Ferronematics are suspensions of magnetic nanoparticles (e.g. magnetite, carbon nanotubes functionalized by ferromagnetic particles) in nematic liquid crystals prepared in an effort to increase the sensitivity of liquid crystals to magnetic field. The electro- and magneto-optical behaviour of these substances is studied in co-operation with the Institute of Experimental Physics, Košice, Slovakia.

 

Participating researchers: Nándor Éber, Katalin Fodor-Csorba, Péter Salamon, Tibor Tóth-Katona and Anikó Vajda

 

Recent results

 

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.

 

N. Tomašovičová, M. Timko, N. Éber, T. Tóth-Katona, K. Fodor-Csorba, A. Vajda, V. Gdovinová, X. Chaud, and P. Kopčanský: Magnetically Induced Shift of the Isotropic-Nematic Phase Transition Temperature in a Mixture of Bent-Core and Calamitic Liquid Crystals Doped with Magnetic Particles, Liquid Crystals, 42, 959-963 (2015) [[pdf]

 

In the work phase transitions in bent-core liquid crystals were studied using differential scanning calorimetry. For the binary mixture of bent-core molecules with 50 wt% of rod-shaped compound, the nematic to smectic transition occured below 40oC and the crystallization temperature shifted to sub-ambient temperatures. The influence of doping of the bent-core liquid crystals with magnetic nanoparticles on the kinetics of observed phase transitions was studied. The phase transition temperatures were shifted depending on the nanoparticle type and changed with varying cooling rate for all studied liquid crystal samples.

 

 

A. Juríková, K. Csach, J. Miškuf, N. Tomašovičová, Z. Mitróová, V. Závišová, M. Koneracká, P. Kopčanský, M. Timko, N. Éber, K. Fodor-Csorba, A. Vajda: Thermal Stability of Bent-Core Liquid Crystals Doped with Magnetic Nanoparticles, Acta Phys. Polon. A 127(2), 638-640 (2015) [pdf]

 

Liquid crystals, due to their large dielectric anisotropy, respond very sensitively to application of an external electric field, whereas they are only weakly sensitive to the magnetic field. A possible way of improving that sensitivity is doping liquid crystals with magnetic nanoparticles. As a result, stable colloidal suspensions of liquid crystals with relatively low concentrations of magnetic nanoparticles (called ferronematics, ferrocholesterics, ferrosmectics, etc.) can be produced. We illustrate some examples of the influence of the magnetic field, as well as of a superposition of magnetic and electric fields on the structural transitions (e.g. on the Fréedericksz transition) in ferronematics based on the calamitic liquid crystal 4-(trans-4'-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT). It is shown that the samples respond to the applied magnetic field of low strength. The effects of the magnetic particles and magnetic field on the nematic to isotropic phase transition temperature are discussed as well.

 

P. Kopčanský, N. Tomašovičová, V. Gdovinová, M. Timko, N. Éber, T. Tóth-Katona, J. Jadzyn, J. Honkonen and X. Chaud: How to Enhance Sensitivity of Liquid Crystals to External Magnetic Field?, Acta Phys. Polon. A 127(2), 157-162 (2015) [pdf]

 

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.

V. Gdovinová, N. Tomašovičová, N. Éber, T. Tóth-Katona, V. Závišová, M. Timko and P. Kopčanský: Influence of the anisometry of magnetic particles on the isotropic–nematic phase transition, Liquid Crystals 41(12), 1773-1777 (2014) [pdf]

 

The contribution is an overview of the observations regarding the structural transitions in ferronematics based on thermotropic nematics doped with magnetic nanoparticles of different shape, and the magnetic field induced shift of the isotropic to nematic phase transition temperature. Due to presence of magnetic particles an increase of the isotropic-nematic phase transition temperature was observed as a function of applied magnetic field. The response of ferronematics to very low magnetic fields is also presented which is important for the construction of various magneto-optical devices.

P. Kopčanský, N. Tomašovičová, M. Timko, V. Gdovinová, T. Tóthkatona, N. Éber, C.-K. Hu, S. Hayryan, X. Chaud: F.1.17. - Increase of the sensitivity of liquid crystals to magnetic field due to doping with magnetic nanoparticles. In. Proceedings of the 9th International Conference on Fundamental and applied MHD, Thermo acoustic and Space technologies,Riga, June 16-20, 2014, pp. 337-341 [pdf]

 

We investigated experimentally the magneto-optical and dielectric properties of magnetic-nanoparticle-doped nematic liquid crystals (ferronematics). Our studies focus on the effect of the very small orienting bias magnetic field Bbias, and that of the nematic director pretilt at the boundary surfaces in our systems sensitive to low magnetic fields. Based on the results we assert that Bbias is not necessarily required for a detectable response to low magnetic fields, and that the initial pretilt, as well as the aggregation of the nanoparticles play an important (though not yet explored enough) role.

