Evolution of local atomic structure accompanying devitrification of amorphous Ni-Zr alloy thin films

Abstract

Thin film metallic glasses undergoing devitrification can form partially crystallized or fully crystallized materials with novel structural and magnetic properties. The development of desired and tunable properties of such systems drives the need to understand the mechanism of their thermal evolution at the atomic level. Co-sputtered amorphous Ni-Zr alloy thin films which were thermally annealed in steps of 200°C from room temperature up to 800°C, were observed to undergo an amorphous-to-crystalline transformation. Evolutions in local atomic structure, including oxide formation and depletion during this devitrification process, were determined using a combination of X-ray reflectivity (XRR), grazing incidence X-ray diffraction (GIXRD) and Extended X-ray Absorption Fine Structure (EXAFS) techniques. The most probable phase of the alloy was determined as Ni7Zr2; undergoing a polymorphous transformation. The slight oxide content in the films was also noted to decrease rapidly as annealing proceeded; leaving the crystallization pathway unhindered. modeling and analyses of XRR and GIXRD data showed that film thickness decreased with annealing while long range order increased. Pair distribution function peak widths were observed to decrease with annealing, indicating a control over structural disorder in the system as it transitioned from the amorphous to crystalline state. Partial atomic distribution in the environment of each constituent was examined at every stage of annealing through EXAFS measurements, which gave proper insight into atomic scale changes to the order present. Detailed analyses through these techniques gave coherent results, thus providing a true picture of the thermal evolution process of devitrification in this technologically useful material.