  Magnetic measurements have been performed on 40-nm sphere-like Fe3O4
nanoparticles using a Quantum Design vibrating sample magnetometer. Coating
Fe3O4 nanoparticles with SiO2 effectively eliminates magnetic interparticle
interactions so that the coercive field HC in the hightemperature range between
300 K and the Curie temperature (855 K) can be well fitted by an expression for
noninteracting randomly oriented single-domain particles. From the fitting
parameters, the effective anisotropy constant K is found to be (1.68 \pm 0.17)
\times 105 erg/cm3, which is slightly larger than the bulk magnetocrystalline
anisotropy constant of 1.35 \times 105 erg/cm3. Moreover, the inferred mean
particle diameter from the fitting parameters is in quantitative agreement with
that determined from transmission electron microscope. Such a quantitative
agreement between data and theory suggests that the assemble of our SiO2-coated
sphere-like Fe3O4 nanopartles represents a good system of noninteracting
randomly-oriented single-domain particles.
