Mössbauer effect determination of chemical segregation in sputtered Co-Cr films

Abstract

Mössbauer spectroscopy has been used to determine several of the features of sputtered Co-Cr films. Isotopically enriched 57Fe was used as a probe in the films and in bulk alloys employed as calibration standards. Film stoichiometries from 14.5 to 32 at. % Cr were measured. Three phases are observed. For films sputtered at 200 °C substrate temperature, one phase (about 40 at. % of the films) has a composition close to the overall film composition. The second and most accurately determined phase is Co rich, showing increasing Cr concentration and decreasing mole fraction with increasing overall Cr content. For example, a 21.3 at. % Cr film contains a Co-rich phase with 6.0 at. % Cr. The third phase is Cr rich and nonmagnetic. Compositions have been determined indirectly through the film saturation moments. Assuming the Fe:Co ratio is constant in the various phases, the third phase contains about 37 at. % Cr for nearly all overall film compositions, close to the σ-phase boundary. The relative amount of this phase increases with increasing total Cr. Segregation occurs for Cr film concentrations at least as low as 14 at. %. The mechanism of segregation seems to be nucleation and growth rather than spinodal. For perpendicularly oriented films, Mössbauer polarization data show no evidence of in-plane moments. Low-temperature Mössbauer data show negligible temperature-dependent relaxation.