Structure and magnetic properties of Cu-10 wt% Co alloys processed by twin roller melt spinning

Nanostructured magnetic Cu-10 wt% Co alloys have been processed by ultra rapid solidification, in a twin roller melt spinning device. This solidification method (two wheels) is known to promote a more uniform microstructure as compared with that resulting in single wheel devices. The structural properties of ribbons processed at tangential wheel speeds of 5 and 23 m/s, in the as-cast condition and after a heat treatment of 1 h at 923 K, are investigated by X-ray diffraction techniques; the magnetic properties are also characterized by M–H loop measurements and the electric resistance measured as a function of the applied magnetic field at 300 K. The M–H loops can be described by the sum of a superparamagnetic like component involving small, interacting clusters of pure Co, and a soft ferromagnetic contribution which is likely to arise in the matrix. The X-ray spectra are consistent with a Cu matrix exhibiting a spinodal like Co concentration profile even in specimens in the as-cast condition. Isothermal annealing above 873 K results in the formation of larger Co clusters, in an improvement of the nominal coercivity and also leads to a higher polarization at a maximum field of 1.5 T. On the other hand, magnetoresistance effects are observed in as-cast samples solidified at 23 m/s but not in the specimen cooled at 5 m/s nor in those heat treated for 1 h at 923 K.