Effects of swift heavy ion irradiation and thermal annealing on nearly immiscible W/Ni multilayer structure

The effect of 120 MeV Au 9 + ion irradiation and thermal annealing on [ W ( 25 Å ) ∕ Ni ( 25 Å ) ] × 10 multilayers, grown on float-glass and silicon substrates, has been studied. Wide-angle x-ray diffraction studies of pristine, as well as irradiated W/Ni multilayers, show deterioration of the superlattice structure, but x-ray reflectivity (XRR) studies reveal a nearly unaffected multilayer structure. Analysis of the XRR data using "Parratt's formalism" does show a significant increase of W/Ni interface roughness. The observed differences in wide-angle and low-angle scattering results of the irradiated W/Ni multilayers suggest significant difference in the interlayer and intralayer mixing induced by swift heavy ion irradiation. XRR results also reveal the fluence dependence of layer densification. Plane, as well as cross-sectional transmission electron microscopy, carried out in imaging and diffraction modes very clearly shows that at higher fluence the intralayer microstructure becomes nanocrystalline (1-2 nm) and at some places amorphous too. But, the definition of the W and Ni layers still remains intact. This has been understood in terms of competition between low miscibility at the W/Ni interface and ion-beam mixing kinetics.