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Journal ArticleDOI

Capacitance changes in ferronematic liquid crystals induced by low magnetic fields.

TL;DR: The response in capacitance to low external magnetic fields of suspensions of spherical magnetic nanoparticles, single-wall carbon nanotubes, and SWCNT functionalized with Fe(3)O(4) nanoparticles in a nematic liquid crystal has been studied experimentally.
Abstract: The response in capacitance to low external magnetic fields (up to 0.1 T) of suspensions of spherical magnetic nanoparticles, single-wall carbon nanotubes (SWCNT), SWCNT functionalized with carboxyl group (SWCNT-COOH), and SWCNT functionalized with Fe${}_{3}$O${}_{4}$ nanoparticles in a nematic liquid crystal has been studied experimentally. The volume concentration of nanoparticles was ${\ensuremath{\phi}}_{1}={10}^{\ensuremath{-}4}$ and ${\ensuremath{\phi}}_{2}={10}^{\ensuremath{-}3}$. Independent of the type and the volume concentration of the nanoparticles, a linear response to low magnetic fields (far below the magnetic Fr\'eederiksz transition threshold) has been observed, which is not present in the undoped nematic.
Citations
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Journal ArticleDOI
01 Apr 2017
TL;DR: In this paper, the authors present experimental realization and behaviour of the ferromagnetic nematic phase, which is observed in different suspensions of magnetic nanoplatelets, and a brief explanation of a simple macroscopic theory which can be used to understand the main features of the Ferromagnetic phase is presented.
Abstract: This review presents experimental realization and behaviour of the ferromagnetic nematic phase, which is observed in different suspensions of magnetic nanoplatelets. After a general introduction, the challenges in the synthesis of magnetic nanoplatelets and preparation of the nematic suspensions are discussed. A brief explanation of a simple macroscopic theory, which can be used to understand the main features of the ferromagnetic phase, follows. In the main part, four different ferromagnetic nematic systems are presented: (i) ferromagnetic suspensions of the platelets in nematic liquid crystals, (ii) dense suspensions in an isotropic solvent – ferromagnetic ferrofluids, (iii) biaxial ferromagnetic nematic suspensions, and (iv) chiral ferromagnetic suspensions. The main focus is on the formation of the ferromagnetic phases and the growth of the magnetic domains. At the end, dynamics of ferromagnetic liquid crystals and methods for their observation are briefly discussed.

87 citations


Cites background from "Capacitance changes in ferronematic..."

  • ...effect was observed at lower magnetic fields compared to the field needed to reorient pure LC [59,60,61,72,73]....

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Journal ArticleDOI
TL;DR: Observations are observed of effects, which are not present in a pure NLC and are a consequence of the coupling between the nematic director and the magnetization.
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.

84 citations

Journal ArticleDOI
TL;DR: In this paper, E7 liquid crystal doped with different amounts of Fe3O4 nanoparticles was studied and dielectric properties of pure and ferro-nematic with special initial ordering (homogeneous and homeotropic orientations) in different temperatures were analyzed.

43 citations

Journal ArticleDOI
TL;DR: In this paper, the shape anisotropy of magnetic particles on the isotropic-nematic phase transition was studied in ferronematics based on the nematic liquid crystal (LC) 4-(trans-4n-hexylcyclohexyl)-isothiocyanato-benzene (6CHBT).
Abstract: The influence of the shape anisotropy of magnetic particles on the isotropic–nematic phase transition was studied in ferronematics based on the nematic liquid crystal (LC) 4-(trans-4-n-hexylcyclohexyl)-isothiocyanato-benzene (6CHBT). The LC was doped with spherical or rod-like magnetic particles of different size and volume concentrations. The phase transition from isotropic to nematic phase was observed by polarising microscope as well as by capacitance measurements. The influence of the concentration and the shape anisotropy of the magnetic particles on the isotropic–nematic phase transition in LC are demonstrated here. The results are in a good agreement with recent theoretical predictions.

