Field Dependence of Magnetic Birefringence of Magnetic Fluid in Low-Magnetic-Field Region

The magnetic-field, H , dependence of the magnetic birefringence of two kinds of magnetic fluid thin-film samples was measured in a region with a low magnetic field of less than 150 Oe. While the magnetic fluid of one sample had been exposed in a vacuum state for several minutes before the optical measurement, that of the other had not. The birefringence Δ n of the former sample is proportional to H 2 , but Δ n of the latter is in proportion to H in the low-field region. The magnetic birefringence of the former sample is attributed to the interaction between an individual magnetic colloidal particle and the external field, but that of the latter is due to the interaction among the colloidal particles exerted by the field.