We investigate the effect of dilute Ir substitution on the magnetism of the trimer-based ruthenate Ba4Ru3O10 using neutron diffraction, magnetic susceptibility measurements, atomistic simulations, and first-principles calculations. 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 Ru3O12 trimer, while the central Ru(1) site remains nonmagnetic. The N\'eel temperature is reduced from ≈105 K 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 shows that Ir preferentially occupies the central Ru(1) site, where its extended 5d orbitals disrupt the Ru-Ru molecular-orbital network and intra/inter-trimer exchange pathways. Atomistic simulations incorporating this paramagnetic dilution reproduce the suppressed ordering temperature and the coexistence of ordered and paramagnetic components.
@article{arxiv.2603.05715,
title = {Coexisting Paramagnetic Spins and Long-Range Magnetic Order in Ba$_4$(Ru$_{0.92}$Ir$_{0.08}$)$_3$O$_{10}$},
author = {Farhan Islam and Jiasen Guo and Wei Tian and Bing Li and Xudong Huai and Thao T. Tran and Gang Cao and Zachary Morgan and Feng Ye},
journal= {arXiv preprint arXiv:2603.05715},
year = {2026}
}