Multiple switching fields and domain wall pinning in single Co nanowires

We have measured the low temperature magnetoresistance (MR) of ferromagnetic cobalt nanowires of 30 nm thickness and widths between 32 and 700 nm. The wires are in situ capped with a 2 nm Pt layer to prevent oxidation. Single wires exhibit sharp resistance peaks at the coercive fields H-c-where H-c increases with decreasing wire width-which can be completely understood in terms of an in-plane switching process. Adding 2 x 2 mu m(2) nucleation pads at the wire ends clearly reduces the coercive fields of the wires. By fabricating wires with two different widths a two-step switching process can be achieved where the wider nanowire switches first. In this case, a single domain wall is introduced in the constriction between the wider and the narrower part of the wire. The MR can be studied in more detail and with higher accuracy by adding additional voltage leads close to the domain wall. The results are discussed to distinguish between contributions resulting from domain wall magnetoresi stance and anisotropic magnetoresistance (AMR).