Surface-induced superparamagnetic relaxation in nanoscale ferrihydrite particles

The effect of surface on superparamagnetic relaxation for nanoscale ferrihydrite particles (d∼5 nm) has been investigated using Mössbauer spectroscopy. With increasing surface chemisorption of ${\mathrm{SiO}}_4^{4\mathrm{-}}$, the superparamagnetic transition temperature (${\mathit{T}}_{\mathit{B}}$) falls from 100 to 40 K and the transition sharpens. This indicates that the ${\mathrm{SiO}}_4^{4\mathrm{-}}$ 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 ∼12 K; the spectra exhibit characteristic size-dependent features. © 1996 The American Physical Society.