# Frustrated spin-1/2 molecular magnetism in the mixed-valence antiferromagnets Ba3MRu2O9 (M = In, Y, Lu)

## TL;DR

This study investigates the magnetic properties of Ba3MRu2O9 compounds, revealing that Ru dimers act as molecular units with spin-1/2 moments on a frustrated triangular lattice, leading to complex magnetic ground states.

## Contribution

It demonstrates that Ru dimers behave as molecular units with distributed spin-1/2 moments, challenging previous charge order and double exchange models.

## Key findings

- Molecular units form a frustrated triangular lattice.
- Lu sample exhibits long-range magnetic order.
- Y and In samples show disordered static magnetism.

## Abstract

We have performed magnetic susceptibility, heat capacity, muon spin relaxation, and neutron scattering measurements on three members of the family Ba3MRu2O9, where M = In, Y and Lu. These systems consist of mixed-valence Ru dimers on a triangular lattice with antiferromagnetic interdimer exchange. Although previous work has argued that charge order within the dimers or intradimer double exchange plays an important role in determining the magnetic properties, our results suggest that the dimers are better described as molecular units due to significant orbital hybridization, resulting in one spin-1/2 moment distributed equally over the two Ru sites. These molecular building blocks form a frustrated, quasi-two-dimensional triangular lattice. Our zero and longitudinal field muSR results indicate that the molecular moments develop a collective, static magnetic ground state, with oscillations of the zero field muon spin polarization indicative of long-range magnetic order in the Lu sample. The static magnetism is much more disordered in the Y and In samples, but they do not appear to be conventional spin glasses.