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 LaMn2S i2 compounds have been reinvestigated by neutron diffraction experimen ts. The ferromagnetic ordering previously proposed to take place on th e manganese sublattice is revised. At high temperature, both compounds are purely collinear antiferromagnets (not detected by magnetic measu rements), characterized by a stacking of antiferromagnetic (001) Mn pl anes. Below T(c) = 310 and 325 K for LaMn2Ge2 and LaMn2Si2, respective ly, both compounds exhibit an easy-axis ferromagnetic behaviour. Howev er, the occurrence of a dominant antiferromagnetic component within th e (001) Mn planes yields a conical magnetic structure for the germanid e (cone semi-angle alpha = 58-degrees at 2 K) and a canted magnetic st ructure for the silicide (phi = 49-degrees). At 2 K, the total Mn mome nts are about 3.0 and 2.4 mu(B) for LaMn2Ge2 and LaMn2Si2, respectivel y. The results are compared with those of closely related RMnSi and RM nGe compounds and the magnetic properties of the ThCrSi2-type structur e RMn2X2 (X = Si, Ge) compounds are discussed.