Magneto-optic effects in colloids of ferromagnetic nanoparticles in nematic liquid crystals

Abstract

This thesis describes theoretical and experimental investigation of the optical and magnetic effects in nematic liquid crystals and in ferronematics, namely suspensions of ferromagnetic nanoparticles in nematic liquid crystals. In the experimental part, the effect of the nanoparticles shape and functionality on the suspension stability and magneto-optic properties were studied. Suspensions with magnetic nanospheres showed a linear response to low magnetic fields (< 100 Oersted) and a decrease in the effective Frederiks threshold. Ferronematics with magnetic nanorods coated by 4-n-Octyloxybiphenyl-4-carboxylic acid were more stable and showed a larger decrease in the Frederiks threshold than the spherical magnetic nanoparticles coated by Oleic acid. No ferronematic effects were detected in the weakly magnetic hematite nanorod suspensions. The aim of the theoretical part was to develop a realistic numerical model that could simulate the experimental results of the magnetic-field-induced Frederiks transition in nematic and ferronematic cells. The modelling was carried out in two steps. The first step involved modelling the Frederiks transition of an undoped liquid crystal cell in the presence of an easy axis pretilt and a bias, in-plane, magnetic field. The nematic model predicted that applying a bias field would lead to a shift of the threshold response, which would be sensitive to the bias field direction. This prediction was confirmed as an excellent agreement between the model and experimental data was achieved. In the second stage, a new approach to modelling of ferronematics was proposed, which involved extending previous ferronematic theories to include both the ferromagnetic effect of the particles and the intrinsic magnetic properties of the nematics. There were two variable parameters in the model, which characterise the effective ferroparticle-field interaction, and the ferroparticle-nematic director interaction. These parameters for experimental suspensions were obtained by comparing the model with experimental data. The fitting parameters were used to estimate an effective coupling energy between a nematic host and doped nanoparticles. Up to one order of magnitude higher coupling energy was obtained in the magnetite nanorod suspension as compared to the spherical magnetic nanoparticles. The research presented in this thesis demonstrates a route to prepare highly sensitive and stable ferronematic suspensions, contributes to better understanding of the magneto-optic effects in these suspensions, and highlights their potential for applications as tailor-made optical materials in magnetically driven devices.

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Identifiers

ORCID for David P. Shepherd: orcid.org/0000-0002-4561-8184

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