The effect of heat treatments on the structure and magnetic properties of melt-spun Co10Cu90 ribbons

The changes in the nanostructure and magnetic and magnetotransport properties of Co 10 Cu 90 melt-spun ribbons upon heating them have been measured. Thermal treatments to 650 K do not produce changes either in the x-ray diffraction patterns or in the magnetization isotherms of the ribbons. However, upon heating the samples above 650 K, a contraction of the Cu lattice, an irreversible decrease of their resistivity and an increase of the giant magnetoresistance (GMR) of the heated samples are observed. The largest value of the GMR (7% at 77 K and in a magnetic field of 8 × 10 5 A m −1 ) is reached in samples heated to 850 K. These results are explained by assuming the nucleation and growth of superparamagnetic Co nanoclusters in the Cu-rich matrix. The magnetic moment of these clusters was estimated to be 10 4 μ B . Thermal treatments above 850 K produce an increase of the magnetic moment of the samples (in magnetic fields of 8 × 10 5 A m −1 ) and of their coercivity, although the value of the GMR decreases. Parallel to these changes, the intensity of the x-ray diffraction peaks for Co increases. Further growth of the Co nanoclusters is responsible for the changes of the magnetic moment and GMR of the samples.