Negative resistance contribution of a domain-wall structure in a constricted geometry

We study the magnetoresistance (MR) of Py/Py, Co/Py, Co/Co, Ni/Ni, and Co/Cu point contacts (Py=permalloy=Ni80Fe20). These devices are narrow constrictions or channels (diameter, length ≈30 nm) between two thin film electrodes. Due to the small size of the constriction, which is comparable to a bulk domain-wall (DW) thickness, a DW can be caught in it. For almost all material combinations studied we find that low resistance contacts show an MR minimum at zero field (H=0) of magnitude 0.4%–1.3%, for temperatures between 1.5 and 293 K. The minimum occurs for all field orientations with respect to the channel axis. When the contact resistance increases beyond the value set by a diameter-to-length ratio for the channel of about unity, the resistance minima at H=0 evolve into a maximum/minimum combination as expected for a predominant anisotropic magnetoresistance (AMR) effect. We use micromagnetic calculations based on magnetostatic and exchange interactions to obtain the magnetization in the constriction. Th...