Direction Dependent Diffusion of Aligned Magnetic Rods by Means of X-Ray Photon Correlation Spectroscopy

Rodlike hematite particles in suspension align perpendicular to an external magnetic field due to a negative anisotropy of their magnetic susceptibility Delta(chi). The diffusion tensor consists of two principal constants D-parallel to and D-perpendicular to for the diffusion parallel and perpendicular to the long particle axis. X-ray photon correlation spectroscopy is capable of probing the diffusive motion in optically opaque suspensions of rodlike hematite particles parallel to the direction of the scattering vector Q. Choosing Q parallel or perpendicular to the direction of an external magnetic field H the direction dependent intermediate scattering function is measured by means of x-ray photon correlation spectroscopy. From the intermediate scattering function in both directions the principal diffusion constants D-parallel to and D-perpendicular to are determined. The ratio D-parallel to/D-perpendicular to increases with increasing aspect ratio of the particles and can be described via a rescaled theoretical approach for prolate ellipsoids of revolution. DOI: 10.1103/PhysRevLett.110.048301