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A long time ago, Brochard and de Gennes predicted the possibility ofsignificantly decreasing the critical magnetic field of the Fredericksztransition (the magnetic Fredericksz threshold) in a mixture of nematic liquidcrystals and ferromagnetic particles, the so-called ferronematics. Thisphenomenon has rarely been measured, usually due to soft homeotropic anchoringinduced at the nanoparticle surface. Here we present an optical study of themagnetic Fredericksz transition combined with a light scattering study of theclassical nematic liquid crystal, 5CB, doped with 6 nm diameter magnetic andnon-magnetic nanoparticles. Surprisingly, for both nanoparticles, we observe atroom temperature a net decrease of the threshold field of the Fredericksztransition at low nanoparticle concentrations, which appears associated with acoating of the nanoparticles by a brush of polydimethylsiloxane copolymerchains inducing planar anchoring of the director on the nanoparticle surface.Moreover the magnetic Fredericksz threshold exhibits non-monotonic behaviour asa function of the nanoparticle concentration for both types of nanoparticles,first decreasing down to a value from 23\% to 31\% below that of pure 5CB, thenincreasing with a further increase of nanoparticle concentration. This isinterpreted as an aggregation starting at around 0.02 weight fraction thatconsumes more isolated nanoparticles than those introduced when theconcentration is increased above $c = 0.05$ weight fraction (volume fraction$3.5 \times 10^{-2}$). This shows the larger effect of isolated nanoparticleson the threshold with respect to aggregates. From dynamic light scatteringmeasurements we deduced that, if the decrease of the magnetic threshold whenthe nanoparticle concentration increases is similar for both kinds ofnanoparticles, the origin of this decrease is different for magnetic andnon-magnetic nanoparticles. For non-magnetic nanoparticles, the behavior may beassociated with a decrease of the elastic constant due to weak planaranchoring. For magnetic nanoparticles there are non-negligible local magneticinteractions between liquid crystal molecules and magnetic nanoparticles,leading to an increase of the average order parameter. This magneticinteraction thus favors an easier liquid crystal director rotation in thepresence of external magnetic field, able to reorient the magnetic moments ofthe nanoparticles along with the molecules.

Influence of a dispersion of magnetic and nonmagnetic nanoparticles on the magnetic Fredericksz transition of the liquid crystal 5CB