Origin of Magnetism in Supposedly Non-magnetic 5d¹ Osmium Oxide

Stefano Agrestini^1*, Samuele Sanna³, Francesco Borgatti⁴, Vesna F. Mitrovic⁵, Patrick M. Woodward⁶, Federico Boscherini³, Christoph J. Sahle⁷, Marco Moretti Sala²

¹Physical Science, Diamond Light Source, Didcot, UK
²Dipartimento di Fisica, Politecnico di Milano, Milano, Italy
³Department of Physics and Astronomy “A. Righi”, University of Bologna, Bologna, Italy
⁴Istituto per lo Studio dei Materiali Nanostrutturati (ISMN), Consiglio Nazionale delle Ricerche (CNR), Bologna, Italy
⁵Department of Physics, Brown University, Providence, Rhode Island, USA
⁶Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio, USA
⁷Physical Science, European Synchrotron Radiation Source, Grenoble, France

* Presenter:Stefano Agrestini, email:stefano.agrestini@diamond.ac.uk

Classical textbooks predict that the strong spin-orbit coupling in an ion with 5d¹ configuration induces a Jeff = 3/2 ground state that is non-magnetic due to the perfect cancellation of spin and orbit momentum. However, experimentally a wide range of magnetic states have been observed in 5d¹ compounds, which has caused a hot debate about the origin of the finite magnetic moment. Here, we report RIXS and XMCD investigations of the cubic double perovskite Ba₂NaOsO₆, a prototype example of magnetic 5d¹ oxide. Strong charge transfer excitations in Os L₃ RIXS indicate that Ba₂NaOsO₆ is a negative charge transfer system with large covalency. Our O K edge RIXS data reveal a breaking of the local symmetry by a compressive distortion of the OsO₆ octahedra, demonstrating that Os7+ is a Jahn-Teller active ion. From sum rules applied to the Os L₂₃ XMCD we found the orbital-to-spin moment ratio to be far from the -1 ratio expected for the Jeff = 3/2 ground state. Full-multiplet ligand-field calculations show that (1) the mixing of eg-t2g orbitals, (2) the strong covalency, and (3) the non-cubic crystal field, all three contribute to bringing the system away from a pure Jeff = 3/2 ground state and generating a finite magnetic moment. These results are not limited to the case of Os7+ but are applicable to any 5d ion with a d¹ configuration.

Keywords: spin-orbit interaction, magnetic materials, RIXS, XMCD, Jahn-Teller