Alenka Mertelj

Ferromagnetism in suspensions of magnetic platelets in liquid crystal

More than four decades ago, Brochard and de Gennes proposed that colloidal suspensions of ferromagnetic particles in nematic (directionally ordered) liquid crystals could form macroscopic ferromagnetic phases at room temperature. The experimental realization of these predicted phases has hitherto proved elusive, with such systems showing enhanced paramagnetism but no spontaneous magnetization in the absence of an external magnetic field. Here we show that nanometre-sized ferromagnetic platelets suspended in a nematic liquid crystal can order ferromagnetically on quenching from the isotropic phase. Cooling in the absence of a magnetic field produces a polydomain sample exhibiting the two opposing states of magnetization, oriented parallel to the direction of nematic ordering. Cooling in the presence of a magnetic field yields a monodomain sample; magnetization can be switched by domain wall movement on reversal of the applied magnetic field. The ferromagnetic properties of this dipolar fluid are due to the interplay of the nematic elastic interaction (which depends critically on the shape of the particles) and the magnetic dipolar interaction. This ferromagnetic phase responds to very small magnetic fields and may find use in magneto-optic devices.

Spontaneous liquid crystal and ferromagnetic ordering of colloidal magnetic nanoplates.

Ferrofluids are familiar as colloidal suspensions of ferromagnetic nanoparticles in aqueous or organic solvents. The dispersed particles are randomly oriented but their moments become aligned if a magnetic field is applied, producing a variety of exotic and useful magnetomechanical effects. A longstanding interest and challenge has been to make such suspensions macroscopically ferromagnetic, that is having uniform magnetic alignment in the absence of a field. Here we report a fluid suspension of magnetic nanoplates that spontaneously aligns into an equilibrium nematic liquid crystal phase that is also macroscopically ferromagnetic. Its zero-field magnetization produces distinctive magnetic self-interaction effects, including liquid crystal textures of fluid block domains arranged in closed flux loops, and makes this phase highly sensitive, with it dramatically changing shape even in the Earths magnetic field.

Magnetodielectric and magnetoviscosity response of a ferromagnetic liquid crystal at low magnetic fields

We report on experimental studies of the viscoelastic, magnetodielectric, and magnetoviscosity properties of ferromagnetic liquid crystals (LCs) prepared by dispersing ferromagnetic nanoparticles in a thermotropic LC. Both the splay elastic constant and rotational viscosity of the ferronematic LCs are found to be considerably lower than that of the pure LC and advantageous to the device applications. The ferromagnetic LCs show unique magnetodielectric and magnetoviscosity response at very low magnetic fields that are useful for smart fluid applications.

Controlled heteroaggregation of two types of nanoparticles in an aqueous suspension.

Composite particles combining nanoparticles of different functional materials, as well as nanoclusters of nanoparticles of controlled size, can be synthesized by the assembly of nanoparticles in an aqueous suspension. Different interactions between the nanoparticles in the suspension can be applied for their heteroaggregation and controlled by engineering the surface properties of the nanoparticles. The heteroaggregation of nanoparticles in a suspension was studied on a model system composed of superparamagnetic carboxyl-functionalized silica-coated maghemite nanoparticles (cMNPs) (24 nm in size) and larger, amino-functionalized, silica nanoparticles (aSNPs) (92 nm). The heteroaggregates formed with electrostatic attractions between the nanoparticles displaying an opposite electrical surface charge, or with chemical interactions originating from covalent bonding between the molecules at their surfaces. The suspensions were characterized with measurements of the zeta-potential and dynamic light scattering (DLS). The heteroaggregates were analyzed by transmission (TEM) and scanning (SEM) electron microscopy. The kinetics of the heteroaggregation was followed by continuous monitoring of the changes in the average hydrodynamic size by DLS. The results show that covalent bonding is much more effective than attractive electrostatic interactions in terms of a much greater and more homogeneous coverage of the larger central aSNP by the smaller cMNPs in the outer layer.

Ferromagnetic nematic liquid crystals

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.

