NEUTRON-DIFFRACTION STUDY OF CEMN2GE2, PRMN2GE2 AND NDMN2GE2 - EVIDENCE OF DOMINANT ANTIFERROMAGNETIC COMPONENTS WITHIN THE (001) MN PLANES IN FERROMAGNETIC THCR2SI2-TYPE MANGANESE TERNARY COMPOUNDS

The magnetic properties of ThCr2Si2-type structure RMn(2)Ge(2) (R=Ce, Pr, Nd) compounds have been reinvestigated by the use of neutron diffraction experiments. The ferromagnetic ordering previously proposed to take place on the manganese sublattice is revised. The three compounds are characterized by mixed (001) Mn planes where ferro- and antiferromagnetic components coexist. In the temperature range 2-300 K, CeMn2Ge2 (T-c=320 K) exhibits an easy axis conical magnetic structure (cone semiangle (alpha approximate to 60 degrees) with a total Mn moment value of about 2.7 mu(B), at 2 K. Ce moments do not order above 1.6 K. At room temperature, PrMn2Ge2 (T-c=330 K) and NdMn2Ge2 (T-c=330 K) compounds exhibit an easy axis canted ferromagnetic structure (phi approximate to 60 degrees) while, at lower temperature (T<250 K), a conical magnetic structure takes place on the Mn sublattice (alpha approximate to 60 degrees). The rare earth sublattices orders ferromagnetically below about 100 K. In PrMn2Ge2, an easy axis prevails in the whole temperature range. At 2 K, the total Mn moment is about 2.8 mu(B), and the Pr moment value is about 29 mu(B). On the contrary, in NdMn,Ge, the easy axis rotates to the (001) plane below 250 K. At 2 K, the total Mn moment is about 2.7 mu(B), and the Nd moment value is about 2.3 mu(B). In all cases, the thermal dependence of the AF component suggests that an ordering temperature T-N not detected by magnetic moments, occurs at a temperature much higher than the Curie point. The results are compared with those of the related RMnSi, RMnGe and RMnSi(2) compounds. The evolution of the magnetic behaviour within the ThCr2Si2-type structure RMn(2)X(2) (X=Si, Ge) compounds is discussed.