COBALT FERRITE – SILICA CORE – SHELL PARTICLES:

A MAGNETIC YUKAWA SYSTEM

T. Autenrieth ¹ , J. Wagner ¹ , R. Hempelmann ¹ , A. Robert ² , W. Hartl ³ .

1. Physical Chemistry, University Saarbrucken, D-66123 Saarbrucken (Germany)

2. European Science Research Foundation, Grenoble (France)

3. Wallburg-Realschule, Eltmann (Germany)

Core-shell particles consisting of a cobalt ferrite core and a silica shell are prepared by precipitation followed by a modified Stoeber synthesis. The preparation starts from the coprecipitation of a stoechiometric mixture of iron (III) chloride and cobalt (II) chloride by addition of sodium hydroxide. The surface of the magnetic precipitate is modified by several steps in order to chloride well dispersed colloidal suspension in alcoholic media. The polycondensation of tetraethoxisilane (TEOS), which forms the silica shell, is induced by the addition of ammoniac. In a last step, core-shell particles are transferred into water as suspending medium.

The particle size of core and shell is determined by dynamic light scattering, Small Angle X-Ray scattering (SAXS), TEM, and magnetogranulometry. Whereas the diameter of the magnetite have been constant around 14 nm, the thickness of the silica shell can be tuned in order to vary the magnetic interaction. Because the polydispersity of these particles is less than 0.02, they self-occupier is colloidal liquids and crystals after removing stray ions by a mixed bed ion exchanger. This efficiently is induced by surface charges of the silica shell arising from dissociation of weakly active-groups as probed by light-scattering electrophoresis in dependence of the pH. The effective number of charges is obtained by analysis of the structure factor of a liquid like ordered suspension.

Due to the existence of well separated magnetic particles, in opposite to uncoated magnetic particles, the magnetization can be described by a simple Langevin model, whereby the mean magnetic diameter and polydispersity are in excellent agreement with the sizes obtained by SAXS and TE-Micrographs.

In presence of a magnetic field gradient, the number density of the particles can be tuned by magnetostriction: the number density of particles increases with increasing field gradient. Thois is visible in an impressive way by the blue shift of Bragg reflexions arising from colloidal crystals. The interdistance of lattice planes in a colloidal crystal decreases with increasing number density. Under the same scattering angle, the ratio of the wavelengths, which fulfill Braggs law corresponds to the ratio of lattice interdistances. A blue Bragg spot indicates a by a factor of approximately 2/3 smaller than a red one, which corresponds to an increase of number density by more than a factor of 3.

Colloid crystals of magnetic core-shell particles.