Dynamics of field-induced ordering in magnetic colloids studied by new time-resolved small-angle neutron-scattering techniques.
The reversal of magnetic moments of nanoparticles in concentrated Co ferrofluids was monitored in an oscillating magnetic field by new time-resolved stroboscopic small-angle neutron-scattering techniques. Time resolution in the micros range was achieved by using a pulsed beam technique, TISANE, while in continuous mode resolution was limited by the wavelength spread to about 1 ms. The frequency dependence of anisotropic scattering patterns has been modeled using Langevin dynamics. The dynamics follows a two step mechanism: field-induced ordering is governed by fast Brownian rotation of nanoparticles with a characteristic time of about 160 micros. Magnetic relaxation of locally ordered domains of about 100 nm in size takes place within a few seconds by Brownian rotation or by Néel type rotation of magnetic moments.
Wiedenmann
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Hahn-Meitner-Institut, Structure Research, Glienicker Strasse 100, D-14109 Berlin, Germany. wiedenmann@hmi.de
Keiderling
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Habicht
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Russina
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Gähler
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