Magnetization studies of RMn₆Ge_6-xGa_x single crystals (R=Sc, Y, Gd-Lu; 0.63≤x≤1.00)

The magnetic properties of single crystals of rare-earth compounds of composition RMn₆Ge_6-xGa_x (x ≈ 1) have been studied. In these compounds, the Mn sublattice orders ferromagnetically with a preferred moment direction perpendicular to the c axis. In the compounds in which also the R component carries a magnetic moment, the latter couple antiparallel to the Mn moments. Upon cooling from room temperature to cryogenic temperatures, the R-sublattice anisotropy becomes the dominant contribution to the total magnetocrystalline anisotropy, which leads to a spin-reorientation transition at intermediate temperatures for R=Tb-Yb. At low temperatures, the preferred moment directions are characterized by an easy axis for R=Tb, Er, Tm, and Yb and by an easy cone for R=Dy and Ho. A satisfactory account of the observed preferred moment directions has been given in terms of crystal-field theory. Measurements of the magnetization in the main crystallographic directions have been made in high magnetic fields, up to about 55 T. The interplay between the antiferromagnetic intersublattice R-Mn coupling and the magnetocrystalline anisotropy leads to field-induced magnetic phase transitions which are particularly pronounced in the compounds with Ho, Er and Tm.