Magnetoresistance of single Permalloy circular rings

We have measured magnetoresistance in single, 1 mm external diameter, Permalloy (Ni80Fe20) circular rings with varied inner holediameter of 150, 300, and 600nm and film thickness of 25 nm. The Permalloy ring structures and the 10-nm-thick, 250-nm-wide Aunanocontacts were fabricated on a SiO2/Si substrate using e-beam lithography. Using a four contact geometry we studied the dependenceof the magnetoresistance on the direction of the applied field. The experimental data are explained by considering only the conventionalanisotropic magnetoresistance effect. Numerical simulations of the current distribution within the samples combined with micromagneticsimulations of the field dependent magnetization profile, yield good agreement with the experimental data. Upon increasing the innerhole diameter (viz. decreasing the ring width) the magnetoresistance measurements show a transition of the reversal process from the‘‘vortex nucleation–displacement–annihilation’’ sequence to the ‘‘onion state–reversed onion state’’ sequence, typical of narrownanorings.r 2007 Elsevier B.V. All rights reserved.