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

The temperature, particle size, and particle concentration dependence of th e zero-field birefringence were investigated using gamma-Fe2O3 and MnFe2O4 ionic magnetic fluids in the range of 290-350 K. Upon heating a sample from below its characteristic temperature T-c, which depends upon the particle size and particle concentration, the zero-field birefringence goes critical ly down to zero. The experimental data are successfully explained when the following two points are considered in the model describing magnetic birefr ingence in magnetic fluids. First, dimers rather than monomers are responsi ble for the zero-field birefringence. Second, thermal disruption of the dim er structure follows a critical behavior. A theory for the zero-field biref ringence is developed using classical statistics to evaluate the orientatio nal order tensor component S-zz. Finally, the zero birefringence signal at zero field, as found in magnetic fluids containing surfactant on the partic le surface, is discussed within the dimer model. (C) 2000 American Institut e of Physics. [S0021-8979(00)04205-5].