Magnetic structures of RMnSi 2 compounds (R = La, Ce, Pr, Nd) from neutron study

Abstract

Investigations by neutron diffraction measurements are reported on the ternary silicides RMnSi 2 (R = La to Nd) with the orthorhombic structure of TbFeSi 2 type (SG:Cmcm). This structure, closely related to the ThCr 2Si 2 type structure, can be described as isolated ThCr 2Si 2 blocks connected via α ThSi 2 slabs. Each of the R, Mn and Si atoms are arranged in alternate layers stacked (along the b-axis) with the sequence RSi(Mn 2)SiRSiSiRSi(Mn 2)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 (∼2μ B) are parallel to the b-axis. At lower temperatures PrMnSi 2 and NdMnSi 2 show an additional magnetic transition which corresponds to the ordering of the rare-earth sublattice. PrMnSi 2 orders antiferromagnetically below T N = 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 (μ Mn = 2.35(10)μ B, μ Pr = 2.04(4)μ B). Below T C = 40 K , NdMnSi 2 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 ( μ Mn = 2.25(10)μ B , μ Nd = 1.80(7)μ B ). At 2 K, a preliminary study shows that, in CeMnSi 2, 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 ( μ Mn = 2.24(15)μ B , μ Ce = 0.23(13)μ B ). The results are discussed in terms of RKKY exchange interactions and compared with those obtained previously on the corresponding ThCr 2Si 2 type ternary silicides and germanides.