Magnetic structures in the ternaryRM2X2compounds (R=Gdto Tm;M=Fe, Co, Ni, or Cu;X=Sior Ge)

Neutron-diffraction measurements were performed on polycrystalline samples of 13 different compounds of the type ${\mathrm{RM}}_2$ $X_2$ (R, a heavy lanthanide from Gd to Tm; M, the 3d element Fe, Co, Ni, or Cu; X, Si or Ge), at room temperature and at low temperatures. The crystallographic structure of these compounds belongs to the tetragonal space group I4/mmm ($D_{4h}^{17}$). It was found that these compounds, paramagnetic at room temperature, undergo a transition to antiferromagnetism at low temperatures. The transition temperatures and most of magnetic structures were determined. Below the transition temperature, magnetic ordering was found only on the R sublattice, and no magnetic ordering was detected on the M sublattice. The magnetic lattices were found to be characterized by propagation vectors of k→=(0,0,1) in the cobalt compounds investigated (${\mathrm{TbCo}}_2$ ${\mathrm{Si}}_2$, ${\mathrm{HoCo}}_2$ ${\mathrm{Si}}_2$, ${\mathrm{DyCo}}_2$ ${\mathrm{Ge}}_2$, and ${\mathrm{ErCo}}_2$ ${\mathrm{Ge}}_2$), k→=((1/2),0,(1/2)) in the copper compounds investigated (${\mathrm{TbCu}}_2$ ${\mathrm{Si}}_2$, ${\mathrm{HoCu}}_2$ ${\mathrm{Si}}_2$, ${\mathrm{TbCu}}_2$ ${\mathrm{Ge}}_2$, and ${\mathrm{HoCu}}_2$ ${\mathrm{Ge}}_2$), and incommensurate k→=(0,0,0.76) in ${\mathrm{HoNi}}_2$ ${\mathrm{Ge}}_2$ (unsolved incommensurate in other nickel compounds and in iron compounds). Together with previously published results on other ${\mathrm{RM}}_2$ $X_2$ compounds, the magnetic lattice is found to vary with M and be almost invariant to R.