Use of ferromagnetic resonance to determine the size distribution of γ-Fe2O3 nanoparticles

Ferromagnetic resonance (FMR) experiments were performed as a function of temperature (10–300 K) on γ-Fe2O3 nanoparticles prepared by a sol–gel method. By measuring at several temperatures the relative intensity of the spectrum due to superparamagnetic particles and the anisotropy field of the spectrum due to ferrimagnetic particles, we determined the size distribution of γ-Fe2O3 nanoparticles. It was found to be a log-normal distribution with a most probable diameter Dm=8.1 nm and a standard deviation σ=0.25. Transmission electron microscopy measurements performed on the same samples yielded a log–normal distribution with Dm=11.2 nm and σ=0.23. The difference is attributed to the existence of a disordered surface layer in the particles.