Relaxation mechanisms in magnetic colloids studied by stroboscopic spin-polarized small-angle neutron scattering

Abstract

Stroboscopic small-angle neutron scattering by using polarized neutrons is presented as a technique for investigations of relaxation phenomena on a nanometer scale. Applying a cyclic perturbation to the sample by an oscillating magnetic field allows the polarization-dependent scattering cross-sections to be extracted as timedependent response. The potential of this special type of "physical" contrast variation technique is demonstrated with three examples of ferrofluids of different particle sizes and viscosities. The dynamics of magnetic moment reorientations turned out to be governed by different time scales. For a temperature-dependent fraction of individual mobile particles, a frequency-dependent phase shift and damping of the intensity modulation have been precisely determined and quantitatively explained using a Debye-like relaxation model. The characteristic relaxation times, τ , in the liquid state of all samples scale with the particle volume and viscosity confirming a Brownian-relaxation mechanism. For the remaining fraction of particles that form locally ordered domains the orientation distribution of the dynamically arrested static moments has been quantified by an order parameter S 1 arr , which is not accessible with nonpolarized neutrons. In a high viscous Co and a magnetite-based ferrofluid, S 1 arr corresponds to maximal alignment of the static moments in the liquid state and to a gradual decrease at lower temperatures when the solvent is frozen. In a low viscosity Co-ferrofluid almost no preferred alignment of the arrested particle moments was observed. In the magnetite-based ferrofluid with the highest dipolar attraction energy an additional phase shift has been detected which increases with frequency and magnitude of the magnetic field. This indicates clearly that the setup of local particle ordering in distorted hexagonal arrangement is delayed with respect to the moment alignment in the time varying magnetic field.

Magnetic small-angle neutron scattering

“…Recent progress in SANS instrumentation regarding time-resolved data-acquisition procedures (TISANE) (Gähler and Golub, 1984;Wiedenmann et al, 2006b) and the integration of spin-echo techniques (MIEZE) opens up the way to kinetic studies and quasi-elastic energy resolution. The chopper-based TISANE technique repre-sents an improvement of conventional stroboscopic timeresolved SANS, which is limited by the neutron timeof-flight spread resulting from the wavelength distribution of the incident neutrons to about 300 Hz time resolution (Wiedenmann et al, 2011). TISANE allows one to probe magnetism up to the µs regime, which permits the investigation of magnetization dynamics of anisometric nanoparticles in oscillating magnetic fields (Bender et al, 2015;Wiedenmann et al, 2008), the dynamics of vortex (Mühlbauer et al, 2011) and sykrmion lattices , or systems out of equilibrium.…”

Effect of agglomeration of magnetic nanoparticles on the Casimir pressure through a ferrofluid

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