Quantized Ferromagnetic Moments of Free Cobalt Clusters

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