Domain nucleation processes in mesoscopic Ni80Fe20 wire junctions

The magnetization reversal process in permalloy (Ni80Fe20) wire junction st ructures has been investigated using magnetoresistance (MR) measurements an d scanning Kerr microscopy. A combination of electron beam lithography and a lift-off process has been utilized to fabricate wires consisting of two 2 00 mu m length regions with distinct widths w(1) and w(2) in the range 1-5 mu m. Longitudinal MR measurements and magneto-optic Kerr effect hysteresis loops demonstrate that the magnetization reversal of the complete structur e is predominantly determined by the wider region for fields applied parall el to the wire axis. Magnetic force microscopy and micromagnetic calculatio ns show that several domain walls nucleate in the wider part and are trappe d in the junction area. This implies that domain nucleation at the junction of the wire initiates magnetization reversal in the narrow half. As a cons equence, the switching fields are found to be identical in both halves in t his case. These results suggest the possibility of designing structures whi ch can be used to "launch" reverse domains in narrow wires within a control led field range. (C) 2000 American Institute of Physics. [S0021-8979(00)013 06-2].