ANOMALOUS MAGNETIC-BEHAVIOR OF CERIUM IN CE2SN5 AND CE3SN7, 2 SUPERSTRUCTURES OF CESN3

The two magnetic compounds Ce2Sn5 and Ce3Sn7 have very similar crystallographic structures. They are both superstructures of CeSn3 with two different cerium sites: one, similar to the cerium site in CeSn3, is ''non-magnetic'' or in intermediate valence state, and the other carries a well-defined moment and dominates the magnetic properties at low temperatures. Although their macroscopic magnetic properties under the effect of an applied field look quite similar, the magnetic structures of the two compounds in zero field are totally different. In fact, Ce2Sn5 presents a modulated structure with moments of maximum value 1.3 mu(B) aligned along a, whereas Ce3Sn7 is an antiferromagnet with moments of only 0.36 mu(B) parallel to c. This difference cannot be attributed to different Crystal Electric Field effects, which, from inelastic neutron spectroscopy measurements, are found to be fairly close in the two compounds. A strong anisotropy of the interionic exchange coupling is evidenced in both compounds from the magnetization curves and neutron data, within a crystal field model. Especially, in Ce3Sn7, a negative value is found for the c component of the exchange coefficient, leading to strong negative interactions when moments are aligned along c. The effects of hybridization of the cerium 4f-electron with the band electrons play an important role in Ce2Sn5 and Ce3Sn7. In both compounds, besides the intermediate-valent behavior of cerium in the CeSn3-type site, Kondo effects are present on the magnetic cerium site, leading to an extra reduction of the cerium magnetic moments. These hybridization effects are also likely to be at the origin of the anisotropy of the exchange interactions and of the resistivity.