Dresden 2009 – wissenschaftliches Programm
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MA: Fachverband Magnetismus
MA 25: Micro- and Nanostructured Magnetic Materials II
MA 25.10: Vortrag
Mittwoch, 25. März 2009, 17:00–17:15, HSZ 403
Freezing dynamics of magnetite ferrofluids studied by time-resolved Small Angle Neutron Scattering — •Sylvain Prévost1, Albrecht Wiedenmann2, Uwe Keiderling3, Dirk Wallacher3, Michael Meissner3, and Joachim Kohlbrecher4 — 1Stranski Lab., TU Berlin, Berlin, Germany — 2Institut Laue Langevin, Grenoble, France — 3Helmholtz-Zentrum Berlin, Berlin-Wannsee, Germany — 4Laboratory for Neutron Scattering, ETH Zurich & PSI, Villigen, Switzerland
The dynamics of particle ordering in ferrofluids has been studied by time-resolved stroboscopic SANS. Two samples are compared, with nearly monodisperse Co and Fe3O4 nanoparticles of similar magnetic moments, dispersed in oil and stabilized by surfactant. The SANS scattering response was measured stroboscopically in an oscillating applied magnetic field, with an optional static field superimposed, the temperature ranging from 100 to 300K.
As long as the magnetic moments follow the applied field, the 2D scattering patterns alternate between fully isotropic and strongly anisotropic. Oscillating behavior with decreasing amplitudes is clearly observed down temperatures lower than the melting point of the pure solvent. Scattered intensities can be successfully fitted in terms of the Langevin statistics. The dynamics of field-induced ordering in the ferrofluid systems is governed by the fast Brownian rotation of individual nanoparticles and small aggregates while the magnetic relaxation of longer dipolar chains and local hexagonal domains is much slower.