A study of the new HfFe6Ge6-type ZrMn6Sn6 and HfMn6Sn6 compounds by magnetization and neutron diffraction measurements

Bulk magnetization and neutron diffraction measurements have been performed on the new HfFe6Ge6-type (Pb/mmm) ZrMn6Sn6 and HfMn6Sn6 compounds. These c ompounds order antiferromagnetically below 580(5) and 575(5) K for R=Zr and Hf, respectively. At 300 K, both compounds exhibit a collinear easy-plane antiferromagnetic arrangement and the magnetic structure consists of ferrom agnetic (001) Mn planes antiferromagnetically coupled along the c-axis with the (+ - - +) sequence, i.e. a doubling of the c-axis of the chemical cell (mu(Mn)=1.98(7) mu(B) and mu(Mn)=2.05(6) mu(B) for ZrMn6Sn6 and HfMn6Sn6, respectively). At approximately 60 K, a commensurate-incommensurate transit ion occurs for both compounds, yielding an helical magnetic structure chara cterized by a thermal dependent k=[00q(z)] wave vector. At 2 K, q(z)=0.491( 1) and 0.481(1) r.l.u., while eta(Mn)=2.11(3) mu(B) and 2.18(2) mu(B) for Z rMn6Sn6 and HfMn6Sn6, respectively. A comparison with other RMn6Sn6 compoun ds involving a non-magnetic R element (R=Mg, Sc, Lu, Y) allows us to underl ine the influence of the R valency on the magnetic behavior of the Mn subla ttice in this class of materials. (C) 1999 Elsevier Science S.A. All rights reserved.