  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<Dr<R where r0 is the linear
size of a spin site and R is the linear size of a cluster.
