Rotational diffusion of magnetic nickel nanorods in colloidal dispersions

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.