Advancements in magnetic nanoparticle design: SiO2@Fe3O4 core/shell nanoparticles with size-tunable magnetic responses

Thin magnetic films conformed to closed surfaces are theoretically predicted to exhibit exotic magnetic ground states at microscopic curvatures. However, there is a general lack of experimental reports on the subject, mostly associated to challenges in achieving the desired geometry at the nanoscale. In this work, we tackle this issue by experimentally probing magnetite nanoshells grown on the surface of bare silica nanospheres of 20 nm - 600 nm in diameter. Such a system can be described as magnetic centers arranged in a closed, non-flat film. Results reveal that the absence of a magnetic core results in shells with a curvature-dependent magnetic response which is markedly different to that of conventional bulk magnetic nanoparticles. We report that a granular aspect in such magnetic nanoshells negatively affects the formation of clear vortex signatures in measurements of macroscopic sample quantities. Micromagnetic simulations suggest this to be the result of the pinning of vortices on different regions of the sphere surface, leading to sharp, field-driven variations of the magnetic response of individual particles.