Effect of an external magnetic field on the gas-liquid transition in the Ising spin fluid

Abstract

The theoretical phase diagrams of the magnetic (Ising) lattice fluid in an external magnetic field is presented. It is shown that, depending on the strength of the nonmagnetic interaction between particles, various effects of external field on the Ising fluid take place. In particular, at moderate values of the nonmagnetic attraction the field effect on the gas-liquid critical temperature is nonmonotoneous. A justification of such behavior is given. If short-range correlations are taken into account (within a cluster approach), the Curie temperature also depend on the nonmagnetic interaction.PACS numbers 64.70.Fx, 77.80.Bh, 75.50.Mm, 64.60.Kw. Anisotropic liquids are very sensitive matters. Such are nematic liquid crystals and ferrofluids. Many efforts have been made in order to investigate effects of shape and flexibility of the molecules, of long and short-range interactions on the properties of anisotropic liquids. External field effects are still worthier of attention, because the application of an external field allows to change properties of anisotropic liquids dynamically (in contrast to the effects of molecules' shape, etc., which are static).In ferrofluids an external magnetic field removes the magnetic order-disorder transition, nevertheless the first order transitions between ferromagnetic phases of different densities remain. The external field deforms the phase diagram of a magnetic fluid shifting coexistence lines between these phases. Kawasaki studied a magnetic lattice gas [1], which implements one of the ways to model properties of magnetic fluids. On temperature-density phase diagrams in Ref.[1] one can see that the gas-liquid binodal of the magnetic fluid significantly lowers after the application of an external magnetic field. Vakarchuk with coworkers [2] and Lado et al. [3,4] studied another model of magnetic fluids -the fluid of hard spheres with embedded Heisenberg spins. They concluded that in such a system the temperature of the gas-liquid critical point (the top of binodal) increases after the application of an external field. The nature of such a discrepancy can be various, because the types of models (continuum fluids [2][3][4] and the lattice gas [1]) as well as approximations used differ.In this letter results of a more detailed investigation of the magnetic (Ising) lattice gas are reported. The first new point is the inclusion of the nonmagnetic interaction between particles. Another one consists in overcoming of limitations applied by the mean field approximation (MFA). It is known that this approximation is good for very long-range potentials only (and becomes accurate for a family of the infinite-ranged ones, the so-called Kac potentials [5,6]). The MFA can not reproduce some essential features of the systems with the nearest-neighbor interaction (such as the percolation phenomena in the quenched diluted Ising model, differences in magnetic properties of the quenched site-disordered Ising model and the annealed one [7]). Such drawbacks can be overcome with the two-site cluste...

XYSpin Fluid in an External Magnetic Field

“…This complexity arises due to a coupling between spin and spatial interactions. Similar phase diagrams are found in binary mixtures [6 -10] with their demixing and G-L transitions, spin lattice gas models [11,12], mixtures of 3 He- 4 He with the superfluid and demixing transitions [13][14][15], and others.…”

Self-consistent approach for magnetic ordering and excited site occupation processes in a two-level system

“…By certain features, the model described by the Hamiltonian (2.5) is close to a well-known Blume-Emery-Griffits (BEG) model [16] introduced to simulate the thermodynamic behaviour of 3 He-4 He mixtures and to numerous modifications of BEG model used for interpretation of diluted magnets properties [17][18][19][20][21][22][23]. The papers in this field of investigation are generally based on the use of the Ising Hamiltonian with a spin equal to unity as well as on some assumption concerning the character of randomness for magnetic particles in a lattice.…”

Density-functional study of model bidisperse ferrocolloids in an external magnetic field

“…In zero magnetic field all fluids display a phase transition from an isotropic gas ͑IG͒ into a ferromagnetic liquid ͑FL͒. 1,2,5 This diversity illustrates that the details characterizing the magnetic interactions are indeed a͒ Electronic mail: gabriel.range@fluids.tu-berlin.de b͒ Electronic mail: sabine.klapp@fluids.tu-berlin.de crucial for the effect of external fields on the fluid. Magnetic fields cause the high-temperature critical line to disappear, and the TCP transforms into a critical point ͑CP͒ separating two magnetized phases with different densities.…”

“…Examples for such fluids are Ising fluids, that is, fluids of spherical particles with short-range isotropic and additional ͑classical͒ Ising interactions, [1][2][3][4][5] Heisenberg fluids, 6-13 where the particle spin can rotate continuously, and ferrocolloids consisting of magnetic dipolar particles. Examples for such fluids are Ising fluids, that is, fluids of spherical particles with short-range isotropic and additional ͑classical͒ Ising interactions, [1][2][3][4][5] Heisenberg fluids, 6-13 where the particle spin can rotate continuously, and ferrocolloids consisting of magnetic dipolar particles.…”