Ferromagnetism in Suspensions of Magnetic Platelets in Liquid Crystal

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To explain the mechanism that prevents aggregation and produces spontaneous magnetization, the following are noted. The structure of the nematic director field along a single disk-like platelet with strong perpendicular anchoring is of quadrupole symmetry with a Saturn ring around the platelet (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 239). The exact structure of the nematic field depends on the size of the particle and the strength of the anchoring. Owing to the anchoring, the nanoplatelets in our system orient so that their plane is perpendicular to the nematic director no and magnetic moments are parallel to no (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 239). The platelets are also different in their diameters and the polydispersity may also play a crucial role in the stability of the suspension (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 240). In terms of the coupling of the director and magnetization, the direct magnetic coupling of the nematic liquid crystals with the magnetic field is very weak. However, the direction of the magnetization is connected with the orientation of the platelets, which is coupled to the orientation of the director n (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 240).

In conclusion, researchers on ferromagnetism in suspensions of magnetic platelets in liquid crystal demonstrated the existence of a ferromagnetic nematic colloidal suspension through experimentations based on past theoretical predictions. The system has the standard properties of a ferromagnet: monodomain samples can be prepared by cooling in an external field, and the system shows hysteretic behaviour, magnetization reversal in a flipped external field, and domain walls and domain-wall motion (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 240). The quadrupolar nature of the nematic elasticity-mediated interaction prevents particle aggregation and causes the magnetic dipolar interaction to favour ferromagnetic ordering (Merteli, Lisjak, Drofenik & Čopič, 2013, p. 240). Further experimentation of the particles in other liquid crystal phases with chiral or smetic order would create opportunities in a new field of research.

Reference

Mertelj, A., Lisjak, D., Drofenik, M., & Čopič, M. (2013). Ferromagnetism in suspensions of magnetic platelets in liquid crystal. Nature, 504(7479), 237-240. doi:10.1038/nature12863