On the interplay of magnetic and molecular forces in Curie-Weiss ferrofluid models

We consider a mean-field continuum model of classical particles in Rd With Ising or Heisenberg spins. The interaction has two ingredients, a ferromagn etic spin coupling and a spin-independent molecular force. We show that a f eedback between these forces gives rise to a first-order phase transition w ith simultaneous jumps of particle density and magnetization per particle, either at the threshold of ferromagnetic order or within the ferromagnetic region. If the direct particle interaction alone already implies a phase tr ansition, then the additional spin coupling leads to an even richer phase d iagram containing triple (or higher order) points.