Nematic-Isotropic Transition: Thermal Hysteresis and Magnetic Field Effects

Using neutron scattering, we have studied the effect of an applied magnetic field on the nematic-isotropic transition temperature Tc in para-azoxy-anisole (PAA). We have, for the first time, been able to measure a thermal hysteresis width, (T₊*-T_-*)/Tc, and find a value 3.7×10^-4 for a field-aligned sample, much lower than any previous estimate, and decreasing to zero for weaker fields. Also Tc decreases with the field, as does the step Qc in the order parameter. The data at the lowest fields indicate that in zero field the transition may be of second order, and we correlate this with an observed increase of the director fluctuations.