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 approximate to 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. These calculations predict that, due to the finite channel length, there are two partial DWs at either side of the channel. For high resistance contacts this agrees with the observed AMR, which results from scattering in the homogeneously magnetized material in the channel. The MR minimum for low resistance contacts arises from the DWs, which cause a resistance decrease. We attribute this decrease to a change of spin-dependent diffuse scattering at the constriction boundary due to the DWs. (C) 2001 American Institute of Physics.