Spin dynamics in hybrid iron oxide-gold nanostructures

We report a broadband 1H NMR study of the spin dynamics of coated maghemite and gold-maghemite hybrid nanostructures with two different geometries, namely dimers and core-shells. All the samples have a superparamagnetic behavior, displaying a blocking temperature (TB ~ 80 K (maghemite), ~105 K (dimer), ~150 K (core-shell)), and the magnetization reversal time follows the Vogel-Fulcher law. We observed three different anomalies in 1H NMR T1 -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 surface-core spin dynamics (T < 25 K). By fitting the T1 -1 data with a heuristic model, we achieved a good agreement with magnetic relaxation data and literature values for methyl group rotation frequencies.