Magnetic behavior of coated superparamagnetic iron oxide nanoparticles in ferrofluids
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Magnetic behavior of coated superparamagnetic iron oxide nanoparticles in ferrofluids W. Voit 1,3, D.K. Kim 2, W. Zapka3, M. Muhammed 2 and K.V. Rao 1 1
Engineering Materials Physics Division, and 2 Materials Chemistry Division, Royal Institute of Technology, SE-100 44 Stockholm, Sweden 3 XaarJet AB, SE-175 26 Järfälla, Sweden
ABSTRACT We present a study on the magnetic behavior of nanosized iron oxide particles coated with different surfactants (sodium oleate, PVA and starch) in a ferrofluid. The effect of the coating material, and different particle concentrations in the ferrofluid have been magnetically investigated to determine the effective magnetic particle size and possible interaction. The superparamagnetic iron oxide particles, synthesized by a controlled co-precipitation technique, are found to contain magnetite (Fe3O4) as a main phase with a narrow physical particle size distribution between 6 and 8 nm. The mean effective magnetic size of the particles in different ferrofluid systems are estimated to be around 4-5 nm which is smaller than the physical particle size. On a 10% dilution in the starch coated ferrofluid we observe a decrease in the blocking temperature.
INTRODUCTION Ferrofluids are stable colloidal suspensions of single domain ferro- or ferrimagnetic particles in a liquid carrier. The particles in ferrofluids are coated with a layer of surfactant, usually consisting of long polymeric chains, to enable a stabilization of the particles against gravimetrical forces and to avoid strong interaction and agglomeration of the particles. However, one cannot neglect interactions among particles in a ferrofluid, especially when the particle density increases. Such interactions mainly arise from dipole-dipole interactions between the particles. The effect of these interactions on the magnetic properties has been studied extensively during the past years [1-3]. These studies show a dependency of the DC-susceptibility data on particle concentration and applied magnetic field. Especially for the calculation of magnetic mean particle diameters and even more for the determination of the size distribution of the particles, the particle-particle interaction effects play an important role [4]. In this paper we present a study of the interaction effects in ferrofluids containing superparamagnetic magnetite particles in a size range of 6 to 8 nm. The ferrofluid were produced using a controlled co-precipitation method, with special precautions to prevent undesirable oxidation of Fe2+. The ferrofluids were synthesized with different coating materials covering fully the particle surface. Ferrofluids with different particle concentrations have also been investigated to determine the possible interaction effects. The resulting data is correlated with the current existing theoretical models.
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SAMPLE PREPARATION AND EXPERIMENTAL METHODS All ferrofluid samples were prepared based on a method of alkalizing an aqueous mixture of ferric and ferrous salts [5], with NaOH as alkali source. A process of N2 gas flow th
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