The effects of the boundary conditions on the surface magnetization of finite (thin films) and semi-infinite Ising ferromagnets are analyzed in detail. We studied the (100) surface of the simple cubic lattice using the cube approximation of the cluster-variation method. Our results show that the magnetization at the surface layer depends strongly on the boundary conditions imposed in the calculation. In particular, for the case in which the Curie temperature at the surface ($T_C$${)}_s$, is higher than the corresponding in the bulk ($T_C$), the derivative of the surface magnetization at $T_C$ is discontinuous if a reduced number of layers is assumed to be different from the bulk.

• Received 27 January 1989

DOI:https://doi.org/10.1103/PhysRevB.39.9746