High-temperature magnetic properties of noninteracting randomly oriented single-domain Fe 3 − δO 4 nanoparticles

Abstract

Magnetic measurements have been performed on 40-nm sphere-like Fe 3 − δ O 4 ( δ = 0.043 ) nanoparticles using a Quantum Design vibrating sample magnetometer. Coating Fe 3 − δ O 4 nanoparticles with SiO 2 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 ) × 10 5 erg / cm 3 , which is very close to the bulk magnetocrystalline anisotropy constant of 1.35 × 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 SiO 2-coated sphere-like Fe 3 − δ O 4 nanoparticles represents a good system of noninteracting randomly-oriented single-domain particles.