Spin waves and spin-state transitions in a ruthenate high-temperature antiferromagnet

Ruthenium compounds serve as a platform for fundamental concepts such as spin-triplet superconductivity 1 , Kitaev spin liquids 2 – 5 and solid-state analogues of the Higgs mode in particle physics 6 , 7 . However, basic questions about the electronic structure of ruthenates remain unanswered, because several key parameters (including Hund’s coupling, spin–orbit coupling and exchange interactions) are comparable in magnitude and their interplay is poorly understood, partly due to difficulties in synthesizing large single crystals for spectroscopic experiments. Here we introduce a resonant inelastic X-ray scattering (RIXS) 8 , 9 technique capable of probing collective modes in microcrystals of 4 d electron materials. We observe spin waves and spin-state transitions in the honeycomb antiferromagnet SrRu 2 O 6 (ref. 10 ) and use the extracted exchange interactions and measured magnon gap to explain its high Néel temperature 11 – 16 . We expect that the RIXS method presented here will enable momentum-resolved spectroscopy of a large class of 4 d transition-metal compounds.