We investigate the effect of dilute Ir substitution on the magnetism of the trimer-based ruthenate ${\mathrm{Ba}}_4{\mathrm{Ru}}_3{\mathrm{O}}_{10}$ using neutron diffraction, magnetic susceptibility measurements, first-principles calculations, and atomistic simulations. Neutron diffraction shows that Ir doping preserves the zigzag antiferromagnetic structure and the ordered-moment magnitude of the parent compound, in which the moments reside exclusively on the two outer Ru(2) sites of each ${\mathrm{Ru}}_3{\mathrm{O}}_{12}$ trimer, while the central Ru(1) site remains nonmagnetic. The Néel temperature is reduced from $\sim 105$ to 84.0(1) K upon 8% Ir substitution, while magnetic susceptibility reveals a pronounced low-temperature Curie-like upturn, indicating the coexistence of paramagnetic spins with long-range antiferromagnetic order. Density-functional calculations show that Ir preferentially occupies the central Ru(1) site, which disrupts the exchange pathways of the affected ${\mathrm{Ru}}_3{\mathrm{O}}_{12}$ trimer state. Atomistic simulations incorporating this paramagnetic dilution reproduce the suppressed ordering temperature and the coexistence of ordered and paramagnetic components.

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• Antiferromagnetism
• Doping effects
• Exchange interaction
• Magnetic interactions
• Magnetic order
• Magnetic susceptibility