Domain wall trapping at mesoscopic ferromagnetic junctions

We have investigated the effects of domain walls (DWs) on electron transport across mesoscopic ferromagnetic junctions. The structures consist of a single 30 nm Ni80Fe20 thick 0.5×24 μm wire bisected by a mesoscopic junction with width w in the range 0.2–1 μm. Using magnetoresistance (MR) measurements, we observed directly an extra resistivity associated with controlled domain wall nucleation process on a complete field hysteresis loop for various junction widths. For the field applied along the wire hard axis, we observed complex MR curves at low fields for wires with w>0 compared with a simple parabolic MR behavior of a single wire of the same lateral dimension. This is attributed to the trapping of DWs at the junction area perpendicular to the current direction. The MR behavior for the field applied along the wire easy axis is strikingly dependent on the junction width due to the variation in spin configurations at the junction area. The field angle dependent MR responses and simple micromagnetic simulations suggest that the magnetization reversal process at the junction area is mediated by domain wall propagation.