The magnetic ordering of ${\mathrm{Sm}}^{3+}$ and ${\mathrm{Gd}}^{3+}$ moments in some T′ and T* copper oxides and oxychlorides has been studied. The temperature-dependent magnetic susceptibility of ${\mathrm{CaGdCuO}}_3$Cl and ${\mathrm{Sr}}_{0.1}$${\mathrm{La}}_{0.9}$${\mathrm{Gd}}_{0.9}$${\mathrm{CuO}}_4$, both with T* structure, showed anomalies at 275 K and 150–275 K, respectively, which are attributed to the antiferromagnetic ordering of the copper spins in each system. A comparison between the magnetic ordering of ${\mathrm{Gd}}^{3+}$ and ${\mathrm{Sm}}^{3+}$ in the oxychlorides revealed that the ${\mathrm{Gd}}^{3+}$ moments order at 6.1 K, while those of ${\mathrm{Sm}}^{3+}$ do not order down to 2 K. In contrast, both the parent T′ oxides ${\mathrm{Gd}}_2$${\mathrm{CuO}}_4$ and ${\mathrm{Sm}}_2$${\mathrm{CuO}}_4$ order at 6.66 and 5.95 K, respectively. A mechanism for superexchange involving the rare-earth f orbitals and the surrounding oxygen or copper orbitals is proposed based on crystal-field arguments to explain the observed differences in the magnetic ordering of the ${\mathrm{Gd}}^{3+}$ and ${\mathrm{Sm}}^{3+}$ moments.

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