Structure and magnetic properties of SiO2-coated Co nanoparticles

SiO 2 -coated Co nanoparticles in a size range of 10 to 50 nm were synthesized by a wet chemical approach, and their structure and magnetic properties were investigated using x-ray diffraction, high-resolution transmission electron microscopy, and a superconducting quantum interference device magnetometer. The structure of the synthesized nanoparticles varied with calcination temperature. When the calcination temperature was as high as 900 °C, the nanoparticles had a core/shell structure: the core was fcc Co and the shell was amorphous SiO2. When the calcination temperature was 800 °C or below, the nanoparticles had a nano-onion structure: the shells from the exterior to the interior were amorphous SiO2, fcc Co, and CoO, and the innermost core was Co3O4. The SiO2 shell had the ability of hindering Co from particle growth during the synthesis procedure and protecting Co against oxidation after the synthesis procedure. The nanoparticles were ferromagnetic. At both low and room temperatures, the saturation magnetization increased with increasing calcination temperature, while the coercivity decreased with increasing calcination temperature. For the nanoparticles calcined at 800 °C or below, the low temperature coercivity was found to be notably higher than the room temperature one due to Co/CoO exchange coupling. For the nanoparticles calcined at 900 °C, the coercivity was relatively low and the saturation magnetization reached the expected values.