Nanosecond time-scale switching of permalloy thin film elements studied by wide-field time-resolved Kerr microscopy

The switching of extended Ni81Fe19 thin film elements with a thickness of 50 nm and various shapes (squared, rectangular, pointed) has been studied by time-resolved stroboscopic Kerr microscopy based on a conventional wide-field optical polarization microscope. The elements are deposited on coplanar strip-lines that generate field pulses driven by electronic pulse generators. Time resolution is obtained by imaging with a gated and intensified charge-coupled device camera. The opening can be varied from 250 ps to continuous exposure, allowing the comparison of fast magnetization processes and quasistatic switching in slowly varying fields. The latter is typically characterized by the formation of a concertina domain pattern that irreversibly decays in a multidomain ground state by the abrupt motion of vortices and domain walls. After excitation with fast field pulses similar blocked patterns are formed. They dissolve by spatially inhomogeneous rotational processes involving cross-tie-wall-like domain boundaries.