In this paper the influence of the crystal structure on the magnetic ordering in ternary RT₂X₂ and RTX₃ (R is the rare earth metal, T the 3d, 4d or 5d transition metal and X = Si or Ge) compounds is studied. These compounds crystallize in three types of crystal structure which have been derived from the tetragonal BaAl₄ structure: in a body-centered ThCr₂Si₂ type structure (space group I4/mmm), in a primitive CaBe₂Ge₂-type (space group P4/nmm) and in a body-centered structure BaNiSn₃ (space group I4mm). The position of R atoms in these crystal structures causes the anisotropic character of magnetic interaction between the R ions, and the Ising-like model can be used to explain the stability of magnetic ordering. In the rare-earth sublattice the different magnetic structures, namely, collinear AFM and non-collinear AFM cosinusoidally modulated, are observed. The analysis of the magnetic structure of RT₂X₂ (X = Si, Ge) compounds reveals a strong dependence of magnetic ordering on the a/c ratio (a, c are the lattice constants). For a/c < 0.415, a simple collinear ordering is observed (for example, in (Nd,Tb,Dy,Ho,Er)Rh₂X₂, PrCu₂X₂, RIr₂Si₂, RCo₂X₂ – AF I type; in NdFe₂Si₂ and (Nd, Pr)Fe₂Ge₂ – AF II type and in (Gd, Tb, Dy, Ho, Er)Cu₂X₂ – AF IV type), while the compounds with a/c > 0.415 exhibit oscillatory magnetic structure (for example, in (Tb, Dy, Ho, Er)Os₂Si₂; in RPd₂X₂ and (Tb, Dy, Er)Pd₂Si₂ – LSW IV; in RRu₂X₂ and TbRu₂X₂ – LSW II. These relations are performed for large numbers of RT₂X₂ compounds. Another types of magnetic structure than result from this relation are observed for compounds with T = Mn, Fe and Au (for example, in TbFe₂X₂ – LSWIV).

The various magnetic structures observed above can be interpreted by the RKKY model of interactions. Such conclusion is supported by the fact that the nearest R – R distance is equal to the lattice parameters a of ~ 0.4 nm (in plane) and ~ 0.6 nm (between plane) and that these compounds are good electrical conductors. In isotropic RKKY model, the Fermi vector is dependent strongly on both the a/c ratio and on the number of free electrons Z per magnetic ion. The analysis of modulated magnetic structures RRu₂Si₂ (R = Tb – Er) based on RKKY model indicates that the order is stable for Z ≈ 3. This is in agreement with discussed covalent bonds in tetrahedron TX₄ in ThCr₂Si₂-type crystal structure.

The Néel temperatures in the RPd₂Si₂, RRu₂Si₂ and RRh₂Si₂ series depend randomly on the number of f-electrons. This fact suggests that the influence of crystalline electric field (CEF) should be also considered in explaining the magnetism in these compounds. The CEF at the R site can strongly affect the magnetic properties of the RT₂X₂ systems. The orientation of magnetic moment with respect to the fourfold axis is connected with the sign of the (CEF parameter). The positive sign at coefficient indicates that the magnetic moment is normal to the tetragonal axis or creates an angle φ with it. Its negative value indicates that the moment is aligned along the c-axis. As an example, the magnetic ordering in TbIrSi₃ and TbIr₂Si₂ will be discussed.

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