Isotropic to nematic transition in alcohol ferrofluids of barium hexaferrite nanoplatelets

Alcohol ferrofluids made of ferrimagnetic barium hexaferrite (BHF) nanoplatelets (NPLs) form a unique example of a dipolar fluid - a liquid magnet. Its formation is induced at a high enough concentration of the NPLs. The key interactions between the NPLs are long-ranged dipolar magnetic and screened anisotropic electrostatic. Herein, we report the results on tuning the isotropic-nematic phase transition (i.e., the NPLs threshold concentration) in 1-butanol ferrofluids of BHF NPLs by affecting the interactions in the ferrofluids. The threshold concentration was determined by polarizing optical microscopy (POM) combined with a system for magnetic field manipulation and was in the range between 4.6 and 6.6 % (v/v) depending on the ferrofluid. We observed that the threshold concentration decreased for 0.6 % (v/v) with a larger mean diameter of the NPLs, up to 0.6 % (v/v) with increased ionic strength of the ferrofluid, and for similar to 2 % (v/v) with higher saturation magnetization of the NPLs. We showed that by tuning the parameters affecting the ferrofluid's interplatelet interactions, we can alter the threshold concentration for the liquid magnet formation. The results elucidate the importance of a delicate balance between the repulsive screened electrostatic and attractive dipolar magnetic interactions for the liquid magnet formation. (C) 2021 Elsevier B.V. All rights reserved.