Using density functional theory in the modified mean-field (MMF) approximation we study the phase behavior of asymmetric binary mixtures of equisized dipolar hard spheres with different dipole moments in the fluid phase regime. We focus on "dipole-dominated" systems where isotropic attractive interactions are absent. Despite these restrictions our results reveal complex fluid-fluid phase behavior involving demixing and first- and second-order isotropic-to-ferroelectric phase transitions the relative importance of which depends on two "tuning" parameters, that is, the parameter Gamma measuring the ratio of the dipolar coupling strengths, and the chemical potential difference Deltamu controlling the composition. The interplay of these effects then yields three different types of phase behavior differing in the degree to which demixing dominates the system. A generic feature of the resulting diagrams is that the isotropic-to-ferroelectric transition is shifted towards significantly higher densities compared to the one-component case, and is therefore destabilized. Furthermore, demixing in the MMF approach turns out to be always accompanied by spontaneous ferroelectricity, which is in contrast to recent integral equation and simulation results for the limiting case of a mixture of dipolar and pure hard spheres (Gamma=0).

The phase behavior of bidisperse ferrocolloids consisting of binary mixtures of dipolar hard spheres (DHS's) with different particle diameters and different dipole moments is investigated using density-functional theory in a modified mean-field approximation. We focus on the fluid phase regime, where we consider both isotropic and anisotropic states. Depending on the parameter Gamma -measuring the asymmetry of the dipolar couplings-the systems display complex fluid-fluid phase behavior involving demixing transitions, as well as first- and second-order isotropic-to-ferromagnetic phase transitions. The topology of the resulting phase diagrams turns out to be similar to those corresponding to monodisperse DHS mixtures investigated previously by us [Phys. Rev. E 69, 041201 (2004)]]. However, additional size asymmetry has a strong impact on the relative importance of the various types of phase transitions. In particular, the demixing transition of bidisperse ferrocolloids is strongly destabilized compared to that of monodisperse DHS's in the sense that demixing critical points are significantly shifted towards lower temperatures.

We present phase diagrams of a model bidisperse ferrocolloid consisting of a binary mixture of dipolar hard spheres (DHSs) under the influence of an external magnetic field. The dipole moments of the particles are chosen proportional to the particle volume to mimic real ferrocolloids, and we focus on dipole-dominated systems where isotropic attractive interactions are absent. Our results are based on density-functional theory in the modified mean-field (MMF) approximation. For one-component DHS fluids in external fields, and for corresponding mixtures dominated by one of the components, MMF theory predicts the tricritical point of the transition between an isotropic gas and a ferromagnetic liquid occurring at zero field to be changed into a critical point separating two magnetically ordered phases of different density. The corresponding critical temperature displays a nonmonotonic dependence on the field strength. Completely different behavior is found for the critical temperature related to the demixing phase transitions appearing in strongly asymmetric mixtures [G. M. Range and S. H. L. Klapp, Phys. Rev. E 70, 061407 (2004)]. For such systems, we find a monotonic decrease of the demixing critical temperature with increasing field. The field strength dependence of the critical temperature can therefore be tuned between nonmonotonic and monotonic behaviors just by changing the composition of the mixture--e.g., by adjusting the chemical potentials. This allows us to efficiently control the influence of external magnetic fields on the phase behavior over a large temperature interval.

Using reference hypernetted chain (RHNC) integral equations and density functional theory in the modified mean-field (MMF) approximation we investigate the phase behavior of binary mixtures of dipolar hard spheres. The two species (A and B) differ only in their dipole moments m(A) and m(B), and the central question investigated is under which conditions these asymmetric mixtures can exhibit demixing phase transitions in the fluid phase regime. Results from our two theoretical approaches turn out to strongly differ. Within the RHNC (which we apply to the isotropic high-temperature phase) demixing does indeed occur for dense systems with small interaction parameters Gamma= m(2)(B)/m(2)(A). This result generalizes previously reported observations on demixing in mixtures of dipolar and neutral hard spheres (Gamma=0) to the case of true dipolar hard sphere mixtures. The RHNC approach also indicates that these demixed fluid phases are isotropic at temperatures accessible by the theory, whereas isotropic-to-ferroelectric transitions occur only at larger Gamma. The MMF theory, on the other hand, yields a different picture in which demixing occurs in combination with spontaneous ferroelectricity at all Gamma considered. This discrepancy underlines the relevance of correlational effects for the existence of demixing transitions in dipolar systems without dispersive interactions. Indeed, supplementing the dipolar interactions by small, asymmetric amounts of van der Waals-like interactions (and thereby supporting the systems tendency to demix) one finally reaches good agreement between MMF and RHNC results.

Using the reference hypernetted chain (RHNC) integral equation theory and an accompanying stability analysis we investigate the structural and phase behaviors of model bidisperse ferrocolloids based on correlations of the homogeneous isotropic high-temperature phase. Our model consists of two species of dipolar hard spheres (DHSs) which dipole moments are proportional to the particle volume. At small packing fractions our results indicate the onset of chain formation, where the (more strongly coupled) A species behaves essentially as a one-component DHS fluid in a background of B particles. At high packing fractions, on the other hand, the RHNC theory indicates the appearance of isotropic-to-ferromagnetic transitions (volume ratios close to one) and demixing transitions (smaller volume ratios). However, contrary with the related case of monodisperse DHS mixtures previously studied by us [Phys. Rev. E 70, 031201 (2004)], none of the present bidisperse systems exhibit demixing within the isotropic phase, rather we observe coupled ferromagnetic/demixing phase transitions.