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/C u point contacts (Py=permalloy=Ni80Fe20). These devices are narrow constric tions or channels (diameter, length approximate to 30 nm) between two thin film electrodes. Due to the small size of the constriction, which is compar able to a bulk domain-wall (DW) thickness, a DW can be caught in it. For al most 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 tempera tures between 1.5 and 293 K. The minimum occurs for all field orientations with respect to the channel axis. When the contact resistance increases bey ond the value set by a diameter-to-length ratio for the channel of about un ity, 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 inter actions 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 wit h the observed AMR, which results from scattering in the homogeneously magn etized material in the channel. The MR minimum for low resistance contacts arises from the DWs, which cause a resistance decrease. We attribute this d ecrease to a change of spin-dependent diffuse scattering at the constrictio n boundary due to the DWs. (C) 2001 American Institute of Physics.