NEEL LINES IN THE BLOCH WALLS OF BUBBLE GARNETS AND THEIR DARK-FIELD OBSERVATION

The technique for optical microscopy, [polarized anisotropic dark-field observation (PADO)], of vertical Néel lines (also called vertical Bloch lines) existing inside the Bloch walls separating magnetic domains in thin epitaxial films of ferrimagnetic garnets (bubble garnets) is studied and applied here. Results of experiments investigating the role of the polarization of light impose severe constraints on the theoretical models. The picture emerging from them excludes a contrast due to light diffracted by the usual magneto-optical effects inside the wall. By varying the angle of incidence of the anisotropic dark-field illuminating beam, line contrast inversions can be obtained. It is shown that the sample's thickness, when well above the wavelength of light, is responsible for that fact. Moreover, to reproduce all line contrast reversals, an effective height which is smaller than the sample's thickness has to be considered for the line structure. A tempting conclusion from this study may be that lines are seen in PADO by means of a wall deformation, the origin of which has to be found.