The material class of rare earth nickelates with high Ni3+ oxidation state is generating continued interest due to the occurrence of a metal-insulator transition with charge order and the appearance of non-collinear magnetic phases within this insulating regime. The recent theoretical prediction for superconductivity in LaNiO3 thin films has also triggered intensive research efforts. LaNiO3 seems to be the only rare earth nickelate that stays metallic and paramagnetic down to lowest temperatures. So far, centimeter-sized impurity-free single crystal growth has not been reported for the rare earth nickelates material class since elevated oxygen pressures are required for their synthesis. Here, we report on the successful growth of centimeter-sized LaNiO3 single crystals by the floating zone technique at oxygen pressures of up to 150 bar. Our crystals are essentially free from Ni2+ impurities and exhibit metallic properties together with an unexpected but clear antiferromagnetic transition.

Motivated by the rich interplay among electronic correlation, spin-orbit coupling (SOC), crystal-field splitting, and geometric frustrations in the honeycomblike lattice, we systematically investigated the electronic and magnetic properties of Li 2 RhO 3 . The material is semiconducting with a narrow band gap of ∼ 78 meV, and its temperature dependence of resistivity conforms to a three-dimensional variable range hopping mechanism. No long-range magnetic ordering was found down to 0.5 K, due to the geometric frustrations. Instead, single atomic spin-glass behavior below the spin-freezing temperature (∼6 K) was observed and its spin dynamics obeys the universal critical slowing down scaling law. A first-principles calculation suggested it to be a relativistic Mott insulator mediated by both electronic correlation and SOC. With moderate strength of electronic correlation and SOC, our results shed light on the research of the Heisenberg-Kitaev model in realistic materials.

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...

Muon-spin relaxation results on the pyrochlore iridate Nd 2 Ir 2 O 7 are reported. Spontaneous coherent muonspin precession below the metal-insulator transition (MIT) temperature of about 33 K is observed, indicating the appearance of a long-ranged magnetic ordering of Ir 4+ moments. With further decrease in temperature, the internal field at the muon site increases again below about 9 K. The second increase of internal field suggests the ordering of Nd 3+ moments, which is consistent with a previous neutron experiment. Our results suggest that the MIT and magnetic ordering of Ir 4+ moments have a close relationship and that the large spin-orbit coupling of Ir 5d electrons plays a key role for both MIT and the mechanism of the magnetic ordering in pyrochlore iridates in the insulting ground state.

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We observe quasi-static incommensurate magnetic peaks in neutron scattering experiments on layered cobalt oxides La2−xSrxCoO4 with high Co oxidation states that have been reported to be paramagnetic. This enables us to measure the magnetic excitations in this highly hole-doped incommensurate regime and compare our results with those found in the low-doped incommensurate regime that exhibit hourglass magnetic spectra. The hourglass shape of magnetic excitations completely disappears given a high Sr doping. Moreover, broad low-energy excitations are found, which are not centered at the incommensurate magnetic peak positions but around the quarter-integer values that are typically exhibited by excitations in the checkerboard charge ordered phase. Our findings suggest that the strong inter-site exchange interactions in the undoped islands are critical for the emergence of hourglass spectra in the incommensurate magnetic phases of La2−xSrxCoO4.

We report on an elastic neutron scattering study of the charge correlations in La2–xSrx CoO4 with x = 1/3, 0.4 and 0.5. We found that the checkerboard charge ordering correlations present in the x = 0.5 sample persist in the x = 0.4 and 1/3 materials. These checkerboard charge ordering correlations are robust and explain the occurrence of nano‐phase separation in layered cobaltates for Sr‐concentrations away from half‐doping. The half‐integer reflections then arise from the nanometer‐sized hole‐rich regions (blue areas in title figure) instead of the undoped ones (red areas in title figure). The appearance of nano‐phase separation is an important ingredient for understanding the formation of hour‐glass shaped magnetic excitation spectra in La2–xSrx CoO4. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

The magnetic ground state of the Rh-doped Kondo semiconductor CeRu 2 Al 10 [Ce(Ru 1−x Rh x ) 2 Al 10 ] is investigated with the muon spin relaxation method. Muon spin precession with two frequencies is observed in the x = 0 sample, while only one frequency is present in the x = 0.05 and 0.10 samples, which is attributed to the broad static field distribution at the muon site. The internal field at the muon site is enhanced from about 180 G in the x = 0 sample to about 800 G in the Rh-doped samples, supporting the spin-flop transition as suggested by the magnetization measurement, and the boundary of different magnetic ground states is identified around x = 0.03. The drastic change of magnetic ground state by a small amount of Rh doping (3%) indicates that the magnetic structure in CeRu 2 Al 10 is not robust and can be easily tuned by external perturbations such as electron doping. The anomalous temperature dependence of the internal field in CeRu 2 Al 10 is suggested to be attributed to the hyperfine interaction between muons and conduction electrons.