Cobalt ferrite-silica core-shell colloids:: a magnetic Yukawa system

Colloidal core-shell particles consisting of a magnetic cobalt ferrite core surrounded by a silica shell are prepared via precipitation and a subsequent modified Stober synthesis. The resulting particles with narrow size distribution interact due to weakly acidic surface groups at the silica shell via a screened Coulomb or Yukawa potential in addition to the magnetic dipole-dipole interaction of the magnetic cores. The structural characterization of the particles is done by transmission electron microscopy and small-angle X-ray scattering (SAXS), and the potential parameters of the electrostatic monopole and magnetic dipole-dipole interactions are determined by laser Doppler electrophoresis and magnetogranulometry, respectively. Owing to the interaction potential, the particles self-organize to a liquid-like ordered structure as determined by SAXS. The magnetic dipole-dipole interaction becomes evident from the anisotropic scattering pattern in the presence of an external magnetic field. Copyright (C) 2004 John Wiley Sons, Ltd.