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

The early growth stages of Co structures on Au(lll) have been analyzed by s canning tunneling microscopy and their magnetic properties simultaneously m easured, in situ, by Ken effect and, ex sial by superconducting quantum int erference device. With increasing cobalt coverage, cobalt clusters, organiz ed on the lattice of the 22 x root 3 zigzag reconstruction coalesce into on e-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 d ictated by the presence of ordered point dislocations at the zigzag reconst ruction of the relaxed gold surface, anomalous nucleation and growth is als o observed when large domains of strained linear reconstructions are presen t. Two families of clusters must then be considered with important conseque nces for magnetic properties. When only the zigzag reconstruction is presen t, 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 mod el describes well transitions from OD cluster superparamagnetism, to 1D Isi ng behavior and finally to 2D ferromagnetism as a function of cobalt covera ge, in good agreement with experimental results. [S0163-1829(99)04617-2].