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

The two magnetic compounds Ce2Sn5 and Ce3Sn7 have very similar crystal lographic 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 carrie s a well-defined moment and dominates the magnetic properties at low t emperatures. 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, Ce 2Sn5 presents a modulated structure with moments of maximum value 1.3 mu(B) aligned along a, whereas Ce3Sn7 is an antiferromagnet with momen ts of only 0.36 mu(B) parallel to c. This difference cannot be attribu ted to different Crystal Electric Field effects, which, from inelastic neutron spectroscopy measurements, are found to be fairly close in th e two compounds. A strong anisotropy of the interionic exchange coupli ng is evidenced in both compounds from the magnetization curves and ne utron data, within a crystal field model. Especially, in Ce3Sn7, a neg ative 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 ba nd electrons play an important role in Ce2Sn5 and Ce3Sn7. In both comp ounds, besides the intermediate-valent behavior of cerium in the CeSn3 -type site, Kondo effects are present on the magnetic cerium site, lea ding to an extra reduction of the cerium magnetic moments. These hybri dization effects are also likely to be at the origin of the anisotropy of the exchange interactions and of the resistivity.