Microspheres of magnetite () are coated with oleic acid and suspended in the ferrofluid, which was a colloidal dispersion of nonmagnetic particles of magnetite in kerosene. The microspheres, being multidomain, have zero magnetization at zero fields. On the other hand, the nano-sized particles are single-domain and hence their domain magnetization at zero fields is saturated. Consequently, they behave as tiny magnets stably dispersed in a liquid. In absence of a field, magnetic moments of these particles are randomly oriented and the system resembles a paramagnetic gas. Since the magnetic moment of particles is much larger than that of molecules of a paramagnetic gas, a ferrofluid is also referred as a superparamagnetic fluid. However, this superparamagnetic nature is different from that of Neel’s superparamagnetism.8 Under stationary conditions a ferrofluid performs as a quasi-continuous medium and its properties, such as density, viscosity, and refractive index depend upon an applied static magnetic field. For a dilute ferrofluid, the field dependence of these physical properties is derived using Langevin theory of noninteracting particles.8 Since the size of microspheres are very large compared to the size of nanomagnets, light scattering will be dominated by these larger-sized particles and the scattered radiation will be confined in the forward, as well as the backward, directions.15 The field-induced variation of refractive index of a ferrofluid () is given by Display Formula
(1)Here, is Langevin function given by , and () is the dimensionless field parameter; , , and are respectively dipole moment of the particles, applied magnetic field, Boltzmann constant and absolute temperature.