Ono et al.[1] showed that current-in-plane giant magnetoresistance (MR) can be used to detect the trapping of a domain wall at a constriction in a long, narrow (sub-mm-wide) F/N/F spin valve, where F and N are ferromagnetic and non-magnetic metals, respectively. We have developed a new geometry (figure on the left) for detecting such domain wall trapping at a constriction in narrow F/N/F exchange-biased spin valves --- applying the current perpendicular to the plane (CPP) and only in the region of the constriction. A small superconducting-Nb contact provides uniform CPP current flow in the constriction region. We tested this idea using simple optical lithography. A Fe50Mn50/Co91Fe9(6 nm)/Cu(20)/ Co91Fe9 (6) spin valve was fabricated with 4-µm width, 40-µm length and a single 2-µm-wide constriction over which was centered a 4-µm-square top Nb contact. The figure on the right shows the CPP-MR during reversal of the "free" F-layer from the anti-parallel (AP) to parallel (P) state. The observed steps in the MR demonstrate the trapping of at least one domain wall in the constriction region. Such CPP-MR measurements should be able to detect small motions of a domain wall in the more ideal case of a sub-mm-wide strip where single walls may be trapped. We have now modified out experiments to use e-beam lithograpy. Using a bilayer to pattern an Al mask on our GMR multiplayer, we can ion-mill the multilayer into our notch-wire geometry.

[1] T. Ono et al., Appl. Phys. Lett. 72, 1116(1998).