Magnetic field induced transitions in soft compensated ferrocholesteric liquid crystals

Within the continuum theory, we study the influence of a magnetic field on the orientational and magnetic properties of a helicoidal liquid-crystal ferrosuspension, the so-called ferrocholesteric. We consider the compensated ferrocholesterics with soft planar coupling of magnetic particles with a liquid crystal matrix. We show that under the influence of a magnetic field applied perpendicularly to the axis of the helical structure of a ferrocholesteric, the unwinding of the helical structure takes place, and the transition to the ferronematic phase occurs. We study the dependence of the phase transition field on the suspension material parameters. The behavior of the average magnetization and the pitch of the helical structure as a function of a magnetic field is studied. It is shown that at soft coupling of particles with a liquid crystal matrix, the magnetization vector and the director have different pitches of the spiral structure in an external magnetic field.