Using resonant magnetic x-ray scattering we address the unresolved nature of the magnetic groundstate and the low-energy effective Hamiltonian of Sm2Ir2O7, 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 DzyaloshinskiiMoriya interaction (D = 4.9(3) meV), which provides a consistent description of the magnetic order and excitations. In establishing that Sm2Ir2O7 has the requisite inversion symmetry preserving AIAO magnetic groundstate, our results support the notion that pyrochlore iridates may host correlated Weyl semimetals.The search for novel electronic and magnetic phenomena has recently been fruitful in the correlated, strong spin-orbit coupling regime [1][2][3][4]. The family of pyrochlore iridates, R 2 Ir 2 O 7 (where R is a rare-earth element), has received much interest since the prediction of topologically non-trivial states, most prominently the Weyl semimetal (WSM) [5][6][7][8]. This is motivated by the observation of metal-insulator transitions as a function of temperature and rare-earth ion radius that occur concomitantly with the onset of magnetic order [9][10][11][12]. As magnetic order breaks time-reversal symmetry, the WSM state in these correlated materials requires the preservation of inversion symmetry, a scenario distinct from the weakly correlated limit where the opposite is true. Theoretical proposals for the magnetic order with the required symmetries in pyrochlore iridates have focused on the antiferromagnetic all-in all-out (AIAO) structure, where the moments either all point towards or away from the center of the corner shared tetrahedra which form the iridium sublattice. The R 2 Ir 2 O 7 system thus offers an outstanding opportunity to study novel topological phases in the presence of electronic correlations.Despite substantial experimental effort, however, the nature of the magnetic order of the Ir ions and the effective spin Hamiltonian have remained elusive in pyrochlore iridates [13][14][15][16][17][18][19][20]. Resonant elastic x-ray scattering at the Ir L 3 edge of Eu 2 Ir 2 O 7 has found k = 0 magnetic order of undetermined type [17]. Due to the small magnetic moment of the Ir ion and its high neutron absorption, neutron diffraction has only been successful in studying the rare-earth sublattice. For R = Tb and Nd rare-earths, AIAO magnetic order was found, which has been argued to provide indirect evidence for identical ordering on the Ir lattice [14,19]. An upper limit on the size of the ordered Ir moment was placed at 0.2 µ B (Tb) [19] and 0.5 µ B (Y) [15].Here, we use resonant elastic and inelastic x-ray scattering (REXS and RIXS) at the Ir L 3 edge to reveal the nature of the magnetic order and excitations of the pyroch...

The collective magnetic excitations in the spin-orbit Mott insulator (Sr 1−x La x ) 2 IrO 4 (x = 0, 0.01, 0.04, 0.1) were investigated by means of resonant inelastic x-ray scattering. We report significant magnon energy gaps at both the crystallographic and antiferromagnetic zone centers at all doping levels, along with a remarkably pronounced momentum-dependent lifetime broadening. The spin-wave gap is accounted for by a significant anisotropy in the interactions between J eff = 1/2 isospins, thus marking the departure of Sr 2 IrO 4 from the essentially isotropic Heisenberg model appropriate for the superconducting cuprates.