Direct determination of the spin structure ofNd2Ir2O7by means of neutron diffraction

Abstract

We report on the spin structure of the pyrochlore iridate Nd 2 Ir 2 O 7 that could be directly determined by means of powder neutron diffraction. Our magnetic structure refinement unravels a so-called all-in/all-out magnetic structure that appears in both, the Nd and the Ir sublattice. The ordered magnetic moments at 1.8 K amount to 0.34(1) µ B /Ir 4+ and 1.27(1) µ B /Nd 3+ . The Nd 3+ moment size at 1.8 K is smaller than that expected for the Nd 3+ ground state doublet. On the other hand, the size of the ordered moments of the Ir 4+ ions at 1.8 K agrees very well with the value expected for a J eff = 1/2 state based on the presence of strong spin-orbit coupling in this system. Finally, our measurements reveal a parallel alignment of the Nd 3+ moments with the net moment of its six nearest neighboring Ir 4+ ions.PACS numbers: 75.25. 75.30.Gw, 75.47.Lx, 71.70.Ej The 5d transition metal oxides have attracted substantial attention due to the interplay between the relatively large spinorbit coupling (SOC) and electron-electron correlations (U), which may result in exotic electronic phases such as topological Mott insulators, Weyl semimetals and axion insulators [1][2][3][4][5][6][7]. The pyrochlore iridates (R 2 Ir 2 O 7 , R = rare earth and Y) are a fertile playground to realize these topological phases. For the pyrochlore structure, the ions at the R site or Ir site form a network of corning-sharing tetrahedra with the two sublattices penetrating each other. These pyrochlore iridates exhibit a metal-insulator transition (MIT) at T MI which continuously decreases with increasing R-ionic radius. The MIT disappears (T MI = 0) between R = Nd and Pr [8][9][10]. No thermal hysteresis effect can be observed at T MI (T N ), thus, indicating a second order transition [9]. At T N , magnetization, µSR and resonant X-ray scattering experiments suggest an ordering of the Ir 4+ moments [9,[11][12][13][14]. At lower temperatures, the d − f interaction may induce the magnetic ordering at the R site (also depending on the single-ion anisotropy at the R site [15] [17,18]. It was shown that a quantum MIT is realized when a magnetic field of ∼10 T is applied only along the [0 0 1] direction, with the ground state changing from an insulating magnetic ordered state to a semimetal state [17,18]. This could be explained by a reconstruction of the band structure concomitant with the change of the magnetic structure in the Nd sublattice from an all-in/all-out (AIAO) to a two-in/two-out magnetic structure.Several theoretical calculations [1][2][3][4][5]7] have demonstrated that the AIAO magnetic structure has the lowest energy when U becomes larger than a critical value U c . The AIAO magnetic ordering is also indispensable for realizing the topologically nontrivial Weyl semimetal state [1][2][3][4][5]7]. Therefore, it is essential to unambiguously determine the magnetic structure of Nd 2 Ir 2 O 7 experimentally in order to understand these emergent phenomena in pyrochlore iridates. Apart from the Nd sublattice where an AIAO magnetic st...

A new mathematical approach to finding global solutions of the magnetic structure determination problem

“…These results are comparable with the reported values of m Nd  = −1.27(1) μ B and m Ir  = 0.34(1) μ B 34 .…”

“…This state theoretically corresponds to the all-in all-out type of magnetic structure described by the magnetic propagation vector k mag  = (0, 0, 0), in which all the magnetic moments are oriented either towards the centre of the participating tetrahedron or in the opposite direction 30,32 . This prediction was experimentally confirmed in previous unpolarized neutron diffraction studies 33,34 . However, the superposition of the nuclear and magnetic reflections observed for the magnetic structures with k mag  = (0, 0, 0) produces ambiguous results during their separation.…”

Weyl and Dirac semimetals in three-dimensional solids

“…Experimentally, several pyrochlore iridates are observed to undergo magnetic ordering, and experiments have now confirmed that the order is of the predicted AIAO form (Disseler, 2014;Donnerer et al, 2016;Guo et al, 2016;Sagayama et al, 2013). The magnetically ordered pyrochlores, such as A=Eu, Nd are seen, in clean samples, to be insulating at low temperatures Tafti et al, 2012;Tian et al, 2016;Ueda et al, 2012Ueda et al, , 2015b.…”

Strong quantum fluctuations from competition between magnetic phases in a pyrochlore iridate

“…25 In the A 2 Ir 2 O 7 compounds studied so far, the ordered magnetic moments on the Ir sites are small (∼ 0.5 µ B ) and point either all towards the center of the tetrahedron or directly away from it -the socalled all-in-all-out (AIAO) structure, see Fig. 1(a) [26][27][28][29][30][31][32][33] .…”