All-in–all-Out Magnetic Order and Propagating Spin Waves inSm2Ir2O7

Abstract

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...

Magnetic-field induced multiple topological phases in pyrochlore iridates with Mott criticality

“…4d–f. Future neutron and X-ray experiments on the magnetic states as done in refs 19, 20, 21 will serve to verify the present interpretation for the field-induced emergent topological states. Another important feature revealed here is that all these topological states appear to merge towards the magnetic quantum critical point; this may enable the further exploration for new topological quantum states and related exotic electromagnetic responses in this ideal system endowed with Mott criticality.…”

“…One can finely tune it by applying hydrostatic pressure1415 or substituting R site1617 that can drive metal-insulator transition (MIT); the Pr compound is a paramagnetic semimetal down to 120 mK18, whereas the paramagnetic or antiferromagnetic AIAO insulating phase shows up with smaller R ionic radius192021, seemingly akin to the correlation-induced MIT as widely observed for 3 d -electron materials152223. On the verge of quantum MIT (in between R =Nd and Pr), however, the unconventional magnetotransport phenomena have been reported, including anomalous Hall effect2425, highly metallic AIAO domain walls2627 and field-induced MIT1011, which may be potentially correlated to the predicted topological states.…”

Topological Magnon Bands and Unconventional Superconductivity in Pyrochlore Iridate Thin Films

“…Reaching the AIAO state in the bilayer does require a finetuning of both DM and Γ, as well as an unusually high value for the DM strength with |DM|/J 1, whereas experiments suggest |DM|/J ∼ 0.1−0.3 [46]. Nevertheless, it is interesting to consider these bilayer mean field states proximate to the AIAO order for |DM|/J ≥ 0.7 as they yield topological lowest magnon bands, whereas those found for the parameters used in the TKT case do not.…”

“…3 [69]. In the topological sector, the gap is 0.3J ≈ 95K, using J = 27.3(6)meV as reported for Sm 2 Ir 2 O 7 [46], giving a flatness ratio that is approximately one. In general we seem unable to achieve a both very flat and topologically non-trivial band in these systems, which would allow for interesting interaction effects [75].…”

Universal geometric frustration in pyrochlores

“…Resonant inelastic X-ray scattering has observed such a feature for Eu 2 Ir 2 O 7 , Pr 2 Ir 2 O 7 , and Sm 2 Ir 2 O 7 8, 46 , though its appearance is likely complicated by other excitations. This feature unexpectedly extends to higher energy for Pr 2 Ir 2 O 7 than Eu 2 Ir 2 O 7 8 , as a result of the larger cationic ratio of Pr/Ir, inducing more disorder for Pr 2 Ir 2 O 7 .…”