Surface-induced superparamagnetic relaxation in nanoscale ferrihydrite particles

The effect of surface on superparamagnetic relaxation for nanoscale ferrihydrite particles (d similar to 5 nm) has been investigated using Mossbauer spectroscopy. With increasing surface chemisorption of SiO44-, the super paramagnetic transition temperature (T-B) falls from 100 to 40 K and the transition sharpens. This indicates that the SiO44- species eliminate the surface unpaired spins and create a nonmagnetic medium, thereby significantly reducing the magnetic coupling between the particles. For ferrihydrite particles impregnated on a silica support, the particle interaction becomes negligible and the superparamagnetic transition occurs at a still lower temperature of similar to 12 K; the spectra exhibit characteristic size-dependent features.