Atomic Scale Characterization and Magneto-Transport Properties of Mechanically Milled Cu-Co Type Alloys

摘要

We have prepared Cu_(80)Co_(20) powders by high-energy ball milling under ambient atmosphere. The evolutions of both the microstructure and the magnetic properties have been investigated as a function of milling time. 3D-focused ion beam was used for microstructural characterization for low milling time (1.5 h). For higher milling time (20 h) atom probe tomography was used to investigate at the atomic scale the elemental distribution of chemical species. In the first steps of milling, the powder presents a ferromagnetic signal due to the presence of micrometric cobalt particles. After 20 h of milling, the powder is composed of CoO particles with diameter around 10—50 nm surrounded by a copper rich matrix (Cu-84 ± 2%, Co-16 ± 2%) and a few cobalt-rich nanoclusters. For this milling time, a typical giant magnetoresistive effect is observed. A positive magnetoresistive effect is also observed at 5 K under low magnetic fields, which is related to the presence of Co oxides clusters. After a heat treatment at 450 °C for 1 h, the precipitation of Co into the copper rich matrix is observed, the giant magnetoresistive effect is enhanced, and the positive magnetoresistive effect disappears.