Veronika Gdovinová

Influence of the anisometry of magnetic particles on the isotropic–nematic phase transition

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.

Magnetically induced shift of the isotropic–nematic phase transition temperature in a mixture of bent-core and calamitic liquid crystals doped with magnetic particles

We have investigated the influence of doping with spherical magnetic nanoparticles on the mixture of a bent-core and a calamitic liquid crystal. Results showed a reduction of the critical field of the magnetic Fréedericksz transition by more than a factor of two after the doping. Moreover, we give for the first time experimental evidence of the theoretically predicted magnetically induced negative shift of the isotropic to nematic phase transition temperature.

Tuning the phase transition temperature of ferronematics with a magnetic field

The influence of magnetic field on the isotropic-to-nematic phase transition temperature is investigated in neat bent-core and calamitic liquid crystals, in their mixture, and in samples doped with spherical magnetic nanoparticles for two different orientations of the magnetic field. A magnetic-field-induced negative or positive shift of the transition temperature was detected depending on the magnetic field orientation with respect to the initial orientation of the nematic phase, and on the type of liquid crystal matrix.

Magnetic Fluids and Their Complex Systems

The presented chapter provides an overview of selected magnetic nanoparticle systems (magnetic fluids, magnetosomes, magnetoferritin and liquid crystals doped with magnetic particles) as the unique materials with potential utilization in the field of biological and biomedical applications as well as in the field of technology. The main idea for magnetic nanoparticles incorporating is the improvement of material properties and to achieve better conditions for a wide range of scientific disciplines such as magnetic hyperthermia, drug delivery, magneto-optics, power engineering and others.

Ferronematics based on the nematic 6CB in combined electric and magnetic fields

ABSTRACT This work is devoted to the study of composite systems of the liquid crystal 4-n-hexyl-4-cyanobiphenyl (6CB) doped with differently shaped magnetite nanoparticles. The ferronematic samples were prepared with the volume concentration of =10 of spherical, as well as of rod-like magnetic particles. The structural transitions in ferronematic samples were observed by capacitance measurements in a capacitor made of indium-tin-oxide-coated glass electrodes in combined electric and magnetic fields.

Alternating current magnetic susceptibility of a ferronematic

We report on experimental studies focusing on the dynamic ac magnetic susceptibility of a ferronematic. It has been shown recently, that in the isotropic phase of a ferronematic, a weak dc bias magnetic field of a few oersteds increases the ac magnetic susceptibility. This increment vanishes irreversibly if the substance is cooled down to the nematic phase, but can be reinduced by applying the dc bias field again in the isotropic phase [Tomašovičová, N. et al. Soft Matter 2016, 12, 5780–5786]. The effect has no analogue in the neat host liquid crystal. Here, we demonstrate that by doubling the concentration of the magnetic nanoparticles, the range of the dc bias magnetic field to which the ferronematic is sensitive without saturation can be increased by about two orders of magnitude. This finding paves a way to application possibilities, such as low magnetic field sensors, or basic logical elements for information storage.

Dielectric Spectroscopy of Ferronematics Based on 6CHBT Liquid Crystal

In our study, the dielectric behaviour of the rod-like liquid crystal (6CHBT) doped with magnetic nanoparticles of spherical shape was investigated by means of dielectric spectroscopy in the frequency range from 20 Hz to 2 MHz. A low frequency dielectric dispersion in the nematic and isotropic phases of the pure liquid crystal (LC) has been assigned to the space charge polarization. After doping the host LC with the magnetic nano particles, a nearly Debye-like relaxation process was observed with the temperature dependence obeying the Arrhenius law. Considering a possible electric double layer formation on the particle surfaces, the detected relaxation process in the doped LC can be associated with the electric double layer polarization. The experimental results point out that in the measured frequency range the space charge and interfacial effects constitute the main dielectric response. Any anchoring effects were not observed and are therefore expected to appear in higher frequencies.