Phase transitions, optical, dielectric and viscoelastic properties of colloidal suspensions of BaTiO3 nanoparticles and cyanobiphenyl liquid crystals

We report experimental studies on the phase transitions and physical properties of colloidal suspensions of BaTiO3 nanoparticles and two cyanobiphenyl liquid crystals (4-pentyl-4ʹ-cyanobiphenyl and 4-octyl-4ʹ-cyanobiphenyl). From the differential scanning calorimetric measurements, we show that the nanoparticles have antagonistic effect on the isotropic to nematic and nematic to smectic-A phase transitions. The birefringence, dielectric anisotropy and splay elastic constant remain almost unchanged, whereas the bend elastic constant and rotational viscosity decrease considerably. The experimental results are discussed based on the possible contribution of BaTiO3 nanoparticles and free surfactant molecules in the suspensions.

Dynamic Magneto-optic Coupling in a Ferromagnetic Nematic Liquid Crystal

Hydrodynamics of complex fluids with multiple order parameters is governed by a set of dynamic equations with many material constants, of which only some are easily measurable. We present a unique example of a dynamic magneto-optic coupling in a ferromagnetic nematic liquid, in which long-range orientational order of liquid crystalline molecules is accompanied by long-range magnetic order of magnetic nanoplatelets. We investigate the dynamics of the magneto-optic response experimentally and theoretically and find out that it is significantly affected by the dissipative dynamic cross-coupling between the nematic and magnetic order parameters. The cross-coupling coefficient determined by fitting the experimental results with a macroscopic theory is of the same order of magnitude as the dissipative coefficient (rotational viscosity) that governs the reorientation of pure liquid crystals.

Monolithic Magneto-Optical Nanocomposites of Barium Hexaferrite Platelets in PMMA

The incorporation of magnetic barium hexaferrite nanoparticles in a transparent polymer matrix of poly(methyl methacrylate) (PMMA) is reported for the first time. The barium hexaferrite nanoplatelets doped with Sc3+, i.e., BaSc0.5Fe11.5O12 (BaHF), having diameters in the range 20 to 130 nm and thicknesses of approximately 5 nm, are synthesized hydrothermally and stabilized in 1-butanol with dodecylbenzenesulfonic acid. This method enables the preparation of monolithic nanocomposites by admixing the BaHF suspension into a liquid monomer, followed by in-situ, bulk free-radical polymerization. The PMMA retains its transparency for loadings of BaHF nanoparticles up to 0.27 wt.%, meaning that magnetically and optically anisotropic, monolithic nanocomposites can be synthesized when the polymerization is carried out in a magnetic field. The excellent dispersion of the magnetic nanoparticles, coupled with a reasonable control over the magnetic properties achieved in this investigation, is encouraging for the magneto-optical applications of these materials.

Anisotropic microrheological properties of chain-forming magnetic fluids

In an external magnetic field, magnetic colloids transform from an isotropic fluid to an anisotropic viscoelastic material. Using passive microrheology, we measured the microrheological properties of a magnetic fluid as a function of direction and magnitude of external magnetic field. The effective microviscosity strongly depends on the magnitude of the external field, while it is almost independent of its direction. The measured effective storage modulus varies significantly within the sample and depends both on the direction and the magnitude of the external magnetic field. It vanishes in a zero field, while in a non-zero field it is larger by a factor of 2 to 4 in the direction along the field than perpendicular to it. The non-zero value of the storage modulus, which indicates the formation of a viscoelastic fluid, appears at magnetic fields at which the dynamic light scattering experiments reveal the formation of elongated structures.

Magnetic-field tuning of whispering gallery mode lasing from ferromagnetic nematic liquid crystal microdroplets

We report magnetic field tuning of the structure and Whispering Gallery Mode lasing from ferromagnetic nematic liquid crystal micro-droplets. Microlasers were prepared by dispersing a nematic liquid crystal, containing magnetic nanoparticles and fluorescent dye, in a glycerol-lecithin matrix. The droplets exhibit radial director structure, which shows elastic distortion at a very low external magnetic field. The fluorescent dye doped ferromagnetic nematic droplets show Whispering Gallery Mode lasing, which is tunable by the external magnetic field. The tuning of the WGM lasing modes is linear in magnetic field with a wavelength-shift of the order of 1 nm/100 mT. Depending on the lasing geometry, the WGMs are red- or blue-shifted.