Magnetic force microscopy and micromagnetic study of cross-tie wall structures in Co91Nb6Zr3 amorphous thin films

High resolution images of cross-tie domain wall structures have been obtained by magnetic force microscopy (MFM) for a 37.5 nm Co91Nb6Zr3 film using a NiFe thin film coated tip. Between successive cross ties, the main or spinal wall was found to be consistently subdivided unequally into pairs of oppositely oriented Néel wall sections separated by circular Bloch lines. Main and wing walls intersect at cross Bloch lines. A reversed-contrast MFM image of the same uneven cross-tie wall structure was obtained after reversing the tip magnetization. MFM images reflect only the field from the divergence of the underlying magnetization M and contain no direct information on curl M. Accordingly they are best interpreted by comparison with the magnetization pattern of a similar cross-tie structure obtained by micromagnetic computation. This enables the cross and circular Bloch line singularities to be distinguished in the MFM images of the cross-tie structure. By combining repeated observations made with opposite tip magnetizations, disturbance of the main and wing wall structures by the tip was extracted from the MFM signal which was then compared with the signal computed for a two-dimensional model wall. The main wall was found to be an asymmetric Néel wall with a weak S shaped magnetic structure. The wing walls were found to be Néel walls of acute angle, decreasing with distance from the spine.