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 obtain ed 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 spin al wall was found to be consistently subdivided unequally into pairs of opp ositely oriented Neel wall sections separated by circular Bloch lines. Main and wing walls intersect at cross Bloch lines. A reversed-contrast MFM ima ge of the same uneven cross-tie wall structure was obtained after reversing the tip magnetization. MFM images reflect only the field from the divergen ce of the underlying magnetization M and contain no direct information on c url M. Accordingly they are best interpreted by comparison with the magneti zation pattern of a similar cross-tie structure obtained by micromagnetic c omputation. This enables the cross and circular Bloch line singularities to be distinguished in the MFM images of the cross-tie structure. By combinin g 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-dimension al model wall. The main wall was found to be an asymmetric Neel wall with a weak S shaped magnetic structure. The wing walls were found to be Neel wal ls of acute angle, decreasing with distance from the spine. (C) 2000 Americ an Institute of Physics. [S0021-8979(00)06603-2].