THE (U(1-X)LN(X))CO2GE2 SYSTEMS - PARALLEL (LN=ND) AND ANTIPARALLEL (LN=TB) MAGNETIC F-MOMENTS IN THE ORDERED STATE, DETERMINED BY NEUTRON-DIFFRACTION

The relative orientation of magnetic moments of uranium (5f-atom) and a lanthanide (Ln, 4f-atom), occupying randomly the same site, was inve stigated by neutron diffraction in the ordered state of the systems (U (1-x)Ln(x))Co2Ge2 with light (Ln = Nd) and heavy (Ln = Tb) lanthanide. These materials crystallize in the ThCr2Si2-type structure. Most of t hem order in the antiferromagnetic AF-I structure, with alternate (+ - ) stacking of (U, Ln) planes having net moment, and f-moments along th e tetragonal axis. It is found that the U and Nd moments in (U1-xNdx)C o2Ge2 are parallel, while the U and Tb moments in (U1-xTbx)Co2Ge2 are antiparallel. This is apparently due to in-plane direct exchange betwe en f-atoms, in agreement with the ''spin-charge'' concept, considering that the U moments behave like light Ln.