnet films with perpendicular anisotropy. A polarizing light microscope with slope illumination of the sample (one-side dark-field observation technique) and a quartz quarter wave plate, placed behind the objective, were used for phase corrections so that some DW (or segments) are shaded. Equations describing intensity distributions of the light diffracted by magnetic inhomogeneities were obtained from diffraction theory.1 The phase relations between complex amplitudes Ep and Es of the diffracted wave were shown to be a function of the DW polarity. According to this theory, the diffracted light from domain walls of opposite polarity will exhibit a dark light for suitable conditions of polarizer and quarter wave plate orientation. The theoretical results were experimentally verified. The proposed magneto-optic (MO) technique where the image contrast depends on the polarity of DW segments differs from that of Refs. 2 and 3 since we used the phase correction (plate λ/4) and an electronic or computer processing of the image were not needed.<ks>

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