Detection of the magnetization reversal in submicron Co particles by GMR measurements

Co particles were obtained by patterning trilayers such as Co/Cu/Co and NiFe/Cu/Co grown by sputtering. Using electronic lithography, we can design on these layers a microsensor for transport measurements and a Co particle on the top layer. The Co particle and the sublayers form a spin-valve structure due to their different coercivity. We used GMR measurements to detect the magnetization reversal in Co particles with sizes between 100 x 50 mu m(2) and 180 x 20 nm(2). We present the size dependence of the GMR ratio and of the reversal magnetic field. For the smallest size, we do not find a Stoner- Wolfahrt-like rotation (i.e. uniform rotation) of the Co particle magnetization, but GMR displays staircase curves indicating discrete variation of this magnetization,