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Statics and dynamics of field induced ordering in Ferrofluids

A. Wiedenmann, U. Keiderling, M. Kammel, A. Heinemann and R.P. May
Hahn-Meitner Institute Berlin GmbH, DE

nanomaterials, ferrofluids, particle ordering, dynamics

Surfactant stabilised ferrofluids have been studied by Small Angle Scattering using polarised neutrons (SANSPOL) and Synchrotron X-rays (SAXS). In concentrated Cobalt ferrofluids samples above 1 vol.% Co a pseudo-crystalline hexagonal ordering of the core-shell nanoparticles was found to be induced by an external magnetic field (H), where magnetic particle moments are aligned along H. The dynamics of ordering and relaxation processes was studied by a new stroboscopic SANSPOL technique. Ferrofluid samples of different particle sizes and concentrations were placed in a homogenous magnetic field the strength of which was cycled between zero and 0.5 T. Time-frame histogram data acquisition has been performed in time slices between 200 and 500 ms synchronised with the cycles of the magnetic field. The analysis of the 2D SANSPOL patterns allowed reliably to extract magnetic and nuclear correlations and to demonstrate the reversibility of the processes. The overall dynamics of the underlying relaxation processes is characterised by an exponential decay of the correlation peaks corresponding to a gradual transformation from local hexagonal order to segments of particle chains. The mechanism of the sluggish kinetics is discussed in terms of relaxation of magnetic moments where dipolar interactions between particles have to be taken into account.

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