Birefringence of magnetite-based magnetic fluids: the effect of the surface-coating layer

Water-based and hydrocarbon-based magnetic fluids (MFs) containing Fe/sub 3/O/sub 4/ nanoparticles are investigated using static magnetic birefringence measurements. A strong dependence of the birefringence signal upon the magnetic permeability is found. In addition to the field dependence of the magnetic permeability, inclusion of dimers in the description of the birefringence signal yields excellent agreement between parameters obtained from birefringence and high-resolution microscopy data.