Neutron diffraction study of the ZrMn6Ge6, LuMn6Ge6 and ScMn6Ge6 compounds

The new hexagonal HfFe6Ge6-type ZrMn6Ge6 compound orders antiferromagnetically below T-N = 640 K and undergoes a second transition at T-t = 50 K. Neutron diffraction experiments were performed on the three ZrMn6Ge6, LuMn6Ge6 and ScMn6Ge6 compounds. At 300 K, they exhibit collinear easy-axis antiferromagnetic arrangements and the magnetic structure consists of ferromagnetic (001) Mn planes antiferromagnetically coupled along the c-axis with a ( + - - +) sequence (mu(Mn) approximate to 2.06, 1.86 and 1.94 mu(B) for Zr-, Lu- and ScMn6Ge6, respectively). At low temperature, spin reorientation processes occur, yielding incommensurate antiferromagnetic arrangements. Below T-t, in both ZrMn6Ge6 and LuMn6Ge6 (T-t similar to 30 K), the Mn moments form a double-cone structure (mu(Mn) approximate to 2.1 mu(B), semi-cone angle alpha approximate to 6 degrees) as already found in YMn6Ge6 (T-t similar to 80 K) whereas only preliminary results are available for ScMn6Ge6 below T-t = 255 K. These results lead us to strongly revise previous conclusions concerning LuMn6Ge6 and ScMn6Ge6 with more reasonable Mn magnetic moment values (similar to 2 mu(B) against the similar to 1 mu(B) value previously published). (C) 1999 Elsevier Science Ltd. All rights reserved.