Unravelling the Surface Local Spin Dynamics in Magnetic Nanoparticles by Means of NMR Relaxometry

We singled out the surface and bulk spin dynamics in magnetic hollow nanoparticles by means of nuclear magnetic resonance relaxometry. Experimental H1-NMR-dispersion curves (NMR-D), measured across a wide frequency range (104 Hz<3×108 Hz), show the presence of a high-frequency contribution to the longitudinal relaxation rate, evidenced for the first time and ascribed to the surface spin dynamics. The nuclear longitudinal relaxation rates were successfully analyzed by means of a phenomenological model accounting for the two spin populations, i.e., surface and core spins. The fit of the longitudinal NMR-D data by means of this model allowed for the estimation of the hyperfine coupling constant of the surface spins, and of the superparamagnetic 1/τN and surface-paramagnetic-like 1/τCsurf spin-spin correlation frequency, the last one being larger by more than 1 order of magnitude. These experimental results provide a substantial contribution to the basic knowledge of spin dynamics in nanoscale systems.