Domain nucleation processes in mesoscopic Ni80Fe20 wire junctions

The magnetization reversal process in permalloy (Ni80Fe20) wire junction structures has been investigated using magnetoresistance (MR) measurements and scanning Kerr microscopy. A combination of electron beam lithography and a lift-off process has been utilized to fabricate wires consisting of two 200 μm length regions with distinct widths w1 and w2 in the range 1–5 μm. Longitudinal MR measurements and magneto-optic Kerr effect hysteresis loops demonstrate that the magnetization reversal of the complete structure is predominantly determined by the wider region for fields applied parallel to the wire axis. Magnetic force microscopy and micromagnetic calculations show that several domain walls nucleate in the wider part and are trapped in the junction area. This implies that domain nucleation at the junction of the wire initiates magnetization reversal in the narrow half. As a consequence, the switching fields are found to be identical in both halves in this case. These results suggest the possibility of designing structures which can be used to “launch” reverse domains in narrow wires within a controlled field range.