Tunable Core-Shell Metal Alloy Pillar Design in Vertically Aligned Nanostructures Toward Multifunctionality.

Multiphase nanostructures provide a powerful platform for multifunctional materials by integrating different phases with unique functionalities for coupling effects. Here, we explore an alloy-based vertically aligned nanocomposite (VAN) thin films consisting of Au, Cu, Co, and Ni alloy nanopillars embedded in a BaTiO3 (BTO) matrix. Core-shell alloy VANs with tunable pillar morphology, ranging from spherical to faceted shapes, are fabricated by pulsed laser deposition by controlling substrate temperature and deposition frequency. These multiphase systems demonstrate tailorable optical anisotropy, tunable hyperbolic response, strong magnetic anisotropy, plasmonic resonances, and magneto-optical coupling, highlighting their potential for spintronic and photonic applications. Calculations show pillar shape morphology is a result of the interplay of surface energies, epitaxial strain, and thermal expansion mismatch between the substrate and the VAN, while spinodal decomposition kinetics of the alloy defines a time-temperature-transformation diagram for controlling the morphology inside the pillar. This work gives insight into the dynamics of phase separation and structure-property relationships in complex alloy VANs allowing for design of next-generation hybrid metamaterials.