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 compounds YMn6Sn6, LuMn6Sn6 and ScMn6Sn6 of HfFe6Ge6-type structure (space 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 temperature range for YMn6Sn6 and ScMn6Sn6 and below room temperature for LuMn6Sn6, the magnetic structure is a flat spiral consisting of ferromagnetic (001) ''Mn-Sn-Sn-Mn'' sheets (with the moments mu(Mn) approximate 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 occurrence of several thermally dependent modulation vectors, the magnetic structure has been refined considering an inhomogeneous mixture of magnetic phases, each of them being characterized by a different wavevector. Under these conditions, the magnetic moments change their orientation 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 degrees-2 x 27 degrees-7 x 114.7 degrees-1 x 160 degrees] along one-half the c' parameter of a supercell with c' = 36c. A discussion and some comparisons 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.