Coexisting Paramagnetic Spins and Long-Range Magnetic Order in Ba$_4$(Ru$_{0.92}$Ir$_{0.08}$)$_3$O$_{10}$

TL;DR

This study explores how dilute Ir substitution in Ba$_4$Ru$_3$O$_{10}$ affects its magnetic properties, revealing coexistence of long-range order and paramagnetic spins due to site-specific Ir occupation disrupting exchange pathways.

Contribution

It demonstrates that Ir doping preserves magnetic structure while introducing paramagnetic spins, and elucidates the atomic-level mechanisms behind this coexistence using combined experimental and computational methods.

Findings

Ir doping preserves the zigzag antiferromagnetic structure.

Paramagnetic spins coexist with long-range magnetic order.

Ir preferentially occupies the central Ru site, disrupting exchange pathways.

Abstract

We investigate the effect of dilute Ir substitution on the magnetism of the trimer-based ruthenate Ba$_4$Ru$_3$O$_{10}$ 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 $\rm Ru_3O_{12}$ trimer, while the central Ru(1) site remains nonmagnetic. The N\'eel temperature is reduced from $\approx\!105$ 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.