MAGNETIC-PROPERTIES OF LARGE ISING CLUSTERS

The magnetic properties of large Ising clusters with cube-octahedral, icosahedral, body-centered cubic and simple cubic structures are studied. The Ising Hamiltonian is solved in the mean field approximation and the magnetization and the Curie temperature are calculated for clusters consisting of as many as 2744 atoms. The dependence of Te with respect to the total number of atoms (N) follows a finite scaling behavior delta = 1 - T-C(N)/T-C(infinity) = alpha N--lambda. We find that for all the structures studied and for the case in which the magnetic coupling constants between spins are assumed independent of their location in the cluster, 0.569 < lambda < 0.597 and 2.192 < alpha < 3.126. Those quantities change considerably when the coupling constants between surface spins are assumed to be different from the rest. These results clearly show the particular behavior of finite aggregates. We present results for the magnetization profiles of sites lying on the central plane and on one of the faces of a body-centered cubic cluster with 1241 spins. We also studied the magnetization decay in the low temperature limit and found that the amplitude and exponent are different from the bulk values. We compare our calculations with other results in the literature.