Spin-lozenge thermodynamics and magnetic excitations in Na$_3$RuO$_4$

TL;DR

This study investigates the magnetic properties of Na$_3$RuO$_4$ using neutron scattering and thermodynamic measurements, finding that the material is better described as a pair of inequivalent dimers rather than the previously proposed lozenge model.

Contribution

The paper provides a detailed analysis of the magnetic excitations in Na$_3$RuO$_4$, challenging the existing lozenge model and proposing a dimer-based description based on experimental data.

Findings

Lozenge model not supported by data

Evidence for a double dimer system

Long-range magnetic order observed below 28 K

Abstract

We report inelastic and elastic neutron scattering, magnetic susceptibility, and heat capacity measurements of polycrystalline sodium ruthenate (Na$_3$RuO$_4$). Previous work suggests this material consists of isolated tetramers of $S=3/2$ Ru$^{5+}$ ions in a so-called "lozenge" configuration. Using a Heisenberg antiferromagnet Hamiltonian, we analytically determine the energy eigenstates for general spin $S$. From this model, the neutron scattering cross-sections for excitations associated with spin-3/2 spin-tetramer configurations is determined. Comparison of magnetic susceptibility and inelastic neutron scattering results shows that the proposed "lozenge" model is not distinctly supported, but provides evidence that the system may be better described as a pair of non-interacting inequivalent dimers, \textit{i.e} double dimers. However, the existence of long-range magnetic order below $T_c \approx 28$ K immediately questions such a description. Although no evidence of the lozenge model is observed, future studies on single crystals may further clarify the appropriate magnetic Hamiltonian.