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Effect of an external magnetic field on the nematic-isotropic phase transition in mesogenic systems of uniaxial and biaxial molecules

Influence of an external magnetic field on the nematic-isotropic ($N-I$) phase transition in a dispersion model of nematic liquid crystals, where the molecules are either perfectly uniaxial or biaxial (board-like), has been studied by Monte Carlo simulation. Using multiple histogram reweighting technique and finite size scaling analysis the order of the phase transition, the transition temperature at the thermodynamic limit and the stability limit of the isotropic phase below the transition temperature for different magnetic field strengths have been determined. The magnetic field dependence of the shift in $N-I$ transition temperature is observed to be more rapid than that predicted by the standard Landau-de Gennes and Maier-Saupe mean field theories. We have shown that for a given field strength the shift in the transition temperature is higher for the biaxial molecules in comparison with the uniaxial case. The study shows that the $N-I$ transition for the biaxial molecules is weaker than the well known weak first order $N-I$ transition for the uniaxial molecules and the presence of the external magnetic field (up to a certain critical value) makes the transition much more weaker for both the systems. The estimate of the critical magnetic field ($\sim 110 T$) for the common nematics is found to be smaller than the earlier estimates.