POLYMERIZATION IN A FERROMAGNETIC SPIN MODEL WITH THRESHOLD

We propose a spin model with a new kind of ferromagnetic interaction, which may be called ferromagnetic coupling with threshold. In this mod el the contribution of a given spin to the total energy has only two p ossible values and depends on the number of parallel spins among its n earest and next to nearest neighbors. By mapping the model onto the Is ing version of the isotropic eight-vertex model, we obtain some eviden ce of a low temperature phase made of alternate parallel plus and minu s polymers, in which the spin is aligned with the majority of its neig hbors.