Magnetic nanoparticle size constraints in ferronematic liquid crystals

Thanks to an induced diamagnetic moment, liquid crystals can be oriented in fields on the order of 1 T. The addition of a few percent of a ferrofluid (FF) whose grain size is 13 nm to a liquid crystal (LC) produces a ferronematic (FN) which can be re-oriented in fields ∼.001 T with the longest axis of the micelles becoming parallel to the field direction (J. Phys. 31 (1971) 691, Phys. Rev. A 34 (1986) 3483, J. Magn. Magn. Mater. 122 ( 1993) 53), but one whose grain size is 3 nm cannot, although the 3 nm FF results in a ferromagnetic moment orders of magnitude larger than the diamagnetic moment of the liquid crystal. We will show that the FN made with the 13 nm particles contain aggregates that are blocked at room temperature, and that these aggregates are responsible for the realignment. Our data suggests that the aggregates mainly consist of two grains.