Amorphous to polycrystalline transition in Co $\mathsf{_x}$ Si $\mathsf{_{1-x}}$ alloy thin films

Abstract

The transition from amorphous to polycrystalline microstructure has been studied in sputtered Co x Si1-x alloy films by structural, magneto-optical and Nuclear Magnetic Resonance measurements. For $x \geq 0.76$ , Si is diluted into Co without significantly altering the polycrystalline microstructure, composed of a mixture of hcp and fcc grains. However, the fraction of Co atoms that contribute to the Nuclear Magnetic Resonance signal is found to decrease steeply (down to about 60 $\%$ at x = 0.76) suggesting a microscopic segregation of a Si rich phase that induces a large degree of disorder. This is reflected in a harder magnetic behavior and a strong anisotropy dispersion. Below x = 0.75, the transition to an amorphous microstructure results in a sudden increase in the fraction of Co atoms within a ferromagnetic phase, indicating the recovery of the microscopic homogeneity. Also a significant enhancement of the macroscopic magnetic anisotropy is found for amorphous films with compositions right below the transition. Within the amorphous phase a second regime of Si segregation appears characterized by a constant Co local environment and constant magnetic properties. Finally, for x = 0.65 there is a significant Si enrichment in the Co environment and the films become non magnetic for compositions below this point.