MAGNETISM IN THE FEW-MONOLAYERS LIMIT - A SURFACE MAGNETOOPTIC KERR-EFFECT STUDY OF THE MAGNETIC-BEHAVIOR OF ULTRATHIN FILMS OF CO, NI, ANDCO-NI ALLOYS ON CU(100) AND CU(111)

The surface magneto-optic Kerr effect (SMOKE) was used to investigate the magnetic properties of epitaxial thin films of Co, Ni, and their a lloys grown on Cu(100) and Cu(111). The Curie temperature T(C) is high er for the same films of a given thickness on Cu(111) than on Cu(100). All the films show a change in the power-law exponent beta of the mag netization density M approximately (1-T/T(C))beta with reducing film t hickness. Ni films on Cu(100) undergo a particularly abrupt crossover at approximately 7 monolayers (ML) from three-dimensional Heisenberg ( beta=0.37) to finite-size two-dimensional XY (beta=0.23) behavior as t he film thickness is reduced. The characteristic power-law exponent be ta=0.23 of these films appears to be an experimental realization of Ko sterlitz-Thouless behavior over a restricted temperature range. A simi lar, but more gradual crossover occurs for the Ni films on Cu(111) at 8 to 12 ML. The finite-size scaling behavior in the few-monolayers-thi ckness range is compared with that reported for Ising thin-film behavi or. In all instances T(C) extrapolates with decreasing thickness to ze ro at one monolayer. The dimensionality crossover and finite-size scal ing behavior is discussed in the light of our current understanding of spin-wave quantization, anisotropy, and film microstructure.