The Mössbauer spectra of monodispersed iron oxide nanoparticles with diameters of 4, 7, 9, and 11 nm have been measured between 4.2 and 315 K and fitted within the formalism for stochastic fluctuations of the hyperfine Hamiltonian. In this model, the hyperfine field is assumed to relax between the six ±x, ±y, and ±z directions in space with a distribution of relaxation rates that is temperature dependent. Muon spin relaxation measurements have been carried out on the 9 nm particles between 4.2 and 295 K. Both techniques reveal three regimes in the magnetic dynamics of these nanoparticles. In the low-temperature regime, between 4.2 and ~30 K, the nanoparticle magnetic moments are blocked and a spin-glass-like state is observed with nearly static hyperfine fields, as is indicated by the well resolved magnetic Mössbauer spectra and the slow exponential decay of the muon asymmetry functions. In the high-temperature regime, above ~125 K, the nanoparticle magnetic moments and, hence, the hyperfine fields, relax rapidly and a typical thermally activated superparamagnetic behavior is observed, as is indicated by the Mössbauer doublet line shape and the muon asymmetry functions that are unquestionably characteristic of monodispersed nanoparticles. In the intermediate regime between ~30 and 125 K, the Mössbauer spectra are the superposition of broad sextets and doublets and the muon asymmetry functions have been fitted with a sum of two terms, one relaxing term similar to that observed at and above 125 K and one term characteristic of static local fields. Hence, in this intermediate regime, the sample is magnetically inhomogeneous and composed of nanoparticles rapidly and slowly relaxing as a result of interparticle interactions. The magnetic anisotropy constants determined from both the Mössbauer spectral and magnetic susceptibility results decrease by a factor ~4 with increasing diameter from 4 to 22 nm and increase linearly with the percentage of iron(III) ions present at the surface of the nanoparticles. The interparticle interaction energy is estimated to be between 89 and 212 K from the temperature dependence of the magnetic hyperfine field measured on the 9 nm nanoparticles.



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