Reentrant Phase Transitions in Ferronematic Liquid Crystals

Abstract

Within the continuum theory we study the influence of codirectional electric and magnetic fields on the orientational structure of a ferronematic layer taking into account the segregation effects. Recently in [Phys. Rev. E 81, 051710 (2010)] the tricritical behavior of the magnetic Freedericksz transition in ferronematics has been theoretically predicted. This paper shows that an external electric field can change the character of this transition. We find reentrant Freedericksz transitions in ferronematic liquid crystals subjected to the electric and magnetic fields. These transitions can be either first-or second-order ones. We derive the analytical expression for the width of the area of the reentrant orientational transitions.

Effect of electric and magnetic fields on the orientation structure of a ferronematic liquid crystal

“…However, in con trast to the magnetic field, the electric field does not induce re entrant transitions in the FN. As shown in [11,12], this is also true for restricted geometries.…”

“…For a finite coupling energy, the magnetization and director tend to orient themselves along the magnetic field upon an increase in its strength, which leads to loss of stability of the homeotropic phase for H ≥ H ⊥ and subsequent transition of the FN to the angular phase. Angles ϕ and ψ of orientation of the director and magnetization in the angular orientation phase of the FN are determined by expressions (11) and (12). In magnetic field H for E = 0, these relations are trans formed into expressions (17) derived earlier in [13], while in electric field E for H = 0, values (15) are observed, which describe only the homeotropic phase (see Fig.…”

“…The presence of an electric field that can directly affect only the LC matrix gives one more mechanism of action, viz., the quadrupole electric mechanism. The competition between these three mechanisms leads to re entrant orientation transitions in the FN in limited geometries [11,12]. In this work, we analyze orientation transition in an inbounded FN in parallel electric and magnetic fields.…”

“…This is possible, for example, when magnetic particles are implanted into the matrix in such a way that the total magnetization of the FN in zero field is perpendicular to the director (so called homeotropic coupling between magnetic particles and the matrix). In this case, the application of a magnetic field induces a variety of physically interesting effects in the suspension (e.g., tricritical or re entrant phenomena [8][9][10][11][12]). It will be shown below that such phenomena are not always connected with a limited geometry, but are the result of competition of the dipole and quadrupole mechanisms of the action of the field on the FN.…”

Interplay between dipole and quadrupole modes of field influence in liquid-crystalline suspensions of ferromagnetic particles

“…The competition between these three mechanisms additionally leads to re-entrant orientational transitions in ferronematics. 9 As is shown in Ref. 10 the re-entrant transitions are possible in ferronematics subjected to the magnetic field without electric field, but under special bistable coupling conditions on the bounding plates.…”