Tricritical Phenomena and Cascades of Temperature Phase Transitions in a Ferromagnetic Liquid Crystal Suspension

Timur Khalilov,Danil Petrov,Dmitriy Makarov

doi:10.3390/cryst11060639

Abstract

We consider temperature-driven phase transitions occurring in a liquid crystal suspension of ferromagnetic particles within the Landau–de Gennes theory. The temperature dependences of the order parameters in the uniaxial model with a vector order parameter for the magnetic subsystem are obtained. The dimensionless expression for the free energy density of the suspension has been used for the study of the phase behavior general regularities of the system. Phase state diagrams of the suspension and temperature dependences of the order parameters of the liquid crystal and the ensemble of magnetic particles for different values of the phenomenological expansion coefficients are constructed. It is shown that the considered model admits the existence of a cascade of temperature phase transitions: isotropic phase–superparamagnetic nematic phase–ferromagnetic nematic phase. We have shown that in the mesomorphic state of the liquid crystal, the spontaneous magnetization can appear in a continuous way or by a jump with decreasing temperature, which corresponds to the tricritical behavior. The values of temperature and expansion coefficients corresponding to the tricritical and triple points are numerically found.

Highlights

Since the appearance of the terms “ferronematics” and “ferrocholesterics” [1], introduced to denote hypothetical liquid crystal (LC) systems that possess a liquid ferromagnetic phase at room temperature, various types of LC-based magnetic nanoparticle suspensions have received considerable research attention [2,3,4,5,6,7,8]
Transitions between the ordinary isotropic magnetic fluid (I), the superparamagnetic nematic (SN) phase, and the ferromagnetic nematic (FN) phase were predicted within the Landau–de Gennes theory generalized to the LC-suspension case
The behavior of the order parameters in the suspension at | a2T | < | a2 | is shown in Within the framework of the phenomenological Landau–de Gennes theory we have studied the temperature-driven phase transitions in the suspension of ferromagnetic particles based on nematic LC

Summary

Introduction

Since the appearance of the terms “ferronematics” and “ferrocholesterics” [1], introduced to denote hypothetical liquid crystal (LC) systems that possess a liquid ferromagnetic phase at room temperature, various types of LC-based magnetic nanoparticle suspensions have received considerable research attention [2,3,4,5,6,7,8]. A theory to describe the appearance of spontaneous magnetization of a suspension induced by the temperature or impurity concentration changes was proposed before the experimental realization of stable ferromagnetic LC suspensions in [15,16] In these studies, transitions between the ordinary isotropic magnetic fluid (I), the superparamagnetic nematic (SN) phase, and the ferromagnetic nematic (FN) phase were predicted within the Landau–de Gennes theory generalized to the LC-suspension case. In [17], the possibility of transitions between the phase states I, SN, and FN presented above is shown, and the expression for the free energy density obtained using the mean field method contains contributions found in the phenomenological approach [15] Another statistical theory was proposed in [18], where, together with the magnetic order parameter M, an additional tensor parameter by analogy with LC was used to describe the orientational structure of the impurity particle ensemble. We have managed to show that in the ferromagnetic liquid crystal suspension the temperature phase transition cascade I–SN–FN includes both first- and second-order transitions, i.e., there is a tricritical behavior, for which the appropriate conditions have been obtained

Theory

Phase Diagrams

Temperature Dependences of Order Parameters

Conclusions