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 290-35 0 K. Approaching a characteristic temperature (T-c) from below, which depen ds upon the sample characteristics, the zero-field birefringence goes criti cally down to zero. Furthermore, the birefringence shows an irreversible pa th upon heating and cooling the samples above T-c. The experimental data ar e successfully explained as long as dimers are included in the model calcul ation and the thermal disruption of them follows a critical behavior. (C) 2 000 American Institute of Physics. [S0021-8979(00)00305-4].