Homonuclear cobalt and iron clusters ${\mathrm{Co}}_N$ and ${\mathrm{Fe}}_N$ measured in a cryogenic molecular beam exist in two states with distinct magnetic moments ($\mu$), polarizabilities, and ionization potentials, indicating distinct valences. The $\mu$ is approximately quantized: ${\mu }_N\sim 2N{\mu }_B$ in the ground states and $\mu _N{}^{*}\sim N{\mu }_B$ in the excited states for Co; ${\mu }_N\sim 3N{\mu }_B$ and $\mu _N{}^{*}\sim N{\mu }_B$ for Fe. At a large size, the average $\mu$ of the two states converges to the bulk value with diminishing ionization potential differences. The experiments suggest localized ferromagnetism in the two states and that itinerant ferromagnetism emerges from their superposition.

• Received 27 June 2008

DOI:https://doi.org/10.1103/PhysRevLett.107.057203