H-1-NMR study of the spin dynamics of fine superparamagnetic nanoparticles

We report a broadband H-1-NMRstudy of the temperature spin dynamics of nearly monodisperse dextran-coated gamma-Fe2O3 magnetic nanoparticles. We observed a maximum in T-1(-1) (T) that decreases in amplitude and shifts toward higher temperatures with increasing field. We suggest that this is related to the progressive superparamagnetic spin blocking of the ferrite core. The data can be explained by assuming a single electronic spin-spin correlation time and introducing a field-dependent distribution of anisotropy energy barriers.