  We study the liquid-gas phase separation observed in a system of repulsive
particles dressed with ferromagnetically aligning spins, a so-called `spin
fluid'. Microcanonical ensemble numerical simulations of finite-size systems
reveal that magnetization sets in and induces a liquid-gas phase separation
between a disordered gas and a ferromagnetic dense phase at low enough energies
and large enough densities. The dynamics after a quench into the coexistence
region show that the order parameter associated to the liquid-vapour phase
separation follows an algebraic law with an unusual exponent, as it is forced
to synchronize with the growth of the magnetization: this suggests that for
finite size systems the magnetization sets in along a Curie line, which is also
the gas-side spinodal line, and that the coexistence region ends at a
tricritical point. This picture is confirmed at the mean-field level with
different approximation schemes, namely a Bethe lattice resolution and a virial
expansion complemented by the introduction of a self-consistent Weiss-like
molecular field. However, a detailed finite-size scaling analysis shows that in
two dimensions the ferromagnetic phase escapes the
Berezinskii-Kosterlitz-Thouless scenario, and that the long-range order is not
destroyed by the unbinding of topological defects. The Curie line becomes thus
a magnetic crossover in the thermodynamic limit. Finally, the effects of the
magnetic interaction range and those of the interaction softness are
characterized within a mean-field semi-analytic low-density approach.
