ACTA ACUSTICA UNITED WITH ACUSTICA Vol. 104 (2018) 48 – 53 DOI 10.3813/AAA.919144 SAW Investigation of Structural Changes in Liquid Crystals Doped with Magnetic Particles P. Bury 1) , M. Veveričík 1) , J. Kúdelčík 1) , P. Kopčanský 2) , M. Timko 2) , V. Závišová 2) 1) Department of Physics, Faculty of Electrical Engineering, University of Žilina, Univerzitná 1, 010 26 Žilina, Slovakia, bury@fel.uniza.sk 2) Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 040 01 Košice, Slovakia Summary The surface acoustic waves (SAW) were used to study the effect of magnetic particles on liquid crystals (6CHBT) structural changes in electric and weak magnetic fields using measurements of the attenuation of SAW propagat- ing along ferronematic liquid crystals. The magnetic nanoparticles in low volume concentration (Φ = 1 · 10 −4 , 1 · 10 −5 ,5 · 10 −5 ) were added to the liquid crystal during its isotropic phase. In contrast to undoped 6CHTB the distinctive SAW attenuation responses induced by both electric and weak magnetic fields in studied ferronematic liquid crystals have been observed suggesting both structural changes and the orientational coupling between both electric and magnetic moments of magnetic particles and the director of the liquid crystal. PACS no. 43.35.Bf, 61.30.Gd, 64.70.Md, 75.50.Mm 1. Introduction Liquid crystalline phases occur as additional, thermody- namically stable states of matter between the liquid state and the crystal state in some materials. They can be characterized by a long-range orientational order of the molecules and, as a consequence, by an anisotropy in their physical properties. Liquid crystals (LCs) can be oriented under electric or magnetic fields due to the anisotropy of dielectric permittivity or diamagnetic susceptibility [1]. In addition, the system of nanoparticles dispersed in LCs have attracted attention for development of novel mate- rials based on the controlled assembly of particles. The properties of such materials, in most cases, significantly differ from those of the host materials. Depending on ex- perimental parameters such as nanoparticle functionaliza- tion and concentration, size and core material, cell surface treatment, as well as applied electric or magnetic field di- verse effects arise in different nematic LC hosts [2, 3, 4, 5]. These effects are due to the anchoring of the liquid crystal molecules at the surface of the particles. So that the doping process of LCs can prepare materials that can have then very interesting properties which are absent in original LC substances. LC suspensions containing nanoparticles have registered additional great attention for many practi- cal applications such as nanosensors, LC display industry, optical processing, biosensors, photonics and magneto- optics [6]. It is already known that ferroelectric particles have the strong effect on the optical and dielectric properties of the Received 27 February 2017, accepted 5 December 2017. nematic matrix and leads in this way to the decrease of driving voltages, increase of the reflection contrast and the steepness of the transition [7]. Next of such materials could be also the suspension of magnetic particles in ne- matic LCs. Stable colloidal suspensions of monodomain ferromagnetic particles in nematic LCs in small concen- trations, called ferronematics, newly attract noticeable in- terest because their response to an external magnetic field oversteps substantially that of pure nematics [8, 9]. As it was already mentioned LCs can be orientated un- der magnetic or electric fields due to their anisotropic properties. However, because of the small value of the anisotropy of the diamagnetic susceptibility, the magnetic fields necessary to align liquid crystals have to reach rather large values (B> 1 T). In an effort to enhance the mag- netic susceptibility of liquid crystals, the idea of doping them with fine magnetic particles was introduced predict- ing a rigid anchoring & m // & n, where the unit vector & n (direc- tor) and the unit vector & m denoting the orientation of the magnetic moment of magnetic particles, would result in the ferromagnetic behaviour of the nematic matrix [10]. The subsequent experiments confirmed the existence of considerable orientational and concentrational effects in liquid crystals doped with magnetic particles as well as the fact that the essential feature of these systems is a strong orientational coupling between the magnetic particles and the LC matrix [11, 12, 13, 14]. Acoustic (ultrasonic) methods are one of the useful tool for the characterization of some particular properties of LCs. In spite of the fact that there are problems why acous- tic techniques have relatively limited use, several success- ful acoustic experiments were realized. Acoustic experi- ments are based on the utilization of tree types of acous- 48 © S. Hirzel Verlag · EAA