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