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 $\vec{\tau }=$ $(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ and a Néel temperature of approximately 20, 28.5, and 15.2 K for $R=$ Gd, Tb, and Dy, respectively. The critical exponent $\beta$ 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.

• Received 8 April 2013
• Revised 27 January 2014

DOI:https://doi.org/10.1103/PhysRevB.89.115103