MAGNETIC-STRUCTURES OF TBMN6SN6 AND HOMN6SN6 COMPOUNDS FROM NEUTRON-DIFFRACTION STUDY

Neutron diffraction measurements have been performed on the ternary stannides TbMn6Sn6 and HoMn6Sn6 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 and manganese atoms is successively distributed in alternate layers stacked along the c axis with the sequence Mn-R-Mn-Mn-R-Mn. Owing to the managanese atom coordination of the rare earth, this structure appears closely related to the CaCu5- and ThMn12-type structures. This study confirms that both the rare earth and the manganese sublattices order simultaneously above room temperature. In the whole temperature range studied (2-300 K), TbMn6Sn6 and HoMn6Sn6 exhibit a collinear ferrimagnetic arrangement. At 300 K, the magnetic structure consists of a stacking of ferromagnetic arrangement. At (001) layers of rare earth and manganese with the coupling sequence Mn(+)R(-)Mn(+)Mn(+)-R(-)Mn(+) (mu-Mn almost-equal-to 2.0(1)mu-B, mu-Tb = 4.9(1)mu-B, mu-Ho = 4.7(1)mu-B). For HoMn6Sn6, the magnetic moments lie in the (001) plane while in TbMn6Sn6, they deviate by phi = 15-degrees from the c axis. Below 300 K, the two compounds exhibit a spin rotation process and at 2 K the magnetic moments are along [001] and at 50-degrees from this axis for TbMn6Sn6 and HoMn6Sn6 respectively (mu-Mn = 2.4(1)mu-B, mu-Tb = 8.6(1)mu-B, mu-Ho = 8.4(1)mu-B). The results are discussed in terms of Ruderman-Kittel-Kasuy-Yoshida exchange interactions and compared with those obtained previously for the parent RFe6Al6 and RCo5 compounds.