Transient grating in a ferrofluid under magnetic field: Effect of magnetic interactions on the diffusion coefficient of translation

Diffusion processes in a magnetic colloid are studied by a forced Rayleigh scattering technique under an applied static magnetic field. A periodic spatial modulation of the particle concentration (transient grating) is induced in the colloid with three different field geometries, B being either parallel or perpendicular to the grating direction. The value of the translational diffusion coefficient of the particles is given by the transient grating relaxation time. It depends on the magnetic field strength and on the field geometry. A theoretical model based on a mean field approximation taking magnetic interactions of particles under a field into account is given which agrees with experimental results.