Magnetic structures ofPrFeSi2andNdFeSi2from neutron and Mössbauer studies

Investigations by neutron diffraction and ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer measurements are reported on the ternary silicides R${\mathrm{FeSi}}_2$ (R=r/rPr,Nd) with the orthorhombic structure of the ${\mathrm{TbFeSi}}_2$-type (space group Cmcm). This structure, which is closely related to the ${\mathrm{ThCr}}_2$ ${\mathrm{Si}}_2$-type structure, can be described as isolated ${\mathrm{ThCr}}_2$ ${\mathrm{Si}}_2$ blocks connected via α-${\mathrm{ThSi}}_2$ slabs. ${\mathrm{PrFeSi}}_2$ orders ferromagnetically below ${\mathit{T}}_{\mathit{c}}$=26 K with magnetic moments [2.58(1)${\mathrm{\mu }}_{\mathit{B}}$ at 1.5 K] parallel to the b axis. ${\mathrm{NdFeSi}}_2$ exhibits a more complex magnetic behavior. Below ${\mathit{T}}_{\mathit{N}}$=6.5 K, it develops a sine-modulated antiferromagnetic structure, q=(0, 0.591, 0), of the Nd moments parallel to the b axis with a maximum value of 2.52${\mathrm{\mu }}_{\mathit{B}}$ at 4.2 K. No local moment was detected at the iron site. ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer data on ${\mathrm{NdFeSi}}_2$ indicate a progressive squaring of the sine modulation of the Nd moments with a pure square wave below 3 K. The comparison of the transferred hyperfine fields at the iron nuclei in ${\mathrm{PrFeSi}}_2$ and ${\mathrm{NdFeSi}}_2$ suggests the occurrence of orbital polarization of the conduction electrons giving rise to anisotropic contributions to the hyperfine field and exchange interactions. The results are discussed in terms of Ruderman-Kittel-Kasuya-Yosida exchange interactions and compared to those obtained previously on the ${\mathrm{ThCr}}_2$ ${\mathrm{Si}}_2$-type ternary silicides.