High-temperature magnetic properties of noninteracting randomly oriented single-domain Fe3-δO4 nanoparticles

Magnetic measurements have been performed on 40-nm sphere-like Fe3-delta O4 (delta = 0.043) nanoparticles using a Quantum Design vibrating sample magnetometer. Coating Fe3-delta O4 nanoparticles with SiO2 effectively eliminates magnetic interparticle interactions so that the coercive field H-C in the high-temperature 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 (1.38 +/- 0.11) x 10(5) erg/cm(3), which is very close to the bulk magnetocrystalline anisotropy constant of 1.35 x 10(5) erg/cm(3). 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 ensemble of our SiO2-coated sphere-like Fe3-delta O4 nanoparticles represents a good system of noninteracting randomly-oriented single-domain particles. (C) 2011 Elsevier B.V. All rights reserved.