Exact numerical study of the ground-state magnetic properties of clusters

The ground states of bcc, fee, hcp, and icosahedral clusters having N= 12-1 4 atoms are calculated exactly in the framework of the Hubbard Hamiltonian by using Lanczos's numerical diagonalization method. The total spin S shows a diversity of remarkable, sometimes surprising behaviors as a function of Coulomb interaction strength U/t and band filling nu/N. The magnetic prope rties are analyzed, in particular, by relating them to the corresponding si ngle-particle spectra. Changes of symmetry close to real-space shell closin gs at N=13 are discussed. The relative stability of the different cluster g eometries is determined and the corresponding structural and magnetic diagr ams are derived. Electron correlation effects close to half-band filling re sult in changes of structure that are often accompanied by important change s in the magnetic behavior. Ferromagnetism is found to be particularly stab le for nu/N similar or equal to 1.2-1.4. The main trends as a function of N , nu and U/t are compared with previous exact diagonalization studies on sm aller clusters. [S0163-1829(99)09907-5].