Surface-induced superparamagnetic relaxation in nanoscale ferrihydrite particles.

Abstract

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