[Paper Review] Ba2YIrO6: A cubic double perovskite material with Ir5+ ions

T. Dey, A. Maljuk+14 more|arXiv (Cornell University)|Feb 4, 2016

Advanced Condensed Matter Physics4 citations

TL;DR

This study reports the synthesis and characterization of cubic Ba2YIrO6, a double perovskite with Ir5+ ions in a 5d4 configuration. Despite theoretical predictions of a nonmagnetic Mott insulator due to spin-orbit coupling and electron correlations, the material exhibits correlated magnetic moments down to 0.4 K without long-range magnetic order, challenging current mean-field theories and suggesting unexplained many-body effects beyond LDA+U frameworks.

ABSTRACT

Materials with a 5d4 electronic configuration are generally considered to have a nonmagnetic ground state (J=0). Interestingly, Sr2YIrO6 (Ir5+ having 5d4 electronic configuration) was recently reported to exhibit long-range magnetic order at low temperature and the distorted IrO6 octahedra were discussed to cause the magnetism in this material. Hence, a comparison of structurally distorted Sr2YIrO6 with cubic Ba2YIrO6 may shed light on the source of magnetism in such Ir5+ materials with 5d4 configuration. Besides, Ir5+ materials having 5d4 are also interesting in the context of recently predicted excitonic types of magnetism. Here we report a single-crystal-based analysis of the structural, magnetic, and thermodynamic properties of Ba2YIrO6. We observe that in Ba2YIrO6 for temperatures down to 0.4 K, long-range magnetic order is absent but at the same time correlated magnetic moments are present. We show that these moments are absent in fully relativistic ab initio band-structure calculations; hence, their origin is presently unclear.

Motivation & Objective

• To investigate the structural, magnetic, and electronic properties of cubic Ba2YIrO6, a double perovskite with Ir5+ ions in a 5d4 configuration.
• To determine whether long-range magnetic order emerges in Ba2YIrO6, contrasting it with the magnetically ordered Sr2YIrO6 that has a distorted structure.
• To assess the role of spin-orbit coupling and electron correlations in stabilizing the insulating ground state and explaining the observed magnetic behavior.
• To resolve the discrepancy between experimental observation of correlated magnetic moments and the nonmagnetic ground state predicted by ab initio calculations.
• To explore whether the observed magnetic behavior arises from excitonic magnetism, chemical disorder, or many-body effects beyond standard LDA+U approaches.

Proposed method

• Single crystals of Ba2YIrO6 were grown via the flux method to ensure high structural quality for precise measurements.
• Single-crystal X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to confirm the cubic double perovskite structure (space group Fm3̄m).
• Magnetic susceptibility, specific heat, and resistivity measurements were performed down to 0.4 K to probe magnetic and electronic behavior.
• Density functional theory (DFT) calculations were performed using the L(S)DA+U method with full relativistic four-component Dirac spinors to include spin-orbit coupling.
• The electronic structure was analyzed via partial density of states (PDOS) and band structure calculations to identify the nature of the gap and orbital splitting.
• Comparative analysis of scalar-relativistic vs. fully relativistic calculations was conducted to isolate the role of spin-orbit coupling in driving the Mott insulating state.

Experimental results

Research questions

• RQ1 Does Ba2YIrO6 exhibit long-range magnetic order down to 0.4 K, despite its cubic structure and Ir5+ 5d4 configuration?
• RQ2 Why do experimental measurements reveal correlated magnetic moments in Ba2YIrO6 when fully relativistic ab initio calculations predict a nonmagnetic ground state?
• RQ3 What is the origin of the observed 0.2 eV gap in the electronic structure, and how does it compare to the experimental resistivity-derived gap?
• RQ4 Can the observed magnetic behavior be attributed to excitonic magnetism, chemical disorder (e.g., Ir4+ or Ir6+), or other many-body effects not captured by LDA+U?
• RQ5 Why does the inclusion of spin-orbit coupling in DFT calculations suppress magnetic order while stabilizing a Mott insulator, contrary to experimental observations?

Key findings

• Ba2YIrO6 crystallizes in a cubic double perovskite structure (space group Fm3̄m), confirmed by single-crystal XRD and SEM, with no evidence of structural distortion.
• Magnetic susceptibility follows the Curie-Weiss law with an effective magnetic moment of 0.44 μB/Ir and a negative Weiss temperature of −8.9 K, indicating weak paramagnetic correlations.
• No long-range magnetic order is observed down to 0.4 K, despite the presence of correlated magnetic moments in the susceptibility data.
• LDA+U calculations without spin-orbit coupling predict a magnetic ground state with ~2 μB moment, but this state becomes unstable when spin-orbit coupling is included.
• Fully relativistic LDA+U calculations predict a Mott insulating state with a 0.2 eV gap, consistent with the experimentally observed resistivity gap, but yield a nonmagnetic ground state with Jz = Sz = 0.
• The discrepancy between experimentally observed magnetic moments and the nonmagnetic prediction of fully relativistic DFT suggests the presence of unaccounted many-body effects beyond mean-field theory.