The relation between giant magnetoresistance (GMR) and phase separated nanostructures in ${\mathrm{Cu}}_{90}{\mathrm{Co}}_{10}$ is studied using magnetotransport measurements together with transmission electron microscopy and x-ray microanalysis. The samples were melt-spun ribbons isochronally annealed up to 873 K, and all show in their grains a homogeneous spinodal decomposition characterized by long parallel Co-excess stripes. These stripes have 40-nm modulation periods and develop along the crystalline directions of each grain. Different anneals do not change appreciably the observed microstructures, while the magnetoresistance is initially enhanced by a factor of 2, followed by a 35% drop above 823 K. The latter coincides with the observation of a secondary lamellar decomposition of 4 nm modulation length. We propose that the GMR effects in CuCo alloys originate from these nanoscopic modulations of the constituents induced by nonequilibrium processes.

• Received 2 May 2002

DOI:https://doi.org/10.1103/PhysRevB.68.014434