T. Tóth-Katona, P. Salamon, N. Éber, N. Tomašoviċová, Z. Mitróová, and P. Kopċanský: High concentration ferronematics in low magnetic fields, J. Magn. Magn. Mat., 372, 117-121 (2014) [pdf]

 

 

 

N. Tomašoviċová, M. Timko, V. Závišová, M. Koneracká, Z. Mitróová, A. Hashim, M. Rajňák, N. Éber, T. Tóth-Katona, K. Fodor-Csorba, A. Vajda, P. Kopċanský: Ferronematics: Combinations of Liquid Crystals with Magnetic Fluids, In Eds. M. Reiffers, P. Rybár, Š. Molokáč, Transformation of Knowledge and Technologies to the Praxis Obtained by Research and Development in the Earth Resources Area. Slovak Physical Society, 2013, pp. 65-73 [pdf]

 

Magnetic Fréedericksz transition was studied in ferronematics based on the nematic liquid crystal 4-(trans-4’-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT). 6CHBT was doped with rod-like magnetic particles of different size and volume concentration. The volume concentrations of magnetic particles in the prepared ferronematics were φ1=104 and φ2=103. Structural changes were observed by capacitance measurements, which evidenced of a significant influence of the concentration, shape anisotropy and/or the size of the magnetic particles on the magnetic response of these ferronematics.

 

 

P. Kopċanský, N. Tomašoviċová, T. Tóth-Katona, N. Éber, M. Timko, V. Závišová, J. Majorošová, M. Rajňak, J. Jadzyn, X. Chaud: Increasing the magnetic sensitivity of liquid crystals by rod-like magnetic nanoparticles, Magnetohydrodynamics 49(3-4), 586-591 (2013) [pdf]

 

The response in capacitance to low external magnetic fields (up to 0.1 T) of suspensions of spherical magnetic nanoparticles, single-wall carbon nanotubes (SWCNT), SWCNT functionalized with carboxyl group (SWCNTCOOH), and SWCNT functionalized with Fe3O4 nanoparticles in a nematic liquid crystal has been studied experimentally. The volume concentration of nanoparticles was φ1 = 104 and φ2 = 103. Independent of the type and the volume concentration of the nanoparticles, a linear response to low magnetic fields (far below the magnetic Fréederiksz transition threshold) has been observed, which is not present in the undoped nematic.

 

N. Tomašovičová, M. Timko, Z. Mitróová, M. Koneracká, M. Rajňak, N. Éber, T. Tóth-Katona, X. Chaud, J. Jadzyn, and P. Kopčanský: Capacitance changes in ferronematic liquid crystals induced by low magnetic fields. Phys. Rev. E 87, 014501/1-4 (2013) [pdf]

 

In the present chapter we introduce the basic field-induced reorientational effect of liquid crystals (the Fréedericksz transition) which serves also as a basis for most experiments on ferronematics. We will sketch the mechanism how magnetic particles may affect the magnetic response. We will describe the preparation of magnetic particles and of the ferronematics made of them.

An overview is given of the experimental results obtained on the behaviour of ferronematics depending on the host liquid crystal, on the type, shape, size and concentration of the magnetic particles.

A special emphasis is given to the magnetic field induced shift of the isotropic-nematic phase transition temperature, both in ferronematics and in polymer dispersed liquid crystals doped with magnetic particles.

N. Tomašovičová, P. Kopčanský, N. Éber: Chapter 11, Magnetically Active Anisotropic Fluids Based on Liquid Crystals, In Ed. H.G. Lemu, Anisotropy Research: New Developments, Nova Science Publishers, 2012, pp. 253-282

 

The thermotropic liquid crystal 4-(trans-4-n-hexylcyclohexyl)-isothiocyanato-benzene (6CHBT) has been doped with magnetic  nanoparticles of different shapes. An increase of the isotropic-to-nematic phase transition temperature was observed in the liquid crystal doped with rodlike particles at high magnetic fields. To our knowledge, this is the first observation of a magnetic field induced shift of the phase transition in ferronematics based on a calamitic liquid crystal.

