Magnetic ordering in Er₂Fe₂Si₂C and Tm₂Fe₂Si₂C

The magnetic ordering of two members of the R₂Fe₂Si₂C (R = rare-earth) series of compounds (monoclinic Dy₂Fe₂Si₂C-type structure with the C2/m space group), Er₂Fe₂Si₂C and Tm₂Fe₂Si₂C, have been studied by neutron powder diffraction and ¹⁶⁶Er Mössbauer spectroscopy, complemented by magnetisation and specific heat measurements. In both cases, antiferromagnetic ordering of the R sublattice is observed, with Neel temperatures of 4.8(2) K and 2.6(3) K for Er₂Fe₂Si₂C and Tm₂Fe₂Si₂C, respectively. The magnetic structures of the Erand Tm-based compounds are quite different from those found for the other members of the R₂Fe₂Si₂C series. Previous studies show that the common magnetic structure of the heavy- R₂Fe₂Si₂C compounds involves ordering of the R sublattice along the b-axis with a propagation vector k = [0, 0, ½]. However, the antiferromagnetic structure of the Er sublattice in Er₂Fe₂Si₂C is described by k = [½, ½, 0] with the Er magnetic moments lying close to the ac-plane. Tm₂Fe₂Si₂C is found to exhibit a more complex magnetic structure that is characterised by a square-wave modulation of the Tm magnetic moments along the a-axis and a cell-doubling along the b-axis with k = [0.403(1), ½, 0]. The differences in the magnetic structures of these compounds are interpreted in terms of the RKKY exchange interaction, which depends on the R-R interatomic distances, and crystal field effects acting on the R³⁺ ions.