A neutron diffraction study of HfFe6Ge6-type LuMn6Ge6-xGax compounds (x=0.4, 0.7 and 1.0)

The LuMn6Ge6-xGax compounds (x=0.4, 0.7, 1.0) were studied by magnetization measurements and neutron diffraction. The LuMn6Ge5Ga compound is ferromagnetic in its whole ordered range (T-c = 352 K). The Mn moments (2.02(8) mu (B) at 2 K) are close to the (001) plane. The LuMn6Ge5.3Ga0.7 compound displays a Neel point at 372 K and is helimagnetic in the whole temperature range investigated by neutron diffraction (2-300 K). At 2 K, the normal to the helical plane makes an angle of 45(1)degrees with the c-axis, and the moment is refined to mu (Mn) = 2.36(5) mu (B). The LuMn6Ge5.6Ga0.4 compound displays a Neel point at 451 K. Neutron diffraction indicates that. above T(t)similar to 290 K, the compound is colinear antiferromagnetic, as characterized by a doubling of the c-axis and with a moment direction parallel to the c-axis. Below T-t' similar to 230 K, the compound becomes helimagnetic. At 2 K, the normal to the helical plane makes an angle of 42(1)degrees with the c-axis, and the moment is refined to mu (Mn) = 2.24(4) mu (B). The corresponding q(z) component of the propagating vector [0.2082(4)] at 2 K is significantly different from that measured in the LuMn6Ge5.6Ga0.4 compound [0.1067(6)]. The effects of Ge substitution on the magnetic properties are discussed. (C) 2001 Elsevier Science BN. All rights reserved.