MAGNETIC-STRUCTURE OF YMN6GE6 AND ROOM-TEMPERATURE MAGNETIC-STRUCTURE OF LUMN6SN6 OBTAINED FROM NEUTRON-DIFFRACTION STUDY

Neutron diffraction measurements have been performed on the ternary compounds YMn6Ge6 and LuMn6Sn6 of HfFe6Ge6-type structure (space group, P6/mmm). This structure can be described as a filled derivative of the CoSn-B35-type structure. Each of the rare earth (R) and Mn atoms are successively distributed in alternate layers, stacked along the c axis with the sequence Mn-R-Mn-Mn-R-Mn. At 300 K, both compounds exhibit collinear antiferromagnetic arrangements and the magnetic structures consist of a stacking of ferromagnetic (001) layers of Mn with the coupling sequence Mn(+)-R-Mn(-)-Mn(-)-R-Mn(+) (mu(Mn) almost-equal-to 1.33(1)mu(B) and 1.82(3)mu(B) for LuMn6Sn6 and YMn6Ge6 respectively). For LuMn6Sn6, the magnetic moments lie in the (001) plane, while they are along the c axis in YMn6Ge6. At low temperature, a spin reorientation process occurs in both compounds, yielding incommensurate antiferromagnetic arrangements. For YMn6Ge, (T(t) almost-equal-to 80 K), the Mn moments form a double-cone structure with a periodicity of about 105 angstrom (mu(Mn) = 1.95(4)mu(B) at 2 K), while only preliminary results are available for LuMn6Sn6 below about 200 K. The results are compared with those obtained on the CoSn-B35-type structure binary compounds FeSn and FeGe, on one hand, and the RMn6Sn6 compounds, on the other hand.