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

摘要

Structural, compositional, and chemical changes during the growth of ion beam-synthesised nanoparticles under electron beam annealing (EBA) at 1,000 °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 Fe _xSi _(33-x)O _y (shell) structures to single 5-30 nm Fe crystalline nanoparticles sitting on the SiO _2 as the annealing time was increased. The stoichiometry of the shell after 60 s EBA at 1,000 °C was found to be (O _2) _x:Fe _(14)Si _(19)O _(52) with x between 1 and 7.5 where x increases with increasing depth. This amorphousshell 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 μB per Fe atom even at 300 K. The low-temperature magnetic hysteresis and magneto-optical Kerreffect 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°.