Colloidal dispersions of Ni nanorods were synthesized by pulsed electrodeposition of Ni into nanoporous aluminum oxide layers followed by dissolution of the templates. Geometrical characterization of the nanorods by transmission electron microscopy and scanning electron microscopy allowed us to determine the average length (100-250 nm) and diameter (20-40 nm) of the rods and to estimate the thickness of the polyvinylpyrrolidone surfactant layer. Due to their acicular shape, nanorods of the given size are uniaxial ferromagnetic single domain particles and exhibit a distinct anisotropic polarizability. These two characteristic properties are the physical basis for magnetic field-dependent optical transmission and allow us to investigate the rotational diffusion of the nanorods in liquid dispersion. In the present study, we employed AC magnetization measurements, dynamical light scattering and optical transmission measurements in a rotating magnetic field to determine the rotational diffusion coefficient. The results from all three methods were consistent and agree with theory within a factor of 2.

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Articles you may be interested inNanoscale rheometry of viscoelastic soft matter by oscillating field magneto-optical transmission using ferromagnetic nanorod colloidal probes

The structural and magnetic properties of a cobalt nanorod array have been studied by means of magnetic field dependent small-angle neutron scattering (SANS). Measurement of the unpolarized SANS cross section dAE/d of the saturated sample in the two scattering geometries where the applied magnetic field H is either perpendicular or parallel to the wavevector k i of the incoming neutron beam allows one to separate nuclear from magnetic SANS, without employing the usual sector-averaging procedure. The analysis of the SANS data in the saturated state provides structural parameters (rod radius and centre-tocentre distance) that are in good agreement with results from electron microscopy. Between saturation and the coercive field, a strong field dependence of dAE/d is observed (in both geometries), which cannot be explained using the conventional expression of the magnetic SANS cross section of magnetic nanoparticles in a homogeneous nonmagnetic matrix. The origin of the strong field dependence of dAE/d is believed to be related to intradomain spin misalignment, due to magnetocrystalline and magnetoelastic anisotropies and magnetostatic stray fields.

The response of a colloidal dispersion of Ni nanorods to an oscillating magnetic field was characterized by optical transmission measurements as well as small-angle neutron scattering (SANS) experiments using the TISANE (Time-dependent SANS experiments) technique. Exposed to a static magnetic field, the scattering intensity of the rod ensemble could be well described by the cylinder form factor using the geometrical particle parameters (length, diameter, orientation distribution) determined by transmission electronmicroscopy and magnetometry. An oscillation of the field vector resulted in a reorientation of the nanorods and a time-dependency of the scattering intensity due to the shape anisotropy of the rods. Analysis of the SANS data revealed that in the range of low frequencies the orientation distribution of the rods is comparable to the static case. With increasing frequency, the rod oscillation was gradually damped due to an increase of the viscous drag. It could be shown that despite of the increased friction in the high frequency range no observable change of the orientation distribution of the ensemble with respect to its symmetry axis occurs.