We have measured the giant magnetoresistance (GMR) and the magnetization of a melt-spun granular sample of ${\mathrm{Cu}}_{87}$${\mathrm{Co}}_{13}$. The Cu matrix contains very small particles of Co that exhibit superparamagnetism. Although the GMR is due to these small superparamagnetic particles, we find that the GMR does not vary quadratically with the magnetization. This unexpected result is attributed to the presence of a range of sizes for the superparamagnetic particles. Assuming a simple distribution of particle sizes, we calculated the magnetic-field dependence of the GMR and find excellent agreement with experiment.

• Received 1 July 1994

DOI:https://doi.org/10.1103/PhysRevB.51.667