期刊
JOURNAL OF MOLECULAR LIQUIDS
卷 267, 期 -, 页码 390-397出版社
ELSEVIER SCIENCE BV
DOI: 10.1016/j.molliq.2017.10.106
关键词
Ferronematic; Liquid crystal; Freedericksz transition; Spindle magnetic particles; Capacitance measurements
资金
- Slovak Academy of Sciences in the framework of project VEGA [2/0016/17]
- Slovak Research and Development Agency [APVV-015-0453]
- Ministry of Education Agency for Structural Funds of EU [26220120033]
- National Academy of Sciences of Ukraine [0116U008318]
In this work we studied new composite ferronematic system based on the liquid crystal 4-n-hexyl-4'-cyanobiphenyl (6CB) doped with hematite spindle-like magnetic particles. We experimentally showed that magnetic moments of the elongated spindle particles are oriented perpendicularly to their main axes. It is a fundamental difference of our systems from ferronematics, which were synthesized before by many authors. For a research of magnetic and orientational properties of new composite materials, the ferronematic samples were prepared with different length of the particles and with the volume concentrations of solid impurity phi(1)=10(-3), phi(2)=10(-4) and phi(3)=10(-5). The structural Freedericksz transition in ferronematic samples were observed by capacitance measurements in a capacitor made of ITO-coated glass electrodes. Experimental results showed a decrease of the critical field in the ferronematic samples in comparison with the pure 6CB, and an increase of the threshold shift with a growth of both the volume fraction of solid impurity and the length of particles. The response of capacitance to low external magnetic field - far below the magnetic Freedericksz transition threshold of prepared samples was also observed experimentally. For a theoretical description of this magnetically induced behavior of composite systems, the continual model of ferronematics with bistable coupling between colloidal particles and the nematic matrix was used. This theory permits the equilibrium tilted orientation of the particles with respect to the nematic director. We showed that it creates a so-called pretilt angle effect which explains the behavior of the ferronematic systems in a low field regime as well as decrease of the threshold field and the dependence of this field on the volume fraction of solid impurity and the length of the particles. (C) 2017 Elsevier B.V. All rights reserved.
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