Magnetic Behavior of Twin Roller Melt Spun Cu $_{90}$ Co $_{10}$ Alloys

Magnetoresistive Cu₉₀Co₁₀ alloys have been produced by twin roller melt spinning at tangential wheel speeds between 10 m/s and 30 m/s to obtain different solute and Co-rich precipitate distributions. X-ray diffraction indicates that the ribbons are polycrystalline with a fcc Cu(Co) (200) textured matrix; no evidence of a spinodal-like composition modulation could be detected with this technique. Transmission electron microscopy observations performed in samples quenched at 10 m/s indicate the existence of small, coherent, Co-rich precipitates of mean size of about 11 nm and bigger (~40 nm) Co-oxide particles. The small coherent precipitates form colonies inside the grains, mediated by precipitate free zones. Samples quenched at 30 m/s are single magnetic phase (superparamagnetic), but the hysteresis loops of the other as cast ribbons are well fitted by a superparamagnetic contribution and a ferromagnetic one, the latter arising from the Co-rich precipitates. Room temperature coercivities, of about 30-50 mT, are lower than those predicted for a mechanism of coherent rotation in the ensemble of non-interacting, Co-rich precipitates.