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 {}. 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 [11{}0] remaining constant with decreasing {Λ}. The Ni [22{}0] 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 {} 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.