We report a broadband 1H-NMR study of the temperature spin dynamics of nearly monodisperse dextran-coated γ-Fe2O3 magnetic nanoparticles. We observed a maximum in T1−1(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.
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