Magnetism in Mn clusters: Interplay between spin ordering and structure

We used a quantum Ruderman-Kittel-Kasuya-Yosida (RKKY)-Heisenberg Hamiltonian to describe the ground- and excited-state spin structures of 13-atom icosahedral Mn clusters. The magnetic properties were obtained exactly as a function of the temperature and pairwise exchange interactions. Our results explain in a natural way the seemingly contradictory results for Mn clusters, where first-principles calculations predict both spin-block and antiferromagnetic arrangements.