Magnetic structure of R2CoGa8 (R = Gd, Tb, and Dy): Structural tuning of magnetic properties in layered Ga-based intermetallic compounds

In this work we have determined the magnetic structure of ${R}_{2}$CoGa${}_{8}$ ($R=$ Gd, Tb, and Dy) intermetallic compounds using x-ray resonant magnetic scattering in order to study the evolution of the anisotropic magnetic properties along the series for $R=$ Gd-Tm. The three compounds have a commensurate antiferromagnetic spin structure with a magnetic propagation vector $\stackrel{P\vec}{\ensuremath{\tau}}=$ $(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ and a N\'eel temperature of approximately 20, 28.5, and 15.2 K for $R=$ Gd, Tb, and Dy, respectively. The critical exponent $\ensuremath{\beta}$ obtained from the temperature dependence of the magnetic peaks suggest a three-dimensional universality class for the three compounds. Comparing the simulated and integrated intensities we conclude that the magnetic moment direction is in the $ab$ plane for the Gd${}_{2}$CoGa${}_{8}$ compound and parallel to the $c$ axis for the Tb${}_{2}$CoGa${}_{8}$ and Dy${}_{2}$CoGa${}_{8}$ compounds. The evolution of the magnetic properties of the ${R}_{2}$CoGa${}_{8}$ series for $R=$ Gd-Tm is discussed taking into account the indirect Ruderman-Kittel-Kasuya-Yoshida interaction and crystalline-electric field effects. The comparison between the reported magnetic properties of the Ga-based compounds with those for the In-based isostructural family reveals differences in their exchange couplings that contribute to the understanding of the role of the $f$-electron magnetism in these classes of materials.