Irreversibility of zero-field birefringence in ferrofluids upon temperature reversal

The temperature dependence of the zero-field birefringence was investigated using acid and basic MnFe2O4 ionic magnetic fluids, in the range of 290-350 K. Approaching a characteristic temperature (T-c) from below, which depends upon the sample characteristics, the zero-field birefringence goes critically down to zero. Furthermore, the birefringence shows an irreversible path upon heating and cooling the samples above T-c. The experimental data are successfully explained as long as dimers are included in the model calculation and the thermal disruption of them follows a critical behavior. (C) 2000 American Institute of Physics. [S0021-8979(00)00305-4].