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Magnetismus Vortrag: Mi., 18:20–18:40 M-V15
Time-resolved SANS studies of field induced ordering in Ferrofluids
Albrecht Wiedenmann 1 , Uwe Keiderling 1 , Klaus Habicht 1 , Margerita
Russina 1 , Roland Gähler 2
1 Hahn- Meitner Institut , Glienickerstr. 100, 14109 Berlin, Germany – 2 Institut Laue-
Langevin, BP 85X, F-38042 Grenoble Cedex
We report on real-time stroboscopic investigations of the kinetics of field induced ordering
processes in concentrated Co-ferrofluids by means of SANS. Ordering of the
magnetic particles was forced by an alternating magnetic field of frequency f(s) with
a maximum amplitude of Hmax=±200 Gs. We applied for the first time the TISANE
method where a fast chopper of frequency f(e) was synchronized to f(s) and adjusted
to a data acquisition frequency f(d) of the detector.A considerable gain in intensity
and resolution was obtained by this technique where a large frame overlap and high
repetition rates could be achieved. Up to about f(s)=1300 Hz the SANS scattering
patterns oscillated as a function of time from isotropic at H=0 to strongly anisotropic
at Hmax. The observed threshold frequency is far below the resolution limit of the
TISANE technique which must therefore result from the characteristic time needed for
re-orientation of the magnetic particle moments. In a conventional stroboscopic SANS
experiment when the data acquisition was triggered by a signal from the ac-field the
periodic response was detected only up to about f(s)=600 Hz. With increasing frequency
the oscillations are smeared out by the different flight times corresponding to
the wavelength resolution of the velocity selector of 0.1 [2].
[1] R. Gähler, R. Golub ILL Scientific council April (1999) SC 99-1, page 73
[2] A. Wiedenmann, U. Keiderling, K. Habicht, M. Russina, R. Gähler sbm. PRL