INCOMMENSURATE MAGNETIC-STRUCTURES OF RMN(6)SN(6) (R=SC,Y,LU) COMPOUNDS FROM NEUTRON-DIFFRACTION STUDY

Neutron diffraction measurements have been performed on the ternary co mpounds YMn6Sn6, LuMn6Sn6 and ScMn6Sn6 of HfFe6Ge6-type structure (spa ce group, P6/mmm). All three compounds order antiferromagnetically at 333 K, 353 K and 384 K for R Y, Lu and Sc respectively. In the whole t emperature range for YMn6Sn6 and ScMn6Sn6 and below room temperature f or LuMn6Sn6, the magnetic structure is a flat spiral consisting of fer romagnetic (001) ''Mn-Sn-Sn-Mn'' sheets (with the moments mu(Mn) appro ximate to 2.15 mu(B) at 2 K in the layers) stacked along the c axis in the sequence Mn-R-Mn-Sn-Sn-Mn-R-Mn. In each case, according to the oc currence of several thermally dependent modulation vectors, the magnet ic structure has been refined considering an inhomogeneous mixture of magnetic phases, each of them being characterized by a different wavev ector. Under these conditions, the magnetic moments change their orien tation by about 40 degrees-60 degrees within the basal plane on going from one R sheet to another. In the case of YMn6Sn6, a complex unique arrangement with a non-constant rotation of the moments has also been considered, i.e. a turn-angle sequence [1 x 160 degrees-7 x 114.7 degr ees-2 x 27 degrees-7 x 114.7 degrees-1 x 160 degrees] along one-half t he c' parameter of a supercell with c' = 36c. A discussion and some co mparisons with the parent CoSn B35-type structure binary compounds, on the one hand, and the other RMn(6)Sn(6) compounds, on the other hand, are given in conclusion.