25 citations

Journal ArticleDOI
TL;DR: An opticalStudy of the magnetic Fredericksz transition combined with a light scattering study of the classical nematic liquid crystal: the pentylcyanobiphenyl (5CB), doped with 6 nm diameter magnetic and nonmagnetic nanoparticles, finds that this magnetic interaction favors an easier liquid crystal director rotation in the presence of external magnetic field.
Abstract: A long time ago, Brochard and de Gennes predicted the possibility of significantly decreasing the critical magnetic field of the Fredericksz transition (the magnetic Fredericksz threshold) in a mixture of nematic liquid crystals and ferromagnetic particles, the so-called ferronematics. This phenomenon is rarely measured to be large, due to soft homeotropic anchoring induced at the nanoparticle surface. Here we present an optical study of the magnetic Fredericksz transition combined with a light scattering study of the classical nematic liquid crystal: the pentylcyanobiphenyl (5CB), doped with 6 nm diameter magnetic and nonmagnetic nanoparticles. Surprisingly, for both nanoparticles, we observe at room temperature a net decrease of the threshold field of the Fredericksz transition at low nanoparticle concentrations, which appears associated with a coating of the nanoparticles by a brush of polydimethylsiloxane copolymer chains inducing planar anchoring of the director on the nanoparticle surface. Moreover, the magnetic Fredericksz threshold exhibits nonmonotonic behavior as a function of the nanoparticle concentration for both types of nanoparticles, first decreasing down to a value from 23% to 31% below that of pure 5CB, then increasing with a further increase of nanoparticle concentration. This is interpreted as an aggregation starting at around 0.02 weight fraction that consumes more isolated nanoparticles than those introduced when the concentration is increased above c=0.05 weight fraction (volume fraction 3.5×10^{-2}). This shows the larger effect of isolated nanoparticles on the threshold with respect to aggregates. From dynamic light scattering measurements we deduced that, if the decrease of the magnetic threshold when the nanoparticle concentration increases is similar for both kinds of nanoparticles, the origin of this decrease is different for magnetic and nonmagnetic nanoparticles. For nonmagnetic nanoparticles, the behavior may be associated with a decrease of the elastic constant due to weak planar anchoring. For magnetic nanoparticles there are non-negligible local magnetic interactions between liquid crystal molecules and magnetic nanoparticles, leading to an increase of the average order parameter. This magnetic interaction thus favors an easier liquid crystal director rotation in the presence of external magnetic field, able to reorient the magnetic moments of the nanoparticles along with the molecules.

25 citations

References
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Book
01 Feb 1974
TL;DR: In this paper, the authors define an order parameter statistical theories of the nematic order phenomonological description of the nematic-isotopic mixtures and describe the properties of these mixtures.
Abstract: Part 1 Liquid crystals - main types and properties: introduction - what is a liquid crystal? the building blocks nematics and cholesterics smectics columnar phases more on long-, quasi-long and short-range order remarkable features of liquid crystals. Part 2 Long- and short-range order in nematics: definition of an order parameter statistical theories of the nematic order phenomonological description of the nematic-isotopic mixtures. Part 3 Static distortion in a nematic single crystal: principles of the continuum theory magnetic field effects electric field effects in an insulating nematic fluctuations in the alignment hydrostatics of nematics. Part 4 Defects and textures in nematics: observations disclination lines point disclinations walls under magnetic fields umbilics surface disclinations. Part 5 Dynamical properties of nematics: the equations of "nematodynamics" experiments measuring the Leslie co-efficients convective instabilities under electric fields molecular motions. Part 6 Cholesterics: optical properties of an ideal helix agents influencing the pitch dynamical properties textures and defects in cholesterics. Part 7 Smectics: symmetry of the main smectic phases continuum description of smectics A and C remarks on phase and precritical phenomena.

9,683 citations