Synthesis and characterization of core–shell structure silica-coated Fe29.5Ni70.5nanoparticles

In view of potential applications of magnetic particles in biomedicine and electromagnetic devices, we made use of the classical Stöber method—base-catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS)—to encapsulate FeNi nanoparticles within a silica shell. An original stirring system under high power ultrasound made it possible to disperse the otherwise agglomerated particles. Sonication guaranteed particles remained dispersed during the Stöber synthesis and also improved the efficiency of the method. The coated particles are characterized by electron microscopy (TEM) and spectroscopy (EDX) showing a core–shell structure with a uniform layer of silica. Silica coating does not affect the core magnetic properties. Indeed, all samples are ferromagnetic at 77 K and room temperature and the Curie point remains unchanged. Only the coercive force shows an unexpected non-monotonic dependence on silica layer thickness.