MAGNETIC-STRUCTURES OF LAMNSI2, CEMNSI2, PRMNSI2, NDMNSI2 COMPOUNDS FROM NEUTRON STUDY

Investigations by neutron diffraction measurements are reported on the ternary silicides RMnSi2 (R = La to Nd) with the orthorhombic structure of TbFeSi2 type (SG:Cmcm). This structure, closely related to the ThCr2Si2 type structure, can be described as isolated ThCr2Si2 blocks connected via α ThSi2 slabs. Each of the R, Mn and Si atoms are arranged in alternate layers stacked (along the b-axis) with the sequence RSi(Mn2)SiRSiSiRSi(Mn2)SiR. The Mn-Mn intralayer distances (∼2.9Å) are considerably shorter than the Mn-Mn interlayers distances (∼9Å). At room temperature, all four compounds order ferromagnetically. The magnetic moments of Mn atoms (∼2μB) are parallel to the b-axis. At lower temperatures PrMnSi2 and NdMnSi2 show an additional magnetic transition which corresponds to the ordering of the rare-earth sublattice. PrMnSi2 orders antiferromagnetically below TN = 35 K; its magnetic structure consists of ferromagnetic (010) layers of Pr and Mn with moments perpendicular to the layers. Each Mn layer is antiferromagnetically coupled to the two adjacent ferromagnetically coupled Pr layers (μMn = 2.35(10)μB, μPr = 2.04(4)μB). Below TC = 40 K, NdMnSi2 is still ferroma gnetic. Nd and Mn magnetic moments lie in the (011) plane, along [001] and at 45° of this direction for Nd and Mn, respectively (μMn = 2.25(10)μB, μNd = 1.80(7)μB). At 2 K, a preliminary study shows that, in CeMnSi2, the cerium sublattice would be ferromagnetic and antiferromagnetically coupled with the manganese sublattice according to the following scheme: -(+)--(+)-, reference to the stacking sequence previously defined (μMn = 2.24(15)μB, μCe = 0.23(13)μB). The results are discussed in terms of RKKY exchange interactions and compared with those obtained previously on the corresponding ThCr2Si2 type ternary silicides and germanides. © 1990.