  Clusters supported by solid substrates are prime candidates for heterogeneous
catalysis and can be prepared in various ways. While mass-selected soft-landing
methods are often used for the generation of monodisperse particles,
self-assembly typically leads to a range of different cluster sizes. Here we
show by scanning tunneling microscopy measurements that in the initial stages
of growth Mn forms trimers on a close-packed hexagonal Ir surface, providing a
route for self-organized monodisperse cluster formation on an isotropic
metallic surface. For an increasing amount of Mn, first a phase with
reconstructed monolayer islands is formed, until at full coverage a
pseudomorphic Mn phase evolves which is the most densely packed one of the
three different observed Mn phases on Ir(111). The magnetic state of both the
reconstructed islands and the pseudomorphic film is found to be the
prototypical antiferromagnetic N\'eel state with 120{\deg} spin rotation
between all nearest neighbors in the hexagonal layer.
