New effects in ferromagnetic nanostructures

We report studies of the magnetoresistance of very narrow Ni wires at low temperatures. Our measurements reveal an unanticipated longitudinal magnetoresistance, which is a function of the width of the wire. This magnetoresistance is also a function of M . H, and can thus be used to discern the behavior of M, and the associated motion of domain walls. Studies of this motion are described which show, not surprisingly, that the walls can be pinned at sites which we believe include structural 'defects' such as width fluctuations. It appears that at high temperatures the walls escape from these pinning sites by thermal activation over the associated potential barrier, while at low temperatures they escape by tunneling through the barrier.