Role of Disorder in Governing the Magnetic Properties of Cu2IrO3

TL;DR

The paper examines how synthesis-driven antisite disorder in Cu2IrO3 influences its magnetic ground state in the context of Kitaev physics. Using a prolonged topotactic synthesis at 320 °C, the authors quantify ~25% Cu–Ir antisite disorder and detect mixed Cu and Ir valence states via XPS/XANES, EXAFS, and PDF. Magnetic measurements reveal competing antiferromagnetic interactions and frustrated motifs that generate dynamically fluctuating AFM clusters around 80 K, which freeze into a spin-glass-like state below 29 K, with cluster dynamics supported by AC susceptibility analyses and aging effects. These results demonstrate that disorder, tuned by synthesis, is a crucial control parameter for the magnetic ground state in Cu2IrO3 and may offer a route to engineering emergent quantum phases in Kitaev-related materials.

Abstract

Cu$_2$IrO$_3$ is a honeycomb iridate which has been studied recently as a candidate Kitaev quantum spin liquid. Its magnetic ground state however, has been reported to be quantum disordered, spin glassy, or magnetically ordered depending on synthesis details. We have prepared a Cu$_2$IrO$_3$ sample with large antisite disorder and studied in detail its structure (global and local), charge states, and thermodynamic properties to try to quantify and characterize the disorder and its connection to the magnetic ground state. X-ray diffraction, Extended x-ray absorption fine structure(EXAFS) and X-ray pair distribution function analysis revealed a large site disorder ($\sim$25\%), while XPS and XANES reveal mixed valence of Cu and Ir following Cu$^{1+}$ + Ir$^{4+}$ $\rightarrow$ Cu$^{2+}$ + Ir$^{3+}$. This combination of site disorder and charge redistribution generates competing antiferromagnetic interactions and magnetic frustration, resulting in dynamically fluctuating AFM clusters near 80K that freeze below 29K. These results demonstrate the crucial role of synthesis dependent disorder in determining the magnetic ground state of Cu$_2$IrO$_3$.

Figures (15)

• Figure 1: Rietveld refinement of polycrystalline Cu$_2$IrO$_3$ using X-ray data collected at $\lambda = 0.207371\,\text{\AA}$, confirming that the synthesized powder sample contains no impurity phases.
• Figure 2: The deconvoluted Cu 2p and O 1s core-level X-ray photoelectron spectra (XPS) of Cu$_2$IrO$_3$ confirming mixed valence state of Cu.
• Figure 3: X-ray absorption spectra at the Cu L-edge for Cu$_2$IrO$_3$ and reference CuO. The comparison clearly demonstrates the presence of both Cu$^{1+}$ and Cu$^{2+}$ oxidation states in Cu$_2$IrO$_3$ . Distinct spectral features corresponding to the unoccupied 3d states of Cu$^{2+}$, along with those characteristic of Cu$^{1+}$, are observed and are marked with arrows in the figure.
• Figure 4: Normalized XANES spectra at the Ir L$_3$-edge, with the inset showing a magnified view of the white-line feature. Vertical dashed lines serve as visual guides.
• Figure 5: Normalized XANES spectra at the Cu K-edge, with the inset highlighting the inflection points of the Cu K-edge spectra used to determine the Cu valence state. The analysis indicates that Cu in Cu$_2$IrO$_3$ is predominantly in the +1 oxidation state, with a minor contribution from Cu$^{2+}$, as confirmed by comparison with reference CuO (marked arrow).