Biasing a ferronematic - a new way to detect weak magnetic field

Giant enhancement and facile tuning of photoluminescence in a soft anisotropic magneto-gel

A soft photoluminescent composite, prepared using a nematic liquid crystal and a fluorescent gelator, exhibits a nearly two orders of magnitude increase in fluorescence on addition of superparamagnetic nanoparticles. The internal magnetic field generated leading to an increase in the population of singlet excitons which affects the radiative efficiency, and enhanced ordering of the LC environment are proposed to be responsible for the large increase seen in fluorescence. Also, the nematic nature of the host liquid crystal medium aids in switching of the fluorescence intensity between its anisotropic limits on application of an external electric field with the switch-off time being faster compared to the field-driven switch-on time.

Ferrocholesteric-ferronematic transitions induced by shear flow and magnetic field

We study the unwinding of the ferrocholesteric helical structure induced by a combined action of a magnetic field and a shear flow. Both influences are able to induce the ferrocholesteric-ferronematic transition independently; however, the differences between the magnetic field orientation and the flow alignment direction lead to a competition between magnetic and hydrodynamic mechanisms of influence on the ferrocholesteric structure. We analyze various orientations of a magnetic field relative to the direction of a shear flow. The pitch of the ferrocholesteric helix is obtained as function of the strength and the orientation angle of the magnetic field, the shear velocity gradient and a reactive parameter. Phase diagrams of ferrocholesteric-ferronematic transition and the pitch of the ferrocholesteric helix as functions of the material and the governing parameters are calculated. We find out that imposing a shear flow leads to a shift of the magnetic field threshold. The value of the critical magnetic field depends on the magnetic field orientation, the velocity gradient, and the viscous coefficients. We show that the interplay of a magnetic field and a shear flow can induce reentrant orientational transitions that are ferrocholesteric-ferronematic-ferrocholesteric.

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 Matter2016, 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.

Structuralization of magnetic nanoparticles in 5CB liquid crystals

This work is devoted to the study of highly stable composite systems of the liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) doped with CoFe₂O₄ magnetic nanoparticles. Ferronematic samples were prepared with two different weight concentrations: sample A 0.085 wt% and sample B 0.062 wt%. The interaction of CoFe₂O₄ nanoparticles with the liquid crystal was investigated by small-angle X-ray-scattering and magnetization measurements. The obtained results reveal aggregates formed by magnetic nanoparticles that are oriented in the nematic phase. Moreover, the prepared samples show unexpected behaviour of a sudden change in magnetization, which is unusual for such ferronematics.

Magneto-induced anisotropy in magnetic colloids of superparamagnetic magnetite nanoparticles in an external magnetic field

Here we report a study of chain formation and the magnetic anisotropy induced by them in suspensions of slightly anisotropic Fe₃O₄ nanoparticles in water and in aqueous V₂O₅ suspensions. An investigation into the magnetization of the fluid and frozen suspensions, the application of dynamic light scattering techniques and the observation of the magnetic anisotropy in the frozen magnetically aligned samples allowed us to confirm the existence of chains of Fe₃O₄ in both suspensions. Our study shows that the magneto-induced anisotropy appearing in magnetic fields in colloids with Fe₃O₄ particles is mainly due to many particle (chain) magneto-induced anisotropy, but not due to single particle magneto-induced anisotropy connected with particle shape anisotropy. In other words, the single particle magneto-induced anisotropy is much smaller than the many particle (chain) anisotropy. The anisometry of the chains provides effective coupling with the nonmagnetic V₂O₅ component of the suspension and results in its strong sensitivity to the magnetic field.