NEUTRON-DIFFRACTION STUDIES OF LAMN2GE2 AND LAMN2SI2 COMPOUNDS - EVIDENCE OF DOMINANT ANTIFERROMAGNETIC COMPONENTS WITHIN THE MN PLANES

The magnetic properties of ThCr2Si2-type structure LaMn2Ge2 and LaMn2Si2 compounds have been reinvestigated by neutron diffraction experiments. The ferromagnetic ordering previously proposed to take place on the manganese sublattice is revised. At high temperature, both compounds are purely collinear antiferromagnets (not detected by magnetic measurements), characterized by a stacking of antiferromagnetic (001) Mn planes. Below T(c) = 310 and 325 K for LaMn2Ge2 and LaMn2Si2, respectively, both compounds exhibit an easy-axis ferromagnetic behaviour. However, the occurrence of a dominant antiferromagnetic component within the (001) Mn planes yields a conical magnetic structure for the germanide (cone semi-angle alpha = 58-degrees at 2 K) and a canted magnetic structure for the silicide (phi = 49-degrees). At 2 K, the total Mn moments are about 3.0 and 2.4 mu(B) for LaMn2Ge2 and LaMn2Si2, respectively. The results are compared with those of closely related RMnSi and RMnGe compounds and the magnetic properties of the ThCrSi2-type structure RMn2X2 (X = Si, Ge) compounds are discussed.