Ferromagnetism in Suspensions of Magnetic Platelets in Liquid Crystal

A Summary of the Article “Ferromagnetism in suspensions of magnetic platelets in liquid crystal”

In less than half a century ago, researchers Brochard and de Gennes suggested that a true fluid ferromagnetic phase could appear at room temperature in a colloid of magnetic nanoparticles in nematic liquid crystal (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 237). First of all, ferrofluids are stable colloidal dispersons of monodomain magnetic nanoparticles in isotropic liquids (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 237). In ferrofluids, the magnetic interaction between the nanopraticles is tuned so that in the absence of an external magnetic field, the particles remain dispersed owing to entropic forces. When an external magnetic field is applied, the chaining of the particles occurs along the external field, thus increasing the viscosity of a ferrofluid (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 237). On the other hand, ferrofluid only becomes anisotropic in an external field where the symmetry of the nematic phase of a liquid crystal is uniaxial. In the nematic phase, nematic liquid crystals are composed of rod-shaped molecules that orient themselves along a common direction, called director, which is usually denoted by a unit vector n (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 237). Particles that are suspended in a liquid crystal and have a given orientation of n at the surface cause a deformation of the orientation in the surrounding liquid crystal. Defect lines or points may occur around the particles, making the liquid-crystal-mediated interaction have a dipolar nature or a quadrupolar nature and either be repulsive or attractive (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 237).

In the isotropic phase of a liquid crystal, a suspension of ferromagnetic nanoparticles behaves in a similar way to any other ferrofluid. However, if the shape of the particles is anisotropic, they adopt a certain orientation with respect to the nematic order (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 237). For ferromagnetic ordering, a strong magnetic interaction between the particles is necessary and also a nematic-mediated repulsive interaction is needed to prevent aggregation. A single elongated particle, prepared so that the orientation of a nematic director at its surface is parallel to the suface and to the particle’s axis, orients itself with its axis parallel to no if placed in a nematic of average director orientation no (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 237). The deformation of the nematic field is small and the resulting nematic interaction is too weak to prevent aggregation. In later experiments, aggregation was avoided by using only very diluted stable suspensions of elongated ferromagnetic particles in nematic liquid crystals, where the magnetic interaction between the particles was negligible, and macroscopic magnetization was not observed (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 238). On the other hand, if the elongated particle has an orientation at its surface that is perpendicular to that surface, then it orients itself with its axis perpendicular to no (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 237). The nematic-mediated interaction is weakly attractive and the relative orientation between pairs of particles depends on their relative positions without the stabilizing of ferromagnetic order (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 238).

        By using ferromagnetic platelets that favour a perpendicular orientation of the nematic director at the particle surface, a stable nematic suspension can be produced with macroscopic spontaneous magnetization along the nematic director no (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 238). Preparing a stable isotopic suspension of magnetic platelets is more demanding than with spherical or elongated particles because there is a large polydispersity and the platelets form very stable aggregates due to their shape. In the experiment where planar glass cells filled with liquid crystalline material pentylcyanobophenyl in the isotropic phase is cooled, it is observed that many elongated aggregrates formed at the nematic-isotropic phase boundary (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 238). This formation is avoided by the immediate suppression into the nematic phase. The samples in the nematic phase are stable and no additional aggregation occurs after several months and many exposures to external magnetic fields. When a small external magnetic field is applied, it is observed that the aggregates in slowly cooled cells orient along the field direction while the surrounding nematic liquid crystal remains unchanged and that the response of the domains to an external magnetic field depends on the breaking of the inversion symmetry of the nematic phase (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 238). When the field is applied along the director and then applied in the direction perpendicular to the director, it is observed that spontaneous magnetization is along the director, and that the two types of domain have magnetization in opposite directions. In an experiment involving the application of an external magnetic field upon the direction, observations included a magnetic hysteresis and the disturbance of the nematic director (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 239).