Magnetic properties of NdMn6Sn6 and SmMn6Sn6 compounds from susceptibility measurements and neutron diffraction study

Investigations by susceptibility measurements and neutron diffraction experiments have been performed on the ternary stannides NdMn6Sn6 and SmMn6Sn6 of HoFe6Sn6- and HfFe(6)Ge6-type structure, respectively. In these structures, each of the rare earth and manganese atoms is successively distributed in alternate layers with the sequence Mn-R-Mn-Mn-R-Mn. Owing to the manganese atom coordination of the rare earth, they appear closely related to the CaCu5- and ThMn12-type structures. NdMn6Sn6 and SmMn6Sn6 are ferromagnetic below 357 and 405 K respectively. Two additional magnetic transitions related to the spin reorientation process are undergone in NdMn6Sn6 at 135 and 35 K. A neutron diffraction study of NdMn6Sn6 shows that both the rare earth and manganese sublattices order simultaneously above room temperature. The magnetic structure of NdMn6Sn6 consists of ferromagnetically coupled Mn and Nd ferromagnetic (100) layers (i.e. positive Nd-Mn interaction). In the temperature range 135-35 K, the moment direction is along the a-axis whereas an easy plane occurs at 300 and 2 K with mu(Nd)=3.30(5) mu(B) and mu(Mn)=2.39(4) mu(B). The results are discussed and compared with those previously obtained for the parent NdMn6Ge6 and RMn6Sn6 (R=Tb-Er, Lu, Y, Sc) compounds.