Magnetic-field-induced changes of the isotropic-nematic phase transition in side-chain polymer liquid crystals

The isotropic-nematic $(I-N)$ phase transition of side-chain polymer liquid crystals is intrinsically a weak first-order transition with a biphasic region spread over a wide temperature interval. In the presence of high magnetic fields we find the $I-N$ transition to become a strong first order. The $I-N$ biphasic region shrinks its temperature window as larger magnetic fields are applied, until it completely disappears and the transition completes at a fixed temperature. We interpret this behavior as a consequence of the nonlinear coupling of the magnetic field to the system free energy, via the suppression of the order fluctuations in the nematic mesophase at the $I-N$ transition crossing.