Bloch-line generation in cross-tie walls by fast magnetic-field pulses

The magnetization dynamics of a constrained cross-tie wall in a 50 - nm -thick Ni 81 Fe 19 element is investigated using static and time-resolved wide-field Kerr microscopy. By applying a series of short magnetic-field pulses to the sample the creation of additional vortex antivortex pairs within the cross-tie wall is observed. The cross-tie spacing decreases by a factor of 2-4 relative to the initial equilibrium state. By using sinusoidal and pulsed magnetic-field excitations, a discrimination between the influence of the repetition rate and the rise time of the applied fields is possible. We found a logarithmic increase of the Bloch-line number with decreasing rise time. A resonant excitation with increased repetition rate decreases the cross-tie spacing even further. From direct stroboscopic observation of the precessional remagnetization process, a mechanism for vortex multiplication is proposed.