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

Magnetic measurements have been performed on 40-nm sphere-like Fe3 − δO4 (δ=0.043δ=0.043) nanoparticles using a Quantum Design vibrating sample magnetometer. Coating Fe3 − δO4 nanoparticles with SiO2 effectively eliminates magnetic interparticle interactions so that the coercive field HCHC 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)×105 erg/cm3(1.38±0.11)×105 erg/cm3, which is very close to the bulk magnetocrystalline anisotropy constant of 1.35×105 erg/cm31.35×105 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 ensemble of our SiO2-coated sphere-like Fe3 − δO4 nanoparticles represents a good system of noninteracting randomly-oriented single-domain particles.

Research highlights► High-temperature magnetic properties of SiO2-coated Fe3O4 nanoparticles. ► Coating the nanoparticles with SiO2 effectively reduces interparticle interactions. ► SiO2-coated Fe3O4 nanoparticles are noninteracting single-domain particles. ► The anisotropy constant of the Fe3O4 nanoparticles is close to the bulk value. ► Data are in quantitative agreement with a theory of noninteracting particles.