P. Kopčanský, N. Tomašovičová, M. Koneracká, V. Závišová, M. Timko, M. Hnatič, N. Éber, T. Tóth-Katona, J. Jadzyn, J. Honkonen, E. Beaugnon, X. Chaud, Magnetic-field induced isotropic to nematic phase transition in ferronematics. IEEE T. Magn. 47, 4409-4412 (2011) [pdf]

 

Stable colloidal suspensions of single-walled carbon nanotubes (SWCNT) and SWCNT functionalized with carboxyl groups as well as with magnetite nanoparticles (SWCNT/Fe3O4) in the nematic liquid crystal 4-(trans-4’-n-hexylcyclohexyl)isothiocyanatobenzene (6CHBT) have been prepared. A significant increase of the saturation magnetization has been measured for magnetite labeled SWCNT. The density of anchoring energy at the nematic–magnetic particle boundary has been determined and it is found higher in liquid crystal doped with SWCNT/Fe3O4 than that in the liquid crystal doped with net SWCNT.

Z.Mitróová, N.Tomašovičová, M.Timko, M. Koneracká, J. Kováč, J. Jadzyn, I. Vávra, N.Éber, T. Tóth-Katona, P. Kopčanský: The sensitivity of liquid crystal doped with functionalized carbon nanotubes to external magnetic fields. New J. Chem. 35, 1260-1264 (2011) [pdf]

 

A nematic liquid crystal of low negative anisotropy of the diamagnetic susceptibility has been doped with spherical Fe3O4 nanoparticles. The surface density of the anchoring energy of liquid crystal molecules on the surface of the particles has been estimated using the Burylov-Raikher’s theory. The obtained estimates indicate a soft anchoring of the liquid crystal on the magnetic particles with a preferred parallel orientation of the magnetic moment of magnetic particles and the director.

P. Kopčansky, N. Tomašovičová, M. Koneracká, M. Timko, V. Závišová, N. Éber, K. Fodor-Csorba, T. Tóth-Katona, A. Vajda, J. Jadzyn, E. Beaugnonand, X. Chaud, The structural instabilities in ferronematic based on liquid crystal with negative diamagnetic susceptibility anisotropy, J. Magn. Magn. Mater. 322, 3696-3700 (2010) [pdf]

 

The influence of Fe3O4 nanoparticles on dielectric properties of planar oriented liquid crystal were studied at the frequency range10-1-106 Hz and in temperature interval of 292 - 345 K. It was shown that the dielectric spectrum of doped liquid crystal can be divided into two areas. Parameters obtained from the dielectric spectrum for frequencies lower than 10 Hz characterize near electrode processes, and parameters obtained for frequencies more than 10 Hz characterize the bulk properties of the sample. The relaxation time and near-electrode area thickness have been estimated. It was experimentally shown that magnetic nanoparticles change the near-electrode area parameters.

 

P. Kopcanský, A. Koval’chuk, O. Gornitska, V. Vovk, T. Koval’chuk, N. Tomasovicová, M. Koneracká, M. Timko, V. Závisová, J. Jadzyn, N. Éber, I. Studenyak: Dielectric spectroscopy of liquid crystal doped with Fe3O4 nanoparticles. Phys. Procedia 9, 36-40 (2010) [pdf]

 

 

The 4-(trans-4’-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) nematic liquid crystal was doped with single walled carbon nanotubes (SWNTs) and with magnetite labeled single walled carbon nanotubes (SWNTs/Fe3O4). Prepared samples were characterized by infrared spectroscopy, transmission electron microscopy, optical microscopy and by magnetic measurements. Electric and magnetic Freedericksz transitions were measured for the pure liquid crystal, and for the liquid crystal doped with SWNTs, as well as with SWNTs/Fe3O4 (volume concentration 10-4). The density of anchoring energy at the nematic - magnetic particle boundary was determined. The value of the anchoring energy density for SWNTs/Fe3O4 is lower than for SWNTs. The critical voltage of the Freedericksz transition depends significantly on the bias magnetic field.

Z. Mitróová, M. Koneracká, N. Tomašovičová, M. Timko, J. Jadzyn, I. Vávra, N. Éber, K. Fodor-Csorba, T. Tóth-Katona, A. Vajda, P.Kopčanský: Structural transitions in nematic liquid crystals doped with magnetite functionalized single walled carbon nanotubes.1Phys. Procedia 9, 41-44 (2010) [pdf]

 

The nematic liquid crystal 6CHBT has been doped with spherical magnetite nanoparticles of different size (4.2 nm, 11.4 nm, 31.4 nm and 112.7 nm). It has been shown that doping with magnetic particles smaller than the Shliomis size increases the threshold of the magnetic Freédericksz transition, in contrast to the case of doping with larger magnetic particles that decreases the threshold. If the size of the magnetic particles is comparable with the Shliomis size, both Néel and Brownian rotation of the magnetic moments are present, however, the Brownian rotation is dominant.

