HIGH RARE-EARTH SUBLATTICE ORDERING TEMPERATURES IN RMNSI COMPOUNDS (R=LA-SM, GD) STUDIED BY SUSCEPTIBILITY MEASUREMENTS AND NEUTRON-DIFFRACTION

Investigations by bulk magnetization and neutron diffraction measurements are reported on the ternary silicides RMnSi (R = La-Nd). All these compounds crystallize in the well-known tetragonal structure of the CeFeSi type (space group P4/nmm). This structure, which is closely related to the ThCr2Si2- and TbFeSi2-type structures, can be described as isolated ''ThCr2Si2 blocks'' connected via R-R contacts. The R, Mn and Si atoms are arranged in alternate layers stacked along the c axis in the sequence RSi(Mn2)SiRRSi(Mn2)SiR. All these compounds are antiferromagnetic below T(N) almost-equal-to 310, 240, 265 and 280 K for La-, Ce-, Pr- and NdMnSi respectively. Their magnetic structures are characterized by a stacking of antiferromagnetic (001) Mn layers. The Mn magnetic moments (about 3 mu(B)) are at 45-degrees to c axis (LaMnSi) or in the (001) plane. At lower temperature PrMnSi and NdMnSi show additional transitions which correspond to an antiferromagnetic ordering of the rare earth sublattice (T1 = 130 and 185 K for Pr and Nd respectively) followed by a spin reorientation process simultaneously with a crystallographic phase transition (tetragonal to orthorhombic symmetry) at about 80 K for both compounds. At 2 K the antiferromagnetic (AF) structure of PrMnSi and NdMnSi consists of a stacking of ferromagnetic and antiferromagnetic (001) layers of R and Mn respectively in the sequence Mn(AF) R(+)R(+) Mn(AF) R(-)R(-); the moments are in the layers, collinear in PrMnSi and in quadrature in NdMnSi. In CeMnSi the ferromagnetic (001) cerium layers order antiferromagnetically below about 175 K and are coupled with the manganese sublattice according to the following scheme: Ce(+) Mn(AF) Ce(+)Ce(-) Mn(AF) Ce(+); at 2 K the Ce moment value is mu(Ce) = 0.78(4) mu(B). The results are discussed and compared with those obtained previously for the corresponding ternary silicides and germanides of the CeFeSi-, TbFeSi2- and ThCr2Si2-type structures.