Temperature dependence of magnetic properties of SiO2-coated Co nanoparticles

SiO2-coated Co nanoparticles were synthesized; the different temperature-dependent magnetic behaviors between Co nanoparticles with and without inner Co-oxide cores were observed. For Co nanoparticles free of Co-oxide cores, the magnetization obeyed Bloch's T3/2 law with a Bloch constant close to the bulk value; the coercivity exhibited a weak and linear dependence on temperature. For Co nanoparticles with Co-oxide cores, the Bloch exponents fell from 1.5 to smaller values and decreased with increasing Co-oxide concentration; the coercivity significantly increased with decreasing temperature due to the presence of exchange anisotropy, which arose from the exchange coupling across the Co/CoO interfaces.