Temperature dependence of superparamagnetic resonance of iron oxide nanoparticles

Nanoparticles of maghemite (gamma -Fe2O3) are formed in a sol-gel silicate glass with a molar ratio Fe/Si of 2% by a treatment at 1000 degreesC for 6h. Electron paramagnetic resonance spectrum at 300K shows a relatively narrow sharp line at g(eff) approximate to 2, As the temperature lowers to 5K, the apparent resonance field decreases and the linewidth considerably increases. We develop a theoretical formalism based on a distribution of diameters or volumes of the nanoparticles following a lognormal. The nanoparticles are considered as single magnetic domains with random orientations of magnetic moments and thermal fluctuations of anisotropic axes. The individual line shape function is derived from the damped precession equation of Landau-Lifshitz. An appropriate linewidth expression is put forward, which account for the averaging of the fluctuations of orientations of the magnetic moments with respect to the magnetic field and to the magnetic anisotropy axes. A single set of parameters provides good fits to the spectra recorded at the different temperatures. The low-temperature blocking of the nanoparticle magnetic moments has been clearly evidenced in the resonance absorption intensity and the blocking temperature of the assembly of nanoparticles (averaged over the distribution in the nanoparticle volume) has been evaluated as 90 K. (C) 2001 Elsevier Science B.V. All rights reserved.