MAGNETIC-RESONANCE OF FERRITE NANOPARTICLES - EVIDENCE OF SURFACE EFFECTS

Ferromagnetic resonance (FMR) experiments at 9.26 GHz on non-interacti ng 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 a xes by solidifying the MF matrix under the external field. On those te xtured suspensions, angular analysis is performed allowing to determin e the anisotropy of the system, which turns out to be uniaxial and pos itive, The dependence of angular variations with respect to particle s ize leads to the conclusion that this anisotropy is induced by the par ticle 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 su rface layer. (C) 1998 Elsevier Science B.V. All rights reserved.