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

Abstract

The changes in the nanostructure and magnetic and magnetotransport propertiesof Co10Cu90 melt-spun ribbons upon heating them have been measured. Thermaltreatments to 650 K do not produce changes either in the x-ray diffractionpatterns or in the magnetization isotherms of the ribbons. However, upon heatingthe samples above 650 K, a contraction of the Cu lattice, an irreversible decreaseof 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 × 105 A m−1) is reached in samplesheated to 850 K. These results are explained by assuming the nucleationand growth of superparamagnetic Co nanoclusters in the Cu-rich matrix.The magnetic moment of these clusters was estimated to be 104 μB.Thermal treatments above 850 K produce an increase of the magnetic moment ofthe samples (in magnetic fields of 8 × 105 A m−1) and of their coercivity, althoughthe value of the GMR decreases. Parallel to these changes, the intensity of thex-ray diffraction peaks for Co increases. Further growth of the Co nanoclusters isresponsible for the changes of the magnetic moment and GMR of the samples.