Spin Dynamics in Hybrid Iron Oxide-Gold Nanostructures

We report a broadband H-1 NMR study of the spin dynamics of coated maghemite and goldmaghemite hybrid nanostructures with two different geometries, namely dimers and coreshells. All the samples have a superparamagnetic behavior, displaying a blocking temperature (T-B similar to 80 K (maghemite), similar to 105 K (dimer), similar to 150 K (coreshell)), and the magnetization reversal time follows the VogelFulcher law. We observed three different anomalies in H-1 NMR T(1)1 versus T that decrease in amplitude when increasing the applied magnetic field. We suggest that the anomalies are related to three distinct system dynamics: molecular rotations of the organic groups (240 < T < 270 K), superparamagnetic spin blockage (100 < T < 150 K), and surfacecore spin dynamics (T < 25 K). By fitting the T(1)1 data with a heuristic model, we achieved a good agreement with magnetic relaxation data and literature values for methyl group rotation frequencies.