Iron-based bimagnetic core/shell nanostructures in SiO2: a TEM, MEIS, and energy-resolved XPS analysis

Structural, compositional, and chemical changes during the growth of ion beam-synthesised nanoparticles under electron beam annealing (EBA) at 1,000 degrees C in high vacuum conditions were investigated using medium energy ion scattering, energy-resolved X-ray photoelectron spectroscopy, and transmission electron microscopy. The nanoparticles evolved from <5 to 30 nm diameter Fe-metal (core)/amorphous FexSi33-xOy (shell) structures to single 5-30 nm Fe crystalline nanoparticles sitting on the SiO2 as the annealing time was increased. The stoichiometry of the shell after 60 s EBA at 1,000 degrees C was found to be (O-2)(x):Fe14Si19O52 with x between 1 and 7.5 where x increases with increasing depth. This amorphous-shell phase has not been previously reported and a reanalysis of the magnetization data shows that it is ferromagnetic with a magnetic moment of about 1.7 mu(B) per Fe atom even at 300 K. The low-temperature magnetic hysteresis and magneto-optical Kerr-effect hysteresis at room temperature are enhanced when the core/shell nanoparticles are present. A maximum coercive field of about 740 Oe was found with a corresponding Kerr rotation of about 0.04 degrees.