P.Kopčanský, N.Tomašovičová, M.Koneracká, V.Závišová, M.Timko, L.Tomčo, N.Éber, K.Fodor-Csorba, T.Tóth-Katona, A.Vajda, J.Jadzyn, E.Beaugnon, X.Chaud: Néel and Brownian rotations in ferronematics. Phys. Procedia 9, 82-86 (2010) [pdf]

 

It has been shown that the inclusion of dodecanethiol functionalized gold nanoparticles (with diameter 3–5 nm) into the nematics liquid crystal (6CHBT) enhances its sensitivity to the external magnetic field, i.e. causes a stronger increase of the electric Freédericksz threshold voltage with magnetic field.

P.Kopčanský, N.Tomašovičová, M.Koneracká, M.Timko, V.Závišová, N.Éber, K.Fodor-Csorba, T.Tóth-katona, A.Vajda, J.Jadzyn, E.Beaugnon, X.Chaud: Magnetic Freedericksz transition in liquid crystal doped with gold nanoparticles. Acta Phys. Pol. A 118, 988-989 (2010) [pdf]

 

Composite systems of a nematic liquid crystal with multi-walled carbon nanotubes (MWNTs) and with magnetically labeled multi-walled carbon nanotubes (MWNTs/Fe3O4) have been prepared. It has been shown that doping with MWNTs and with MWNTs/Fe3O4 shifts the threshold voltage of the electric Freédericksz transition to lower values.

Z. Mitróova, N. Tomašovičová, M. Koneracká, V. Závišová, M. Timko, J. Kovač, P. Kopčansky, L. Tomčo, N. Éber, K. Fodor-Csorba, T. Tóth Katona, A. Vajda, J. Jadzyn, E. Beaugnon, X. Chaud: The structural transitions in a nematics liquid crystal doped with magnetically labeled carbon nanotubes. Magnetohydrodynamics 45, 353-360 (2009) [pdf]

 

A nematic liquid crystal has been doped with differently shaped magnetite nanoparticles (spherical, rod-like, chain-like). Experimental results indicate soft anchoring in the case of spherical magnetic particles and rigid anchoring in the case of rod-like and chain-like magnetic particles, with parallel initial orientation between the magnetic moments of the magnetic particles and the director.

  

P. Kopčanský, N. Tomašovičová, M. Koneracká, V. Závišová, M. Timko, A. Džarová, A. Šprincová, N. Éber, K. Fodor-Csorba, T. Tóth-Katona, A. Vajda, J. Jadzyn: Structural changes in the 6CHBT liquid crystal doped with spherical, rodlike, and chainlike magnetic particles. Phys. Rev. E 78, 011702/1-5 (2008) [pdf]

 

Ferronematic droplets have been observed in mixtures of the nematic 6CHBT with phenyl isocyanate and fine magnetic particles. The phase diagram of the system shows that the transition from the isotropic to the nematic phase occurs via a two-phase droplet state. The type of anchoring of the nematic molecules on the surface of the magnetic particles in the droplets has been estimated based on magneto-dielectric measurements.

N. Tomašovišová, P. Kopčanský, M. Koneracká, L. Tomčo, V. Závišová, M. Timko, N. Éber, K. Fodor-Csorba, T. Tóth-Katona, A. Vajda and J. Jadzyn: The structural transitions in 6CHBT-based ferronematic droplets. J. Phys.: Condens. Matter 20, 204123/1-5 (2008) [pdf]

 

HAB (4,4’-di-n-heptylazoxybenzene) based ferronematics have been prepared by addition of spherical magnetic nanoparticles and investigated by DSC. Magneto-dielectric measurements indicate a soft anchoring with the magnetic moment of magnetic particles parallel with the director.

N. Tomašovičová, M. Koneracká, P. Kopčansky, M. Timko, V. Závišová, A. Vajda, K. Fodor-Csorba, N. Éber, T. Tóth-Katona, J. Jadzyn: The Anchoring Energy of Liquid Crystal Molecules to Magnetic Particles in HAB-Based Ferronematics. Acta Physica Polonica A, 113, 591-594 (2008) Acta Phys. Pol. A 113, 591-594 (2008) [pdf]