Magneto-optic and converse magnetoelectric effects in a ferromagnetic liquid crystal

Abstract

We have studied the response of ferromagnetic liquid crystals to external magnetic and electric fields, and compared it to the usual response of nematic liquid crystals (NLCs). We have observed effects, which are not present in a pure NLC and are a consequence of the coupling between the nematic director and the magnetization. The electro-optic effect, which is in the ferromagnetic phase the same as in the pure NLC, is accompanied by a converse magnetoelectric effect. The magneto-optic effect differs completely from the one observed in the pure NLC, where it is a quadratic effect and it only appears when a magnetic field larger than a critical field is applied perpendicular to the director. In the ferromagnetic NLC in addition to the response to the perpendicular field, there is also a qualitatively different response to the parallel field. Contrary to the pure NLC no critical field needs to be exceeded for the system to respond to a perpendicular field, but a critical field needs to be exceeded to observe a response to the field parallel to the director and antiparallel to the magnetization. The critical field is in this case two orders of magnitude smaller than the critical field of the magnetic Frederiks transition in the pure NLC. The experimental observations are well described by a simple macroscopic theory.

Biaxial ferromagnetic liquid crystal colloids

“…2E) cells with n 0 perpendicular and parallel to confining substrates, respectively, as well as for orientations of applied field B and measured M-components parallel and perpendicular to n 0 . Resisted by elastic energy costs of director distortions, magnetic switching of single-domain BFLCCs is threshold-free for fields applied in directions parallel and perpendicular to n 0 and is different from that of uniaxial ferromagnetic LC colloids (6)(7)(8)(9)(10)(11)(12). The field Bkn 0 applied to a BFLCC with n 0 along the cell normal z, with a tilted M spontaneously along one of the orientations on the up-cone, rotates M toward B and thereby tilts the director away from the confining surface normal, leading to light transmission through the sample placed between crossed polarizers (Fig.…”

“…At the same time, there is a great potential for guiding low-symmetry assembly in hybrid LC-colloidal systems, in which the molecular LC is a fluid host for colloidal particles (18). Different types of LCmediated ordering of anisotropic particles can emerge from elastic and surface-anchoring-based interactions and can lead to the spontaneous polar alignment of magnetic inclusions (6), although the orientations of the magnetic dipoles of colloidal particles were always slave to the LC director n, orienting either parallel or perpendicular to it without breaking uniaxial symmetry (6)(7)(8)(9)(10)(11)(12).…”

“…Fluids with polar ordering were envisaged by Born a century ago (3)(4)(5), with their study recently guided by prescient theories of Brochard and de Gennes (6)(7)(8)(9)(10)(11)(12). An experimental search for small-molecule biaxial nematic fluids has gone on for decades (2,13).…”

Tunable structures of mixtures of magnetic particles in liquid-crystalline matrices

“…[8][9][10][11] Correspondingly, even more complex behavior is expected for suspended particles with internal degrees of freedom such as magnetic particles. As predicted in a famous work by Brochard and de Gennes the coupling between ferrocolloids and liquid crystal molecules increases the sensitivity to external magnetic field significantly; 12,13 a phenomenon with many potential applications in magneto- 14,15 or electro-optical devices. 16,17 So far, suspensions of ferrocolloids in a nematic LC matrix composed of much smaller LC molecules (also referred to as low molecular mass LCs) have been studied experimentally and theoretically (see, e.g., ref.…”

Electric-field-induced transport of microspheres in the isotropic and chiral nematic phase of liquid crystals

“…Numerous different particles have been employed, dielectric [8,9], ferroelectric [10,11] and magnetic [12,13] nanoparticles, gold nanoparticles, which are incorporated into the mesogenic structures [14], one-dimensional nanotubes in calamitic [15,16], discotic [17], ferroelectric [18] and lyotropic [19] phases, two-dimensional graphene-based materials, most often graphene oxide [20][21][22], just to name a few. Further materials are quantum dots [23], nanorods [24], and even biological structures [25].…”