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 manganese atom coordination of the rare earth, this structure appears closely related to the CaCu6- 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 (001) layers of rare earth and manganese with the coupling sequence Mn(+)R(−)Mn(+)Mn(+)-R (−)Mn(+) (μMn ≈ 2.0(1)μB, μTb = 4.9(1)μB, μHo = 4.7(1)μB). For HoMn6Sn6, the magnetic moments lie in the (001) plane while in TbMn6Sn6, they deviate by φ = 15° 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° from this axis for TbMn6Sn6 and HoMn6Sn6 respectively μMn = 2.4(1)μB, μTb = 8.6(1)μB, μHo = 8.4(1)μB). The results are discussed in terms of Ruderman-Kittel-Kasuy-Yoshida exchange interactions and compared with those obtained previously for the parent RFe6Al6 and RCo6 compounds.