Magnetically induced decomposition in Co-Cr thin-film and bulk alloys

Abstract

It has been reported in recent years that composition modulations on a ten nanometer scale with unknown origin occur in Co-Cr alloy thin films. In the present investigation, we intend to analyze the origin of these concentration modulations. Sputtered thin films as well as bulk samples both with compositions of about 20 at. % Cr were analyzed by transmission electron microscopy and by field-ion microscopy equipped with atomic-probe mass spectrometry. The experimental results show that composition modulations are present in both types of alloys, with the concentration varying between 8 and 40 at. % Cr. The length scale of the composition modulations is approximately 10 and 40 nm for the thin film and the bulk samples, respectively. The results are interpreted in terms of the thermodynamics and kinetics of the system. Calculations of the thermodynamic functions of the system show that at high Co concentrations, the occurrence of magnetism leads to a downwards curvature of the Gibbs energy vs concentration curves. As a result of this magnetic stabilization at Co-rich concentrations, a metastable miscibility gap occurs that explains the tendency for phase separation into ferromagnetic and paramagnetic phases. The diffusivity of the system was examined using multilayer thin-film diffusion couples, and the results indicate that grain-boundary diffusion is responsible for the fast decomposition process observed. It is concluded that decomposition takes place by a discontinuous precipitation process that starts from grain boundaries.