Magnetism in the few-monolayers limit: A surface magneto-optic Kerr-effect study of the magnetic behavior of ultrathin films of Co, Ni, and Co-Ni alloys on Cu(100) and Cu(111)

TLDR

Finite‑size scaling in ultrathin Co, Ni, and Co‑Ni films on Cu(100) and Cu(111) is compared to that of Ising thin films. The study investigates dimensionality crossover and finite‑size scaling in these ultrathin films on Cu(100) and Cu(111). Surface magneto‑optical Kerr effect measurements were performed.

Abstract

The surface magneto-optic Kerr effect (SMOKE) was used to investigate the magnetic properties of epitaxial thin films of Co, Ni, and their alloys grown on Cu(100) and Cu(111). The Curie temperature ${\mathit{T}}_{\mathit{C}}$ is higher 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 \ensuremath{\beta} of the magnetization density M\ensuremath{\sim}(1-T/${\mathit{T}}_{\mathit{C}}$${)}^{\mathrm{\ensuremath{\beta}}}$ with reducing film thickness. Ni films on Cu(100) undergo a particularly abrupt crossover at \ensuremath{\sim}7 monolayers (ML) from three-dimensional Heisenberg (\ensuremath{\beta}=0.37) to finite-size two-dimensional XY (\ensuremath{\beta}=0.23) behavior as the film thickness is reduced. The characteristic power-law exponent \ensuremath{\beta}=0.23 of these films appears to be an experimental realization of Kosterlitz-Thouless behavior over a restricted temperature range. A similar, 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-thickness range is compared with that reported for Ising thin-film behavior. In all instances ${\mathit{T}}_{\mathit{C}}$ extrapolates with decreasing thickness to zero at one monolayer. The dimensionality crossover and finite-size scaling behavior is discussed in the light of our current understanding of spin-wave quantization, anisotropy, and film microstructure.