Using resonant magnetic x-ray scattering we address the unresolved nature of the magnetic ground state and the low-energy effective Hamiltonian of ${\mathrm{Sm}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, 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 [$J=27.3(6)\mathrm{meV}$] and Dzyaloshinskii-Moriya interactions [$D=4.9(3)\mathrm{meV}$], which provides a consistent description of the magnetic order and excitations. In establishing that ${\mathrm{Sm}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ has the requisite inversion symmetry preserving AIAO magnetic ground state, our results support the notion that pyrochlore iridates may host correlated Weyl semimetals.

• Received 15 January 2016

DOI:https://doi.org/10.1103/PhysRevLett.117.037201