Magnetic resonance of nanoparticles in a ferrofluid: evidence of thermofluctuational effects

Ferromagnetic resonance (FMR) experiments on non-interacting maghemite (gamma-Fe2O3) nanoparticles in ferrofluids, are performed in X- and Q-bands as a function of the particle diameter (4.8-10 nm) and the temperature (3.5-300 K). The colloidal stability and grain size are controlled through a chemical synthesis, the polydispersity being reduced by a phase separation method. The dependencies of spectral characteristics with temperature T, particle volume V and experimental frequency omega, reveal a generic behavior of the FMR lines due to the effect of thermal fluctuations. Their scaling with the Langevin parameter xi(0) = MV omega/gamma yk(B)T is explained in the framework of the theory of FMR for an assembly of independent single-domain anisotropic particles. (C) 1999 Elsevier Science B.V. All rights reserved.