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 studi ed. The Ising Hamiltonian is solved in the mean field approximation an d the magnetization and the Curie temperature are calculated for clust ers consisting of as many as 2744 atoms. The dependence of Te with res pect to the total number of atoms (N) follows a finite scaling behavio r 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 cou pling constants between spins are assumed independent of their locatio n in the cluster, 0.569 < lambda < 0.597 and 2.192 < alpha < 3.126. Th ose quantities change considerably when the coupling constants between surface spins are assumed to be different from the rest. These result s clearly show the particular behavior of finite aggregates. We presen t 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 1 241 spins. We also studied the magnetization decay in the low temperat ure limit and found that the amplitude and exponent are different from the bulk values. We compare our calculations with other results in th e literature.