Effects of structural nonplanarity on the magnetoresistance of Permalloy circular rings

We have measured magnetoresistance in single, 1 micron external diameter, Permalloy Ni80Fe20circular rings with inner diameters of 150, 300, and 600 nm and film thickness of 25 nm. The ringstructures were deposited on 25-nm-thick, 250-nm-wide Au nanocontacts previously fabricated ona SiO2 /Si substrate by e-beam lithography. Using a four contact geometry, we studied thedependence of the magnetoresistance on the direction of the applied field. The experimental datawere compared with simulations based on the conventional anisotropic magnetoresistance effect andon numerical simulations of the current distribution within the samples combined withmicromagnetic simulations of the field dependent magnetization profile. Two different approacheswere used for the simulations: one assumes flat magnetic structures with the Au contacts depositedon top of them; in the second approach the simulations were carried out using nonplanar magneticstructures deposited on the Au contacts. The magnetoresistance curves calculated using the secondapproach differ considerably from those obtained assuming flat magnetic structures and reproducethe experimental curves. The results presented demonstrate that the nonplanarity of the magneticstructures affects profoundly the magnetoresistive behavior of the ring structures. © 2007 AmericanInstitute of Physics.