Droplets of Colloidal Ferromagnetic Nanoplates

Disk-shaped, ferromagnetic barium hexaferrite nanoplates in isotropic solvents exhibit a first-order transition from a paramagnetic isotropic (I) phase to a ferromagnetic nematic (N_F) phase for sufficiently high volume fractions [Nat Comm, 7: 10394, 2016]. In samples prepared at a volume fraction within the I – N_F coexistence range, it is possible to create metastable dispersions of oblate spheroidal N_F droplets in an isotropic background. Magnetostatic interactions strongly favor tangential alignment of the N_F magnetization field and the corresponding nematic director field along lines of latitude. However, such a circumferential director configuration requires the formation of a +1 disclination line along the symmetry axis of the droplet, with a correspondingly large elastic free energy cost. The formation of a disclination line is avoided through the escape of the polar director field into the third dimension, giving rise to spontaneously chiral droplets whose handedness is determined by the sign of twist of the magnetization. Coalescence of droplets leads to highly complex morphologies that depend on the polar and chiral structure of the constituent droplets.