Template-Directed FeCo Nanoshells on AuCu

A synthetic route is reported to achieve a precise control of FeCo shell growth on AuCu cores, leading to AuCu/FeCo core-shell nanoparticles, which could potentially open up the metastable tetragonal FeCo growth. Due to nonmagnetic AuCu core, the FeCo shell exhibits a transition from single domain to magnetic vortex state, which is determined by the particle radius and the thickness of the spheroidal shell. In our system, the transition occurs when the outer radius of the shell reaches about 8.9 nm. Furthermore, our FeCo shells exhibit excellent passivation against oxidation, due to an annealing-induced thin graphite shell. This work shows the uniqueness and versatility of the colloidal core-shell nanotechnology to manipulate nanostructures with desirable functionality.