A neutron diffraction study of HfFe6Ge6-type ErMn6Ge6-xGax compounds (x-0.2, 0.4 and 1.0)

The HfFe6Ge6-type ErMn6Ge6-xGax compounds (x=0.2, 0.4, 1.0) have been studied by powder neutron diffraction. The ErMn6Ge5.8Ga0.2 compound orders antiferromagnetically at 460 K in a collinear easy axis magnetic structure characterized by a doubling of the c-axis. Below T-1=270 K, a helimagnetic structure takes place and persists down to 50 K. Below this temperature, a ferrimagnetic structure with moments aligned along the c-axis is stable (mu(Mn) = 2.21(5) mu(B) and mu(Br) = 8.60(8) mu(B)). The ErMn6Ge5.0Ga0.4 compound orders antiferromagnetically at 394 K. From 300 to 120 K, it is characterized by a helimagnetic arrangement. In both cases, the normal to the c-axis is perpendicular to the [001] direction. Below 110 K, a ferrimagnetic structure takes place and the direction of the ferromagnetic component is aligned along the c-axis. The Mn and Er moments refined at 2 K are mu(Mn)=2.21(6) mu(B) and mu(Br)=9.06(11) mu(B). The ErMn6Ge5Ga compound orders ferri magnetically at 373 K and a moment reorientation takes place around 200 K. At high temperature the moments are close to the (001) plane whereas at low temperature they align along the c-axis. The Mn and Er moments refined at 2 K are mu(Mn)=2.23(6) mu(B) and mu(Er)=9.40(12) mu(B). Evolutions of the moment magnitudes, magnetocrystalline anisotropy and interlayer couplings, as a function of the Ga content and the nature of the rare earth element. are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.