NEUTRON-DIFFRACTION STUDY OF CEMN2GE2, PRMN2GE2 AND NDMN2GE2 - EVIDENCE OF DOMINANT ANTIFERROMAGNETIC COMPONENTS WITHIN THE (001) MN PLANESIN 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 diffr action 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 antiferro magnetic components coexist. In the temperature range 2-300 K, CeMn2Ge 2 (T-c=320 K) exhibits an easy axis conical magnetic structure (cone s emiangle (alpha approximate to 60 degrees) with a total Mn moment valu e 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) compou nds exhibit an easy axis canted ferromagnetic structure (phi approxima te to 60 degrees) while, at lower temperature (T<250 K), a conical mag netic structure takes place on the Mn sublattice (alpha approximate to 60 degrees). The rare earth sublattices orders ferromagnetically belo w about 100 K. In PrMn2Ge2, an easy axis prevails in the whole tempera ture 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 mom ent 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 a n ordering temperature T-N not detected by magnetic moments, occurs at a temperature much higher than the Curie point. The results are compa red with those of the related RMnSi, RMnGe and RMnSi(2) compounds. The evolution of the magnetic behaviour within the ThCr2Si2-type structur e RMn(2)X(2) (X=Si, Ge) compounds is discussed.