NONMONOTONIC CROSSOVER OF THE EFFECTIVE SUSCEPTIBILITY EXPONENT

We have numerically determined the behavior of the magnetic susceptibi lity upon approach of the critical point in two-dimensional spin syste ms with an interaction range that was varied over nearly 2 orders of m agnitude. The full crossover from classical to Ising-like critical beh avior, spanning several decades in the reduced temperature, could be o bserved. Our results convincingly show that the effective susceptibili ty exponent gamma(eff) changes nonmonotonically from its classical to its Ising value when approaching the critical point in the ordered pha se. In the disordered phase the behavior is monotonic. Furthermore, th e hypothesis that the crossover function is universal is supported.