SURFACE-INDUCED SUPERPARAMAGNETIC RELAXATION IN NANOSCALE FERRIHYDRITE PARTICLES

The effect of surface on superparamagnetic relaxation for nanoscale fe rrihydrite particles (d similar to 5 nm) has been investigated using M ossbauer spectroscopy. With increasing surface chemisorption of SiO44- , the super paramagnetic transition temperature (T-B) falls from 100 t o 40 K and the transition sharpens. This indicates that the SiO44- spe cies eliminate the surface unpaired spins and create a nonmagnetic med ium, 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; th e spectra exhibit characteristic size-dependent features.