Origin of Enhancement of Orbital Magnetic Moment in SiO 2 -Coated Fe 3 O 4 Nanocomposites Studied by X-ray Magnetic Circular Dichroism.

In this study, magnetic Fe 3 O 4 nanoparticles (NPs) were dispersed uniformly by varying the thickness of the SiO 2 coating, and their electronic and magnetic properties were investigated. X-ray diffraction confirmed the structural configuration of monophase inverse-spinel Fe 3 O 4 NPs in nanometer size. Scanning electron microscopy revealed the formation of proper nonporous crystallite particles with a clear core-shell structure with silica on the surface of Fe 3 O 4 NPs. The absorption mechanism studied through the zeta potential indicates that SiO 2 -coated Fe 3 O 4 nanocomposites (SiO 2 @Fe 3 O 4 NCs) possess electrostatic interactions to control their agglomeration in stabilizing suspensions by providing a protective shield of amorphous SiO 2 on the oxide surface. High-resolution transmission electron microscopy images demonstrate a spherical morphology having an average grain diameter of ∼11-17 nm with increasing thickness of SiO 2 coating with the addition of a quantitative presence and proportion of elements determined through elemental mapping and electron energy loss spectroscopy studies. Synchrotron-based element-specific soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) techniques have been involved in the bulk-sensitive total fluorescence yield mode to understand the origin of magnetization in SiO 2 @Fe 3 O 4 NCs. The magnetization hysteresis of Fe 3 O 4 was determined by XMCD. At room temperature, the magnetic coercivity ( H c ) is as high as 1 T, which is about 2 times more than the value of the thin film and about 5 times more pronounced than that of NPs. For noninteracting single-domain NPs with the H c spread from 1 to 3 T, the Stoner-Wohlfarth model provided an intriguing explanation for the hysteresis curve. These curves determine the different components of Fe oxides present in the samples that derive the remnant magnetization involved in each oxidation state of Fe and clarify which Fe component is responsible for the resultant magnetism and magnetocrystalline anisotropy based on noninteracting single-domain particles.