Exchange bias effects in ferromagnetic wires

Abstract

We have investigated the exchange bias effect in micron-sized ferromagnetic wires made from Co and Ni 80Fe 20 films. The wires were fabricated using optical lithography, metallization by sputtering and lift-off technique. Magnetotransport measurements were performed at temperatures ranging from 3 to 300 K. We observed marked changes in the magnetoresistance (MR) properties as the temperature is varied. At 300 K, the field at which the sharp peak occurs corresponding to the magnetization reversal of the Co wires is 167 Oe and is symmetrical about the origin. As the temperature was decreased to 3 K, we observed a shift in the peak positions of the MR characteristics for both the forward and reverse field sweeps corresponding to a loop shift of 582 Oe in the field axis. The asymmetric shift in the MR loops at low temperatures clearly indicates the exchange bias between ferromagnetic (Co) and antiferromagnetic parts (Co-oxide at the surfaces) from natural oxidation. Ni 80Fe 20 wires of the same geometry showed similar effect with a low exchange bias field. The onset of exchange biasing effect is found to be 70 and 15 K for the Co and Ni 80Fe 20 wires, respectively. A striking effect is the existence of exchange biasing effect from the sidewalls of the wires even when the wires were capped with Au film.