Transition from zero-dimensional superparamagnetism to two-dimensional ferromagnetism of Co clusters on Au(111)

The early growth stages of Co structures on Au(111) have been analyzed by scanning tunneling microscopy and their magnetic properties simultaneously measured, in situ, by Kerr effect and, ex situ by superconducting quantum interference device. With increasing cobalt coverage, cobalt clusters, organized on the lattice of the $22\times \sqrt{3}$ zigzag reconstruction coalesce into one-dimensional (1D) chains at a coverage of 1.0 monolayer (ML) and then into a nearly continuous film at 2 ML. While this well-defined growth mode is dictated by the presence of ordered point dislocations at the zigzag reconstruction of the relaxed gold surface, anomalous nucleation and growth is also observed when large domains of strained linear reconstructions are present. Two families of clusters must then be considered with important consequences for magnetic properties. When only the zigzag reconstruction is present, the onset of long-range, 2D ferromagnetism is observed at a coverage of 1.6 ML of cobalt. A Monte Carlo simulation of an anisotropic Heisenberg model describes well transitions from 0D cluster superparamagnetism, to 1D Ising behavior and finally to 2D ferromagnetism as a function of cobalt coverage, in good agreement with experimental results.