Low-temperature dynamics of magnetic colloids studied by time-resolved small-angle neutron scattering

Abstract

The dynamics of ordering and relaxation processes in magnetic colloids has been studied by means of stroboscopic small angle neutron scattering techniques in an oscillating magnetic field. Surfactant stabilized ferrofluids ͑FFs͒ of Fe 3 O 4 and Co nanoparticles have been investigated as a function of temperature and frequency and compared to a solid Cu alloy with 0.8 vol % Co precipitates. This technique allowed elucidating the dynamical nature of the locally ordered domains in both ferrofluids as "living objects" becoming arrested below the freezing temperature of the solvent. The time-dependent intensities have been analyzed in terms of Langevin statistics including dynamical interparticle structure factors, which scale with the square of the Langevin function. The local ordering is mainly determined by the effective dipole-dipole interaction, which is enhanced by the partial alignment of the particle moments in an external magnetic field. Starting from the frozen state, the amount of freely rotating particle moments increases continuously with increasing temperature. The dynamical structure factors describing the hexagonal ͑Fe 3 O 4 -FF͒ or chainlike ͑Co-FF͒ ordering reach a maximum around the melting temperature. The alignment of particle moments along the applied field 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. In contrast, no field-induced interparticle correlations occur in the diluted solid CuCo alloy where the moment relaxation is purely of fast Néel type, which-due to a low anisotropy constant-follows the oscillating field at all temperatures.

Theory and simulation of anisotropic pair correlations in ferrofluids in magnetic fields

“…Looking at the simulation results for S(k ) first, upon application of a magnetic field the primary peak at k σ 2π increases and narrows due to the enhancement of near-neighbor nose-to-tail correlations in the field direction. The narrowing of the primary peak is in good correspondence with that seen in experimental [11][12][13][14][15] and previous simulation studies. 20,22,23 For DHSs, the peak position does not shift because the distance of closest approach is fixed by the hardsphere potential.…”

“…10 Field-induced anisotropies in particle correlations have been examined previously by scattering experiments, [11][12][13][14][15] theory, 14,[16][17][18][19][20] and computer simulation. [20][21][22][23] There are, of course, existing theories for the structures of ferrofluids.…”

Static and dynamic correlations in magnetic nanomaterials studied by small angle neutron scattering techniques

“…15), which is basically in agreement with AC susceptibility measurements [42]. For about 40% of the particles, moment rotation is blocked already below 100 Hz while between 600 and 1300 Hz the remaining particle moments continuously freeze out in a random orientation.…”

“…Ferrofluid samples have been cooled down from 300 K to 100 K either in zero magnetic field (ZFC) or by applying a static field B st of 20 mT (FC) [42]. Starting from 100 K, SANS measurements have been performed at each temperature in a static field of B st 20 mT and in an oscillating field at frequencies of 100 Hz-300 Hz where the amplitude of the AC field was B 0 ± 20 mT .…”

“…Therefore, the dynamical structure factor S(Q, , t) depends on L 2 (x) in this model. The experimental data have been simultaneously fitted with the constraints of common structural parameters (see Table 1 of ref [42]). As a result, the values for the dynamical structure factors S(Q, , t) can be determined for each temperature.…”