X-ray-diffraction characterization and sound-velocity measurements of W/Ni multilayers

We present detailed structural characterization and shear elastic constant measurements of sputtered W/Ni multilayers. Structural refinement of the reflection x-ray-diffraction spectra was used to determine the out-of-phase lattice spacings of the constituent layers. The Ni(111) lattice spacing expands ∼3% and the W(110) lattice spacing is constant with decreasing modulation wavelength Λ down to Λ∼30 A. Transmission x-ray diffraction was used to determine the in-plane structure. The W layers undergo an anisotropic contraction in plane with the [002] directions contracting ∼2% and the [1 bar 10] remaining constant with decreasing Λ. The Ni [2 bar 20] expands ∼1.5% with decreasing Λ. The Ni layer expands both in plane and out of plane, contradicting Poisson ratio arguments relating in-plane and out-of-plane strains. Below Λ=35 A the multilayers undergo a structural transition in which both layers transform into a random close-packed structure. The shear velocity decreases ∼22% with decreasing Λ down to the disorder transition and then is Λ independent. The results imply that there is a correlation between the origin of the elastic anomalies and amorphization