We report detailed magnetic, transport, heat-capacity, and neutron-diffraction measurements of ${\mathrm{Ba}}_5{\mathrm{Ru}}_3{\mathrm{O}}_{12}$, a compound consisting of isolated ${\mathrm{Ru}}_3{\mathrm{O}}_{12}$ trimers. We show that this system develops long-range antiferromagnetic ordering at $T_{\mathrm{N}}\sim 60\mathrm{K}$ without structural distortion or metal-insulator type transition, which is in sharp contrast to other barium ruthenate trimer systems such as $9\mathrm{R}\text{−}{\mathrm{BaRuO}}_3$ and ${\mathrm{Ba}}_4{\mathrm{Ru}}_3{\mathrm{O}}_{10}$. A complex magnetic structure is revealed which is attributable to the magnetic frustration due to competing exchange interactions between Ru ions on different crystallographic sites within the ${\mathrm{Ru}}_3{\mathrm{O}}_{12}$ trimer and different degree of orbital hybridization on different Ru sites. We have also investigated the magnetic structure of ${\mathrm{Ba}}_4{\mathrm{Ru}}_3{\mathrm{O}}_{10}$ for comparisons.

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• Frustrated magnetism
• Magnetism
• Ruthenates
• Magnetization measurements
• Neutron diffraction
• Specific heat measurements