Emergence of antiferromagnetic ordering in Mn clusters

First-principles density-functional-theory investigations of small Mnₙ $(n=2--7,13)$ clusters reveal a competition between ferromagnetic and antiferromagnetic ordering of atomic magnetic moments. For smaller sizes $(n<~6),$ this competition results in a near degeneracy between the two types of orderings, whereas AF arrangements are clearly favored for larger clusters. The calculations thus predict a size-dependent transition in the magnetic ordering of Mn clusters.