Spontaneous Ferromagnetic Ordering of Nanoplatelets in Isotropic Solvent*

Room-temperature ferromagnetic fluids were first experimentally demonstrated by Mertelj, \textit{et al} (\textit{Nature}, \textbf{504}: 237--241, 2013), by suspending surfactant wrapped coated barium hexaferrite (BHXF) nanoplates in the liquid crystal 5CB. We have studied the liquid crystal phase behavior of BHXF magnetic platelets suspended in isotropic solvent (1-butanol) at high volume fraction, where simulations predict an N-I transition for monodisperse hard plates. In these suspensions, the anisotropic particles can be aligned by magnetic fields as weak as 2 gauss, leading to a state with substantial birefringence and dichroism. When the volume fraction of the magnetic platelets is higher than 28{\%}, we observe a phase co-existence, with an isotropic state at the top of a capillary and a birefringence phase at the bottom. In the lower phase, domains are found to have different magneto-optical response from each other and the response is dependent on the \textit{sign} of the magnetic field, showing broken time-reversal symmetry and ferromagnetism. Spike structures are observed at the interface between the isotropic and ferromagnetic states.