We present a detailed experimental and theoretical study of the optical response of suspensions of ferromagnetic nanoparticles (''ferroparticles'') in nematic liquid crystals (''ferronematics''), concentrating on the magnetic field-induced Frederiks transition. Even extremely low ferroparticle concentrations (at a volume fraction between 2 Â 10 À5 and 2 Â 10 À4 ), induce a significant additional ferronematic linear response at low magnetic field (<100 G) and a decrease in the effective magnetic Frederiks threshold. The experimental results demonstrate that our system has weak ferronematic behavior. The proposed theory takes into account the nematic diamagnetism and assumes that the effective magnetic susceptibility, induced by the nanoparticles, no longer dominates the response. The theory is in good agreement with the experimental data for the lowest concentration suspensions and predicts the main features of the more concentrated ones. The deviations observed in these cases hint at extra effects due to particle aggregation, which we have also observed directly in photographs.

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International audienceAbstract— The photoaligning properties of the popular photoaligning material polyvinyl-4(fluorocinnamate) (PVCN-F) are presented. The aligning quality and azimuthal and zenithal anchoring energy were measured and the drift of the easy orientation axis (gliding effect) on the PVCN-F surface, depending on UV exposure, was studied. Special attention is paid to unraveling the contribution of the adsorption liquid-crystal molecules onto the aligning surface to the anchoring properties of PVCNF and measuring the drift of the easy orientation axis over the PVCN-F surface. It is shown that a relatively weak azimuthal anchoring energy (Waz ∼ 10−7 − 10−5 J/m2) leads to strong drift of the easy axis in the azimuthal plane that was observed in a moderate (∼0.1–0.3 T) magnetic field. A much stronger polar anchoring (Wzen ∼ 10−4 J/m2) allowed us to observe the essential gliding of the easy axis in the zenithal plane in a rather strong electric field (∼5 V/μm)