Core shell particles consisting of cobalt ferrite and silica as model ferrofluids [CoFe2O4-SiO2 core shell particles]

Nearly monodisperse core shell particles consisting of a magnetic core of cobalt ferrite (CoFe2O4) and a shell of silica NOD are prepared via a modified Stober synthesis. The core shell structure is confirmed by TEM, the size distribution of the whole particles was determined by means of photon correlation spectroscopy and small angle X-ray scattering. Due to charged surface groups of the silica shells and the magnetic moments of the cores, these particles interact both via a screened Coulomb potential and via a magnetic dipole potential. If stray ions are removed in the presence of a mixed bed ion exchanger, the electrostatic repulsion induces colloidal crystallisation. The lattice constant is influenced by the interaction with a magnetic field gradient, indicated by a blue shift of the Lane spots towards increasing field gradient. (C) 2002 Elsevier Science B.V. All rights reserved.