Controlled magnetic switching in single narrow rings probed by magnetoresistance measurements

We present anisotropic magnetoresistance measurements of magnetic switching processes in narrow ferromagnetic permalloy rings fabricated with six nonmagnetic electrical contacts. We demonstrate that measuring the resistance between different contacts allows the determination of the domain wall positions. Furthermore, these measurements also yield the possibility of determining the local switching fields of different parts of the ring. This provides a very useful tool to explore the complete switching process of a single ring. We show that, by using notches of suitable size and by applying fields along appropriate directions, it is possible to select the circulation direction of the vortex state using a uniform field only. (C) 2002 American Institute of Physics.