Neutron diffraction studies of LaMn2Ge2Si2 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 =58° at 2 K) and a canted magnetic structure for the silicide (φ=49°). At 2 K, the total Mn moments are about 3.0 and 2.4 μ_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 ThCr₂Si₂-type structure RMn₂X₂ (XSi, Ge) compounds are discussed.