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 (Tc) 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 Tc. 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.