Hexagon-stripe transition at the magnetic field induced phase transformations

The thermodynamical model of the magnetic colloid with competing attractive and repulsive long-range forces in the Hele--Shaw cell under the action of the normal field is considered. The boundary in the parameter space between the stripe, the hexagonal and the unmodulated phases is calculated. Calculated phase diagram is in qualitative accordance with the numerical simulation data for the kinetics of the magnetic field induced phase transformations obtained by the pseudospectral technique. Transformation of the stripe pattern to the hexagonal found in the numerical experiments allows to propose that the mechanism of the labyrinthine pattern transformation to the hexagonal observed experimentally for the concentrated phase can be connected with the shift of the critical point at the magnetic field increase. Intermediate mixed states found numerically are in the agreement with the calculated phase diagram and results of the experimental investigations.