Magnetoresistance of single Permalloy circular rings

We have measured magnetoresistance in single, 1 mu m external diameter, Permalloy (Ni80Fe20) circular rings with varied inner hole diameter of 150, 300, and 600 nm and film thickness of 25 nm. The Permalloy ring structures and the 10-nm-thick, 250-nm-wide Au nanocontacts were fabricated on a SiO2/Si substrate using e-beam lithography. Using a four contact geometry we studied the dependence of the magnetoresistance on the direction of the applied field. The experimental data are explained by considering only the conventional anisotropic magnetoresistance effect. Numerical simulations of the current distribution within the samples combined with micromagnetic simulations of the field dependent magnetization profile, yield good agreement with the experimental data. Upon increasing the inner hole 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 narrow nanorings. (c) 2007 Elsevier B.V. All rights reserved.