Quantized Ferromagnetic Moments of Free Cobalt Clusters

The magnetic moments μ (N) of free Co&_slash;N(20<=N<=200) clusters has been measured in a cryogenic molecular beam. Besides the ground states with μ (N)/N ∼ 2 μ B, an electronic state has been found in all clusters studied. The magnetic moments μ *(N)/N of this state is approximately 1/2 μ (N)/N. Ionization potential of Co clusters at each of the two states are determined from photo-ionization efficiency measurements. The ionization potentials for the excited states are systematically lower than that for ground states by about 100meV for small clusters, and merge for larger clusters(N>100). This suggests that molecular magnetism of small clusters evolves to itinerant (band) ferromagnetism in the bulk (μ (N)/N ∼ 1.7 μ B) when the energy gap between these two states vanishes.