Ferrofluids Magnetically Controllable Fluids and Their Applications
Magnetic control of the properties and the flow of liquids is a challenging field for basic research and for applications. This book is meant to be both an introduction to and a state-of-the-art review on this topic. Written in the form of a set of lectur
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Small angle neutron scattering (SANS) is known to be well suited for studying density- and concentration fluctuations on a length scale between 0.5 nm and 300 nm which corresponds to typical sizes of microstructural features in nanoscaled (n-) materials [1,2]. This technique allowed different types of inhomogeneities to be identified in crystalline, amorphous and liquid materials. In addition, magnetisation fluctuations in domain like structures resulting from strong intergranular correlations have been monitored by SANS in compacted n-Fe, n-Co, or n-Ni alloys [3,4]. In some diluted systems such as n-Fe3 O4 embedded in a glass ceramic [5] and n-Fe3 Si in an ferromagnetic amorphous Fe-Si-B-Nb-Cu matrix [6,7] magnetic single domain behaviour of the nanocrystalline grains and the nature of interfaces have been evaluated. Here we’ll focus on the determination of the crystalline and magnetic microstructures of magnetic colloids. Magnetic colloids are stable dispersions of ferromagnetic materials. In such “Ferrofluids” (denoted as FF) nanoscaled magnetic particles are stabilized against coagulation either by electrostatic repulsion or by coating the core with organic chain molecules acting as surfactants [8]. Currently great effort is undertaken to prepare new bio-compatible Ferrofluids for potential biomedical applications [9]. Such applications are based on the superparamagnetic behaviour of nanosized particles, which disappears when aggregation takes place as the consequence of an inefficient screening [10,11]. Therefore a precise knowledge of the microstructural parameters is a pre-requisite for the interpretation of macroscopic phenomena and for a tailored fabrication of FF. Stefan Odenbach (Ed.): LNP 594, pp. 33–58, 2002. c Springer-Verlag Berlin Heidelberg 2002
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A. Wiedenmann
Only few methods give access to these parameters. From macroscopic techniques such as magnetisation M(H,T) measurements [12] or magneto-viscous effects [13] average parameters of concentration, size and arrangements of magnetic particles are derived. However, the actual values obtained depend strongly on the basic assumptions for the underlying processes [14]. Wide angle and small angle X-ray scattering as well as transmission electron microscopy [15] are sensitive to the particle core only, since the light elements of the organic shell give no sufficient contrast. The advantages of neutrons in such liquids are twice: First, the strong scattering power of hydrogen contained in the organic surfactants gives access to the shell and second, the interaction of the neutron spin with magnetic moments allows to visualise the magnetism of the core. Some SANS studies have been performed mainly on concentrated systems FF, which allowed the stability phase diagram to be established [16,17]. However, complications arise in poly-disperse systems when different constituents are present. Then we face the problem that weak magnetic scattering signals have to be analysed beside strong nuclear contributions from other sources or vice versa which can lead to cons
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