Approaching the ideal limit for spin-orbit-coupled quantum moments in iridium halides

Jacob Ruff - CHEXS

New research from a large team from 5 Universities and 2 National Labs reports on a new family of iridium halide materials that provide the best-yet realization of the ideal quantum limit for spin-orbit coupled magnetic moments. These Jeff=½ magnets are promising systems to engineer new kinds of superconductors and realize new kinds of magnetic interactions.

antifluorite structure

Unlike the more well-studied Ruddlesden-Popper iridate materials (such as Sr2IrO4), several iridium halides crystallize into the antifluorite structure shown here. They show only small distortions away from cubic point symmetry at the iridium site (red). This results in the smallest value for Δ (the non-cubic crystal field splitting) observed in any iridium-containing quantum magnet. These new halides are therefore in closest proximity yet to a pure quantum Jeff=½ ground state.

There is a fundamental (quantum) limit to the smallest value of angular momentum that a particle can have. This value is typically denoted as ½ - the value of the spin angular momentum of a single electron. Recently, researchers have realized that materials made up of heavy elements (like iridium) can exhibit a state where the spin and orbital angular momentum of 5d electrons couple strongly together in a total angular momentum Jeff, which nevertheless still attains the quantum limit of ½. These Jeff=½ magnets are promising systems to engineer new kinds of superconductors and realize new kinds of magnetic interactions. However, discovering real materials that have these ideal properties is a challenge. In a recent paper, Reig-i-Plessis et al report a new family of iridium halide materials which are closer to the ideal Jeff= ½ limit than any previously known materials. The large team, hailing from 5 Universities and 2 National Labs, has synthesized new samples and employed a wide range of experimental techniques including neutron scattering, muon spin rotation, x-ray diffraction and x-ray spectroscopy, to demonstrate a uniquely “pure” realization of spin-orbit coupled quantum spins in iridium halides.

Importance 
This work suggests new materials design principles for identifying Jeff= ½ electronic ground states that have not yet been widely explored. Specifically, increasing the spatial separation of the Ir ions and incorporating IrX6 octahedra into the crystal structure, where X is an anion with a low electronegativity, can now be understood to be key. These design principles can also be pursued in fluoride analogs which offer promise in the search for model Jeff=½  systems on the triangular lattice, and have shown no signs of magnetic order to date. Learning to design new materials to exhibit desired quantum phenomenology is a key aspect of the “Quantum Leap”, one of NSF’s 10 Big Ideas.


Need for CHEXS Experimental Capabilities 
CHESS-U X-ray beams deliver exceptionally high flux within narrow-bandwidth beams at energies tuned to the iridium L edges, all of which is necessary to enable precision studies of these materials. Understanding the phenomenology of iridium-containing magnets is a key science driver for the CHEXS facility, including both the resonant x-ray scattering capabilities at the QM2 beamline and the resonant x-ray spectroscopy capabilities at the PIPOXS beamline. Follow-up work is ongoing on these and related systems on both beamlines. The paper by Reig-i-Plessis et al clearly highlights the value of combining distinct experimental perspectives from different beamlines and facilities to fully understand new materials, and the important role for x-rays in understanding exotic quantum magnets.

Branching ratio measurements
Branching ratio measurements using x-rays tuned to the L2 and L3 resonances of the iridium atoms in K2IrBr6. The ratio of the shaded red areas is directly proportional to the expectation value of the quantum spin-orbit coupling operator. These curves are consistent with a branching ratio close to 4, and significant spin-orbit coupling. Tuning to absorption edges allows x-rays to precisely probe the single-ion properties of quantum magnets.

Funding
This research used resources at the High Flux Isotope Reactor (DOE Office of Science User Facility operated by Oak Ridge National Laboratory) and the Advanced Photon Source (DOE Office of Science User Facility operated by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.) Research conducted at CHESS was supported by the National Science Foundation via Awards DMR-1332208 and DMR-1829070. Synthesis, crystal growth, powder x-ray, magnetization, and heat-capacity measurements were carried out in part in the Materials Research Laboratory, Central Research Facilities, University of Illinois. Research was supported by the National Science Foundation Division of Materials Research under Awards DMR-1455264 and DMR-2003117.

Reference
D Reig-i-Plessis, TA Johnson, K Lu, Q Chen, JPC Ruff, MH Upton, TJ Williams, S Calder, HD Zhou, JP Clancy, AA Aczel, and GJ MacDougall.  
Structural, electronic, and magnetic properties of nearly ideal Jeff = ½ iridium halides.  
Physical Review Materials 4, 124407 (2020); https://doi.org/10.1103/PhysRevMaterials.4.124407