Magnetic properties of SiO 2 -coated Fe nanoparticles

SiO 2 -coated Fe nanoparticles were synthesized using a wet chemical method, and their structural and magnetic properties were studied. The SiO 2 material was in an amorphous state. The Fe nanoparticles were in a bcc state and contained an inner ferrihydrite core whose size decreased with increasing calcination temperature. The nanoparticles were basically in the ferromagnetic state. Their saturation magnetization increased with increasing calcination temperature, whereas their coercivity decreased with increasing calcination temperature. Different from bulk Fe, the nanoparticles exhibited strong temperature-dependent magnetic behaviors. The Bloch exponent fell from 1.5 to smaller values and decreased with increasing ferrihydrite content, while the Bloch constants were much bigger than that for bulk and increased significantly with ferrihydrite content. The value of coercivity decreased notably with increasing temperature. The exchange anisotropy arising from the exchange coupling across the Fe/ferrihydrite interfaces was examined and was used to interpret the observed temperature behaviors. © 2002 American Institute of Physics. @DOI: 10.1063/1.1518757#