HORIZONTAL BLOCH LINES AND ANISOTROPIC-DARK-FIELD OBSERVATIONS

Anisotropic-dark-field optical imaging is shown to reveal the presence of horizontal Bloch lines (HBL's) in the moving walls of bubble garnets. It relies on the deformation of the wall shape under dynamic conditions due to the presence of a HBL. The wall tilt is detected because it affects the intensity of the wall images much in the same way as localized wall tilts reveal the location of vertical Bloch lines (VBL's) in static conditions. Quantitative time-resolved measurements of wall intensity are presented and shown to compare very favorably with numerical computations. These use a simplified spin dynamics equation (Slonczewski's equations) describing the wall position and wall-core magnetization in-plane angle, as a function of position across the thickness, and time. Dynamic reversals of VBL's contrast have also been observed. The fact that they occur for one half of the possible configurations is puzzling. They are tentatively explained on the basis of a dynamically induced charge distribution at VBL location, which arises from a wall wave developing around the moving VBL.