Initial susceptibility, flow curves, and magneto-optics of inverse magnetic fluids.

We introduce inverse magnetic fluids, consisting of gibbsite [Al(OH)(3)] platelets and alumina (Al2O3) spheres dispersed in a magnetic fluid, studied together with silica (SiO2) dispersions based on the same magnetic fluid matrix. Atomic force microscopy, optical microscopy, and alternate gradient magnetometry confirm the remarkable stability of the samples. Optical microscopy shows aggregation of nonmagnetic spheres, which, surprisingly, strongly depends on the concentration of the magnetic fluid rather than the concentration of nonmagnetic particles. Our model for the initial susceptibility of inverse magnetic fluids agrees very well with experimental data for systems containing spherical particles. The flow curves in an external magnetic field are strongly influenced by the aggregation of nonmagnetic particles or preformed nonmagnetic particle clusters, and by their disruption due to the shear flow. Static linear magnetobirefringence and magnetodichroism of all samples are investigated both experimentally and theoretically. These effects, which occur in all magnetic fluids, can be enhanced by the additional anisotropy due to the magnetic holes. The experiments we performed showed that, at a wavelength of 820 nm, the magnetodichroism is increased while the magneto-birefringence decreases when nonmagnetic particles were dispersed in the magnetic fluid. Magneto-birefringence is expected to be increased at large enough wavelengths only.