Zero-field birefringence in magnetic fluids: Temperature, particle size, and concentration dependence

The temperature, particle size, and particle concentration dependence of the zero-field birefringence were investigated using γ-Fe2O3 and MnFe2O4 ionic magnetic fluids in the range of 290–350 K. Upon heating a sample from below its characteristic temperature Tc, which depends upon the particle size and particle concentration, the zero-field birefringence goes critically down to zero. The experimental data are successfully explained when the following two points are considered in the model describing magnetic birefringence in magnetic fluids. First, dimers rather than monomers are responsible for the zero-field birefringence. Second, thermal disruption of the dimer structure follows a critical behavior. A theory for the zero-field birefringence is developed using classical statistics to evaluate the orientational order tensor component Szz. Finally, the zero birefringence signal at zero field, as found in magnetic fluids containing surfactant on the particle surface, is discussed within the dimer model.