Magnetic resonance of ferrite nanoparticles: evidence of surface effects

Ferromagnetic resonance (FMR) experiments at 9.26 GHz on non-interacting maghemite (gamma-Fe2O3) nanoparticles of ferrofluids are performed as a function of temperature (3.5-300 K) and particle diameter (4.8-10 nm). The orientational mobility of the particles inside the fluid is employed to monitor the orientational distribution of the anisotropy axes by solidifying the MF matrix under the external field. On those textured suspensions, angular analysis is performed allowing to determine the anisotropy of the system, which turns out to be uniaxial and positive, The dependence of angular variations with respect to particle size leads to the conclusion that this anisotropy is induced by the particle surface, its density being K-s = 2.7 x 10(-2) erg/cm(2) at 3.5 K. Extra isotropic contributions to internal fields at low temperature appear as the signature of the disordered magnetic structure of the surface layer. (C) 1998 Elsevier Science B.V. All rights reserved.