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We present the theoretical phase diagrams of the classical Heisenberg fluid in an external magnetic field. A consistent account of correlations is carried out by the integral equation method. A nonmonotonic effect of fields on the temperature of the gas-liquid critical point is found. Within the mean spherical approximation this nonmonotonic behavior disappears for short-range enough spin-spin interactions. ͓S1063-651X͑99͒51204-X͔ PACS number͑s͒: 64.70.Fx, 75.50.Mm, 05.70.Jk, 61.20.Gy The behavior of magnetic fluids in an external field has commanded more and more attention in recent years and has some peculiarities. In the presence of an external magnetic field the orientational ͑magnetic͒ phase transition is absent, but there are the first order transitions between ferromagnetic phases of different densities ͑e.g., gaseous and liquid phases͒. Physical properties of anisotropic fluids ͑to these belong also, besides magnetic fluids, nematic liquid crystals͒ are determined by the interplay between orientational and translational degrees of freedom. Therefore, by varying the magnetic field it is possible to effect structural properties of magnetic fluids, in particular, to change the region of the gas-liquid coexistence. Such investigations with a calculation of phase diagrams were carried out for model spin systems within the mean field ͑MF͒ approximation ͓1,2͔. It was found that for fluids of hard spheres carrying Ising spins an external magnetic field decreases the temperature of the gas-liquid critical point. On the other hand, the presence of isotropic van der Waals attractions between molecules can lead to the inverse effect ͓2͔. In Ref. ͓2͔ the fluid of hard spheres with the classical Heisenberg spins and strong isotropic attractions was considered also. It was shown that at weak magnetic fields there can be two first order phase transitions in this model: gas-liquid and liquid-liquid. In strong fields the weak liquid-liquid transition disappears.The need to take into account orientational-translational correlations for the description of physical properties of magnetic fluids stimulated studies of the external field effects by more complex techniques. The effect of an external field on the gas-liquid critical point was studied by the functional integration and Green function methods ͓3͔ for the quantum Heisenberg ferrofluid and by the Monte Carlo and integral equation methods for the classical one ͓4,5͔. The pair potentials of those models consisted of contributions of hard spheres and of the spin-spin interaction ͑the so-called ideal Heisenberg fluid͒. In these works the conclusion was that an external magnetic field favors the phase separation, i.e., the application of the external field increases the gas-liquid critical temperature. Let us note that the results of Refs. ͓4,5͔ are obtained for quite strong fields. In our point of view, it was the effect of small fields that is of special interest. This follows from the fact that at small fields orientational fluctuations are large and the corresponding...

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We describe an integral equation method for obtaining the distribution of a nematic fluid near a wall and interacting with a uniform orienting field. Complete density-orientational profiles are calculated for a model nematic with different wall-particle interactions and different orientations of the wall with respect to the field. For orienting walls we identify particular long-range correlations that are responsible for reorientation of the bulk nematic at zero external field. These correlations become stronger as the wall-particle interaction is increased in range; they become longer ranged as the orienting field is weakened. Special attention is focused on systems where the wall-particle interaction favors orientations perpendicular to the surface. The local director orientation can vary discontinuously with the distance from the surface when the orienting influences of the field and the wall are antagonistic. At high densities smectic-like structures appear. Adsorption phenomena are also discussed. For inert hard walls, the ordered fluid avoids the surface, and a surface layer where the particles tend to orient perpendicular to the bulk director appears. Experimentally, this might be seen as wetting of the wall by a less-ordered fluid.

An explicit expression for the wall-nematic direct correlation function (DCF) is obtained for any orientation of the wall with respect to an external orienting field. It is found that inside the surface of the wall, the DCF rapidly tends to a function of the nematogen orientation and depends only on parameters of the bulk fluid. We suggest that the wall-nematic DCF can be used as an ansatz for the colloid-nematic DCF in dilute nematic colloids. The reliability of this ansatz is investigated at different field strengths in both isotropic and nematic regions. Our calculations for spherical colloidal particles show that this approximation is valid for colloidal particles that are large, but well within the physically realistic size range. The ansatz could also be applied to nonspherical colloidal particles.