Journal of Magnetism and Magnetic Materials 122 (1993) 62-65 North-Holland AI41 Magnetic Fredericksz transition in a ferronematic S.V. Burylov and Yu.L. Raikher Laboratory of Kinetics of At~isotropic Fhtids, lnstitttte of Contimeo~s Medi~ Mechanics, UB of the" Rus.~'i~n A~'Ltd. Sci., Perm 614061. Russia On the basis of a recently developed theory, taking into account the finite energy of the orientation coupling between the suspended ferroparticles and the nematic carrier, the magnetic Fredericksz transition (the instability of a uniform texture) taking place in flat layers of ferronematic mixtures is studied. The dependence of the threshold field strength upon the main reference parafneters surface energy density, particle size and concentration - is determined. 1. Introduction Ferronematics (FN) is the name for suspen- sions of monodomain ferro- or ferrimagnetic par- ticles in nematic liquid crystals (NLC). The most essential feature of these systems is a strong orientational coupling between the dispersed phase (ferroparticles) and the liquid-crystalline matrix. The applied magnetic field H, changing the orientation of the particles, via them affects the texture of the NLC matrix. Due to that, the presence of the ferromagnetic admixture en- hances the magnetic susceptibility of FN, in com- parison with pure NLC, by several orders of magnitude. A continuum theory for FN had been devel- oped by Brochard and de Gennes [1] prior to the first experimental realizations of these systems [2,3]. The authors of ref. [1] had proposed and considered magnetic NLC suspensions, whose solid phase consists of needle- or rod-like ferrite particles with length L ~ 0.1 ~m >> a (where a is the NLC molecule size) and diameter d ~ L/IO. Such a distinctive anisometricity imparts to the particles a substantial magnetic rigidity, thus making them into small permanent magnets. In order to avoid strong interparticle magnetic dipole-dipole interaction, and, hence, prevent the disperse phase from agglomeration, the volume Correspondence to: Dr. Yu.L. Raikher, Laboratory of Kinetics of Anisotropic Fluids, Institute of Continuous Media Mechan- ics, UB of the Russian Academy of Science. 614061 Perm, Russia. fraction J" of particles in FN should be suffi- ciently small (f< 10 4). In a composite medium, which FN actually is, each component - the ferroparticle assembly and the NLC matrix - possesses its own set of the orientational degrees of freedom. So, the form of the equilibrium equations essentially depends upon the origin of coupling between the order parameters of the subsystems. In a magnetized FN, where the parallelism of the particle axes is tantamount to the alignment of their magnetic moments, it is reasonable to characterize the orientational state by two intrinsic variables - director n(r) and local magnetization M(r) - averaged over a spatial scale large in comparison with L. In our papers [4,5] we have highlighted a new modification of the continuum model of FN des- tined to describe the experimentally available [3,6] thermotropic systems. Our approach, unlike the basic theory [1], takes into account that the an- choring energy, governing the orientation of ne- marie on the very surface of a ferroparticle, is finite. This difference manifests in a fundamental fact that in FN with finite homeotropic anchoring on particles, the equilibrium orientational state is n(r)±M(r), not n(r)ll M(r), as it had been as- sumed in ref. [1]. The type of symmetry of the internal structure is very important, since it pro- vides easily verified qualitative conclusions. In particular, if n ± M, then an analog of the classi- cal NLC effect - the Fredericksz transition , entirely forbidden in the Brochard-de Genncs model, should exist in real FN. Actually, it was just the observation of this transition [3] that had 0304-8853/93/$06.00 © 1993 ElsevierScience Publishers B.V. All rights reserved