Two members of the isostructural $R_2{\text{CoGa}}_8$ intermetallic series, ${\text{Er}}_2{\text{CoGa}}_8$ and ${\text{Tm}}_2{\text{CoGa}}_8$, have been studied by powder neutron diffraction. Antiferromagnetic ordering of the rare-earth sublattices was confirmed to occur at 3.0 K and 2.0 K, respectively. Furthermore, determination of the critical exponent showed ${\text{Er}}_2{\text{CoGa}}_8$ to adopt a three-dimensional universality class. In spite of a common magnetic easy axis and similar structural characteristics, the antiferromagnetic structures were found to be different for the erbium- and thulium-based compounds. The corresponding magnetic space groups were determined to be $P_{2a}mm{m}^{\prime }$ and $P_{C}mmm$. The difference in magnetic structures is discussed based on crystal electric field effects that are known to be prevalent in such materials.

• Received 29 March 2010

DOI:https://doi.org/10.1103/PhysRevB.82.104407