The purpose of this proposal is to study the ferro/para phase transition in a mesoscopic Ising-like lattice and in particular demonstrate the existence of a negative magnetic susceptibility in the fixed magnetization ensemble. To this aim we will use the correlation < S(r) S(r+Dr) > = < M2>/N2 where N is the total number of spins for a single cluster, M the total magnetization of the cluster, and the equality holds if we choose r0