Ориентационные переходы в магнитокомпенсированных ферронематиках с гомеотропным сцеплением частиц с матрицей

Danil A Petrov,Pavel Skokov

doi:10.17072/1994-3598-2017-1-31-40

Abstract

In the framework of the continuum theory the orientational transitions induced by an external magnetic field in the ferronematic i.e. suspension of submicron particles of ferromagnetic material on the basis of a nematic liquid crystal were studied. The ferronematic was assumed to be compensated i.e., having equal fractions of ferroparticles with oppositely directed magnetic moments. In the absence of magnetic field that suspension is not magnetized and it is a liquid-crystalline analog of an antiferromagnetic. We took into account the following contributions of the free energy to the bulk density: the potential of elastic deformation of the director field, the interactions of diamagnetic matrix and magnetic moments of ferroparticles with the magnetic field, as well as contribution of the entropy of mixing of the ideal solution of suspension particles. Soft homeotropic coupling of ferroparticles with the molecules of the liquid crystal and soft planar coupling of the director with boundaries of the compensated ferronematic layer were considered. It was shown that in considered geometry the applying magnetic field leads to a redistribution of the magnetic impurity in the layer of ferronematic without appearance of orientational structure distortions. The initial antiferromagnetic ordering of the magnetic particles is replaced by a ferrimagnetic one with increasing of the fraction of particles oriented in the magnetic field direction. With increasing of magnetic field the uniform ferrimagnetic state becomes unstable and Freedericksz transition appears. The subsequent increasing of the magnetic field leads to transition to the uniform angular phase, in which the director and magnetization are oriented in the direction of the magnetic field and distortions of orientational structure have been disappeared. Expressions for the threshold transition fields between orientational phases of ferronematic as functions of material parameters were found analytically.Received 16.01.2017; accepted 10.05.2017

Highlights

В рамках континуальной теории изучены индуцированные внешним магнитным полем ориентационные переходы в ферронематике – суспензии субмикронных частиц ферромагнетика, приготовленной на основе нематического жидкого кристалла
In the framework of the continuum theory the orientational transitions induced by an external magnetic field in the ferronematic i.e. suspension of submicron particles of ferromagnetic material on the basis of a nematic liquid crystal were studied
It was shown that in considered geometry the applying magnetic field leads to a redistribution of the magnetic impurity in the layer of ferronematic without appearance of orientational structure distortions

Summary

Hq Hd

Здесь z – безразмерная координата (далее знак тильда будем опускать), g+ и g− – приведенные объемные доли магнитных частиц с магнитными моментами, направленными параллельно и антипараллельно оси Z соответственно. Параметр сегрегации κ феррочастиц представляет собой меру интенсивности концентрационного расслоения магнитной примеси [3]. При κ ≫ 1 распределение феррочастиц в слое ФН можно считать однородным, а при κ ≲ 1 концентрационное расслоение дисперсной фазы становится существенным. Параметр b определяет относительный вклад квадрупольного и дипольного механизмов влияния магнитного поля на ориентационную структуру ФН и представляет собой отношение двух характерных магнитных полей b = Hq⁄Hd. Таким образом при b > 1 появление деформации ориентационной структуры обусловлено преимущественно дипольным механизмом влияния магнитного поля, а при b < 1 – квадрупольным. Воспользуемся оценками безразмерных параметров, приведенных в работах [20, 21, 24]: κ ≈ 0.1 ÷ 1, σ ≈ 10−2 ÷ 1, b ≈ 10, ω ≈ 1 ÷ 10, k ≈ 1

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