Superparamagnetic FexOy@SiO2 core-shell nanostructures:: Controlled synthesis and magnetic characterization

We present the results of controlled synthesis of core-shell nanostructures and the influence of silica coating on the magnetic properties of formed hybrid nanoparticles. The core-shell nanoarchitectures composed of magnetic iron oxide cores and amorphous silica shells have been synthesized through a sol-gel approach and characterized by transmission electron microscopy, energy dispersive X-ray analysis, and magnetometry. It is found that many of the hybrid nanoparticles contain a single core. A good control of the silica shell thickness (10-100 nm) has been achieved by adjusting the silane concentration. From temperature-dependent zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements and ZFC model study, it is found that the mean deblocking temperature and effective anisotropy constant remain similar after the iron oxide nanoparticles are coated with about 12 nm thick silica shells. However, the ZFC peak temperature and the ZFCIFC branching point decrease significantly by over 100 K. To understand these interesting phenomena, a specific sample whose surface is modified with a small amount of tetraethoxysilane was prepared. By studying the ZFC/FC and AC behavior of these three kinds of samples, relative contribution from surface anisotropy and magnetic interparticle interactions to the blocking behavior was evaluated. The magnetic core size effect is studied also by comparing two core-shell samples with slightly different core sizes.