Phase Separation of Ferromagnetic Nematic and Isotropic Colloidal Suspensions*

Suspensions of disk-shaped ferromagnetic barium hexaferrite nanoplates in isotropic solvent spontaneously form a ferromagnetic nematic phase for nanoplate concentrations higher than the Onsager isotropic–nematic phase transition point for hard disks [Nat Comm, 7: 10394, 2016]. At an overall concentration below this value and within the coexistence region, such suspensions phase separate into ferromagnetic nematic and isotropic domains. Under these conditions, the suspension can be driven into a uniform state by mechanical or magnetic stirring, and undergoes a dynamic process of phase separation immediately after the removal of stirring forces, which we have investigated by polarized optical microscopy. Starting from a diffuse, birefringent mesh, nematic regions rapidly coalesce into a three-dimensional network of fine, short, straight filaments. If left undisturbed, these filaments coarsen, combining with each other until the isotropic phase between them is eliminated. In the presence of a weak external magnetic field, filaments along the field direction tend to grow most quickly. We also observe ferromagnetic nematic domains with exotic topological structures, particularly under sudden changes in external field.