We illustrate the magnetic differences between the triangular-dimer-lattice compounds ${\mathrm{Rb}}_2{\mathrm{Ni}}_2{\left({\mathrm{SeO}}_3\right)}_3$ and ${\mathrm{K}}_2{\mathrm{Co}}_2{\left({\mathrm{SeO}}_3\right)}_3$. Synthesized ${\mathrm{Rb}}_2{\mathrm{Ni}}_2{\left({\mathrm{SeO}}_3\right)}_3$ is magnetically ordered at 5.8 K and undergoes two-step antiferromagnetic (AFM) transitions when magnetic field is applied along the easy $c$ axis $(H//c)$. The high-field magnetization at 2 K for $H//c$ demonstrates a 1/3 magnetization plateau, followed by a possible narrow 1/2 plateau. Density functional theory calculations reveal that the exchange within the structural dimer $\left(J_0\right)$ is significantly smaller than the exchange in the triangular-lattice layer $\left(J_1\right)$ and, thus, ${\mathrm{Rb}}_2{\mathrm{Ni}}_2{\left({\mathrm{SeO}}_3\right)}_3$ behaves like an easy-axis triangular lattice antiferromagnet. However, for ${\mathrm{K}}_2{\mathrm{Co}}_2{\left({\mathrm{SeO}}_3\right)}_3$, which is disordered down to 2 K, the high-field magnetization at 1.6 K shows two wide 1/3 and 1/2 magnetization plateaus as well as a narrow 2/3 plateau for $H//c$. The magnetic characterization results show that ${\mathrm{K}}_2{\mathrm{Co}}_2{\left({\mathrm{SeO}}_3\right)}_3$ is a coupled dimer antiferromagnet. The competing interplay between the AFM dimer and triangular lattice, as well as additional anisotropic exchanges (for the Co compound), might lead to the emergence of multiple magnetization plateaus in triangular-dimer-lattice antiferromagents.

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• High magnetic fields
• Magnetic phase transitions
• Magnetism
• Magnetization measurements
• Sample preparation