Evolution of the Magnetic Excitations in NaOsO_{3} through its Metal-Insulator Transition.

The temperature dependence of the excitation spectrum in NaOsO_{3} through its metal-to-insulator transition (MIT) at 410 K has been investigated using resonant inelastic x-ray scattering at the Os L_{3} edge. High-resolution (ΔE∼56 meV) measurements show that the well-defined, low-energy magnons in the insulating state weaken and dampen upon approaching the metallic state. Concomitantly, a broad continuum of excitations develops which is well described by the magnetic fluctuations of a nearly antiferromagnetic Fermi liquid. By revealing the continuous evolution of the magnetic quasiparticle spectrum as it changes its character from itinerant to localized, our results provide unprecedented insight into the nature of the MIT in NaOsO_{3} [J. G. Vale, S. Calder, C. Donnerer, D. Pincini, Y. G. Shi, Y. Tsujimoto, K. Yamaura, M. M. Sala, J. van den Brink, A. D. Christianson, and D. F. McMorrow, Phys. Rev. B 97, 184429 (2018)PRBMDO2469-995010.1103/PhysRevB.97.184429].

Band Filling Control of the Dzyaloshinskii-Moriya Interaction in Weakly Ferromagnetic Insulators.

We observe and explain theoretically a dramatic evolution of the Dzyaloshinskii-Moriya interaction (DMI) in the series of isostructural weak ferromagnets, MnCO_{3}, FeBO_{3}, CoCO_{3}, and NiCO_{3}. The sign of the interaction is encoded in the phase of the x-ray magnetic diffraction amplitude, observed through interference with resonant quadrupole scattering. We find very good quantitative agreement with first-principles electronic structure calculations, reproducing both sign and magnitude through the series, and propose a simplified "toy model" to explain the change in sign with 3d shell filling. The model gives insight into the evolution of the DMI in Mott and charge transfer insulators.

All-in-all-Out Magnetic Order and Propagating Spin Waves in Sm_{2}Ir_{2}O_{7}.

Using resonant magnetic x-ray scattering we address the unresolved nature of the magnetic ground state and the low-energy effective Hamiltonian of Sm_{2}Ir_{2}O_{7}, a prototypical pyrochlore iridate with a finite temperature metal-insulator transition. Through a combination of elastic and inelastic measurements, we show that the magnetic ground state is an all-in-all-out (AIAO) antiferromagnet. The magnon dispersion indicates significant electronic correlations and can be well described by a minimal Hamiltonian that includes Heisenberg exchange [J=27.3(6) meV] and Dzyaloshinskii-Moriya interactions [D=4.9(3) meV], which provides a consistent description of the magnetic order and excitations. In establishing that Sm_{2}Ir_{2}O_{7} has the requisite inversion symmetry preserving AIAO magnetic ground state, our results support the notion that pyrochlore iridates may host correlated Weyl semimetals.