PHASE-TRANSITIONS IN A CONTINUUM MODEL OF THE CLASSICAL HEISENBERG MAGNET - THE FERROMAGNETIC SYSTEM

We present a detailed description of the rich variety of phase transit ions exhibited by the continuum Heisenberg model of spin fluid, treate d by means of integral equation methods (mean spherical approximation and reference hypernetted chain equation) and simulation techniques (c anonical ensemble and Gibbs ensemble Monte Carlo). We focus here on fe rromagnetic interactions. An order-disorder transition and a gas-liqui d transition are characterized. Both transitions are coupled at low te mperature, but near the gas-liquid critical point only ordered states are involved in the gas-liquid transition. Our data preclude the exist ence of a tricritical point, but there is evidence that the line of Cu rie points may end up in a critical end point at the gas-liquid spinod al. At low densities there is a strong tendency to clustering, and the system organizes in magnetized droplets. The reference hypernetted ch ain equation yields a spinodal line (locus of Curie points) for modera te to high densities, whereas at low densities the equation breaks dow n at a singularity with the characteristic behavior of a square root b ranch point.