Coexistence of different magnetic moments in CeRuSn probed by polarized neutrons

We report on the spin densities in CeRuSn determined at elevated and at low temperatures using polarized neutron diffraction. At 285 K, where the CeRuSn crystal structure contains two different crystallographic Ce sites, we observe that a Ce site with larger nearest-neighbor distances is clearly more susceptible to the applied magnetic field, whereas the other is hardly polarizable. This finding clearly documents that different local environment of the two Ce sites causes the Ce ions to split into magnetic Ce3+ and nonmagnetic Ce(4-delta)+ valence states. With lowering the temperature, the crystal structure transforms to a structure incommensurately modulated along the c axis. This leads to new inequivalent crystallographic Ce sites resulting in a redistribution of spin densities. Our analysis using the simplest structural approximant shows that in this metallic system Ce ions coexist in different valence states. Localized 4f states that fulfill the third Hund's rule are found to be close to the ideal Ce3+ state (at sites with the largest Ce-Ru interatomic distances), whereas Ce(4-delta)+ valence states are found to be itinerant and lsituated at Ce sites with much shorter Ce-Ru distances. The similarity to the famous gamma-alpha transition in elemental cerium is discussed.