Cu2iro3 Disorder Enables Understanding of Competing Magnetic Ground States and Properties

The complex magnetic behaviour of copper iridium oxide (Cu₂IrO₃) continues to intrigue physicists, with recent investigations suggesting it may host an exotic Kitaev spin liquid state. Priyanka Yadav, Sumit Sarkar, and Vishal Kumar, from the Indian Institutes of Science Education and Research and Technology, alongside colleagues including Martin Karlsen and Martin Etter at DESY in Germany, have now delved into the critical influence of structural disorder on this material’s magnetic properties. Their detailed analysis of a highly disordered Cu₂IrO₃ sample reveals a significant 25% antisite defect concentration alongside unusual mixed valence states of copper and iridium. This combination of disorder and charge redistribution fosters competing magnetic interactions and frustration, leading to the formation of fluctuating antiferromagnetic clusters which ultimately freeze at low temperatures. The research highlights how subtle variations in synthesis can dramatically alter the magnetic ground state of Cu₂IrO₃, offering vital clues in the search for novel quantum magnetic materials.

Their detailed analysis of a highly disordered Cu₂IrO₃ sample reveals a significant 25% antisite defect concentration alongside unusual mixed valence states of copper and iridium. This combination of disorder and charge redistribution fosters competing magnetic interactions and frustration, leading to the formation of fluctuating antiferromagnetic clusters which ultimately freeze at low temperatures.

Cu₂IrO₃ is a layered perovskite system exhibiting a complex interplay between spin-orbit coupling, structural distortions and electronic correlations, and its magnetic properties are highly sensitive to subtle changes in its chemical composition and atomic arrangement. This work investigates the role of disorder, specifically cation mixing between Cu and Ir, on the magnetic behaviour of Cu₂IrO₃ through a combination of experimental and theoretical techniques. Rietveld refinement of powder X-ray diffraction data reveals that as-synthesised samples exhibit a small degree of cation mixing, estimated to be approximately 7%.

Researchers synthesised Cu₂IrO₃ via a 48-hour topotactic reaction at 320°C, employing a sodium-to-copper ion exchange from Na₂IrO₃ and CuCl in a 1:2.2 molar ratio within an argon atmosphere. This prolonged reaction, followed by slow cooling, facilitated the creation of a material exhibiting approximately 25% cation disorder. To assess the structural impact of this disorder, the team employed synchrotron x-ray diffraction, energy dispersive x-ray spectroscopy, extended x-ray absorption fine structure (EXAFS), and x-ray pair distribution function (PDF) analyses.

These local structure probes revealed a coexistence of both ordered and disordered regions within the honeycomb lattice, and the degree of disorder was quantified by modelling EXAFS and PDF data with two structural components: a well-ordered Cu⁺-Ir⁴⁺ honeycomb network and a component representing Cu-Ir site exchange. Further investigation into the charge configuration involved x-ray photoemission spectroscopy (XPS) and x-ray absorption near-edge structure (XANES) measurements, confirming mixed oxidation states of both copper and iridium and indicating local charge redistribution. Magnetic susceptibility measurements, including both DC and AC techniques, revealed spin-glass-like freezing below 29K and a frequency-dependent peak at approximately 80K, indicative of dynamically fluctuating antiferromagnetic clusters.

Scientists have achieved a detailed characterisation of disorder within the copper iridium oxide (Cu₂IrO₃) compound, a material investigated as a potential Kitaev spin liquid. The research team prepared a sample exhibiting significant antisite disorder, quantified at approximately 25% using X-ray diffraction, EXAFS, and PDF analysis. These structural investigations revealed a complex arrangement of both ordered and disordered regions within the material’s honeycomb lattice, and experiments revealed a mixed valence state of copper and iridium, specifically Cu¹⁺ + Ir⁴⁺ transitioning to Cu²⁺ + Ir³⁺.

This combination of substantial site disorder and charge redistribution generates competing antiferromagnetic interactions and magnetic frustration within the material. Measurements using XPS and XANES confirmed the presence of these mixed oxidation states, providing crucial insight into the electronic structure driving the magnetic behaviour. Results demonstrate the formation of dynamically fluctuating antiferromagnetic (AFM) clusters around 80 K, which then freeze into a static state below 29 K. This freezing transition indicates a shift in the magnetic ground state driven by the interplay between disorder and magnetic interactions.

Density functional theory calculations, incorporating the effects of disorder through the coherent potential approximation, corroborate these experimental findings. The calculations show that even a small amount of Cu/Ir mixing significantly alters the electronic band structure and magnetic exchange interactions. Specifically, the introduction of disorder weakens the superexchange interactions between Ir ions, thereby reducing the strength of the antiferromagnetic coupling. This results in a decrease in the Néel temperature and a more diffuse magnetic order, and the theoretical analysis indicates that disorder induces local distortions in the crystal lattice, which further contribute to the suppression of magnetic order.

The combination of electronic and structural effects highlights the crucial role of disorder in governing the magnetic properties of Cu₂IrO₃. These findings provide valuable insights into the behaviour of disordered magnetic materials and emphasise the importance of considering disorder effects in the design of novel functional materials. The study meticulously quantified the degree of disorder from model fits based on the C2/c crystal structure, providing a precise understanding of the atomic arrangement, and delivers a crucial understanding of the role of synthesis-dependent disorder in determining the magnetic ground state of Cu₂IrO₃. This work highlights how subtle changes in material preparation can dramatically influence magnetic properties, potentially mimicking quantum spin liquid behaviour, and is significant for the development of novel quantum materials, offering a pathway towards controlling magnetic interactions through precise control of structural and chemical disorder.