Phase transformations and magnetism in patterned FePd thin films

Abstract

Local surface curvature and effects associated with a large surface-to-volume ratio are of great importance in thin films and can influence the character of temperature-induced phase transitions. In particular, these effects have a substantial influence on the transformation of thin multilayers into nano-alloys where the introduction of template-assisted patterning can shift the transformation temperature, induce a crystallographic texture, and change the phase composition. In this work, we study the phase transformations in patterned Fe/Pd multilayers leading to the formation of FePd alloys. We used two different template-assisted patterning methods, nanosphere lithography and anodization process, to study the influence of film morphology along with local curvature on the transformation process, structure, and magnetism of the resulting alloy. We combined Mössbauer spectroscopy, X-ray diffraction, SEM imaging, and magnetometry to track these changes at short- and long-range scales. We show that film morphology is one of the most important factors determining the physical properties of the alloy formed after thermal treatment. It allows for patterning-controlled solid-state dewetting and rearrangement of atoms that in turn governs the phase transformation as well as structural and magnetic properties. We identified two counter-effective processes responsible for FePd L1₀ phase formation, the effect of patterning-induced interatomic diffusion and the patterning-limited diffusion radius.