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Description
A QENS study on IN16B (ILL) together with elastic and inelastic fixed-window-scans up to 380 K on humid citrate-coated iron oxide nanoparticles with a diameter of 6 nm show the presence of two diffusive species in the interface region. A comparison between hydrogenated and deuterated samples collected on EMU (ANSTO) allows for an assignment to ligand- and water dynamics. In a global fit approach, we find the citrate molecules to rotate with an activation energy of Ea = 240 meV and a prefactor of τ0 = 0.21 ps, while surface water diffuses translationally with an activation energy of Ea = 190 meV and a prefactor of τ0 = 0.12 ps, significantly slower than bulk water.
Further, various magnetic QENS signals are identified when considering the entire Q-range from Q=0.2 Å-1 up to Q=1.7 Å-1. Based on structure factor evaluations, dynamic features, and the temperature behavior, we identify to our knowledge for the first time in QENS data uniquely superparamagnetic relaxation in the low Q-regime for Q< 0.3 Å-1, a weak signal with spin glass behavior for 0.6 < Q < 1.0 Å-1 originating from the magnetic shell near the surface of the nanoparticles, and a magnon excitation with an energy of 22 µeV at 150 K in the range of the magnetic Bragg peak 1.2<Q<1.4 Å-1.
