Materials with a 5d 4 electronic configuration are generally considered to have a nonmagnetic ground state (J = 0). Interestingly, Sr2YIrO6 (Ir 5+ having 5d 4 electronic configuration) was recently reported to exhibit long-range magnetic order at low temperature and the distorted IrO6 octahedra were discussed to cause the magnetism in this material. Hence, a comparison of structurally distorted Sr2YIrO6 with cubic Ba2YIrO6 may shed light on the source of magnetism in such Ir 5+ materials with 5d 4 configuration. Besides, Ir 5+ materials having 5d 4 are also interesting in the context of recently predicted excitonic types of magnetism. Here we report a single-crystal-based analysis of the structural, magnetic, and thermodynamic properties of Ba2YIrO6. We observe that in Ba2YIrO6 for temperatures down to 0.4 K, long-range magnetic order is absent but at the same time correlated magnetic moments are present. We show that these moments are absent in fully relativistic ab initio band-structure calculations; hence, their origin is presently unclear.

We report susceptibility, specific heat, and neutron diffraction measurements on NaCu2O2, a spin-1/2 chain compound isostructural to LiCu2O2, which has been extensively investigated. Below 12 K, we find a long-range ordered, incommensurate magnetic helix state with a propagation vector similar to that of LiCu2O2. In contrast to the Li analogue, substitutional disorder is negligible in NaCu2O2. We can thus rule out that the helix is induced by impurities, as was claimed on the basis of prior work on LiCu2O2. A spin Hamiltonian with frustrated longer-range exchange interactions provides a good description of both the ordered state and the paramagnetic susceptibility.PACS numbers: 75.10. Pq, 75.40.Cx, 75.25.+z Copper oxides are excellent model systems for lowdimensional spin-1/2 quantum antiferromagnets. In particular, copper oxides with magnetic backbones comprised of chains of CuO 4 squares have been shown to exhibit quasi-one-dimensional behavior. Two classes of copper oxide spin chain materials are known. Compounds in which adjacent squares share their corners are excellent realizations of the one-dimensional (1D) spin-1/2 Heisenberg Hamiltonian [1, 2, 3]. Linear Cu-O-Cu bonds along the spin chains give rise to a large antiferromagnetic nearest-neighbor exchange coupling. In compounds built up of edge-sharing squares, on the other hand, the Cu-O-Cu bond angle is nearly 90 • , so that the nearest-neighbor coupling is more than an order of magnitude smaller [4]. Because of the anomalously small nearest-neighbor coupling, longer-range frustrating exchange interactions have a pronounced influence on the physical properties of these materials. Edge-sharing copper oxides thus provide uniquely simple model systems to test current theories of spin correlations in frustrated quantum magnets.At low temperatures, the ground state of edge-sharing copper oxides is either a 3D-ordered antiferromagnet [5,6,7] or a spin-Peierls state [8], depending on whether interchain exchange interactions or spin-phonon interactions are dominant. In the former case, the magnetic order is almost always collinear. An interesting exception was recently discovered in LiCu 2 O 2 [9, 10, 11], which undergoes a transition to a magnetic helix state at low temperatures. While such a state is expected for classical spin models with frustrating interactions, quantum models predict a gapped spin liquid state in the range of exchange parameters that was claimed to describe the spin system in LiCu 2 O 2 . Since the ionic radii of Li + and Cu 2+ are similar, chemical disorder was identified as a possible solution to this puzzle. Indeed, a chemical analysis of the sample used in the neutron scattering study of Ref. [11] showed that about 16% of the Cu 2+ ions in the spin chains were replaced by nonmagnetic Li + impurities. Since even much lower concentrations of nonmagnetic impurities are found to induce magnetic long-range order in other quasi-1D spin-gap systems, the authors of Ref.[11] attributed the unexpected helix state to the Here we report magnet...

We report a comprehensive neutron diffraction study of the crystal structure and

We study the evidence for spin liquid in the frustrated diamond lattice antiferromagnet CoAl2O4 by means of single crystal neutron scattering in zero and applied magnetic field. The magnetically ordered phase appearing below TN =8 K remains nonconventional down to 1.5 K. The magnetic Bragg peaks at the q=0 positions are broad and their lineshapes have strong Lorentzian contributions. Additionally, the peaks are connected by weak diffuse streaks oriented along the <111> directions. The observed short-range magnetic correlations are explained within the spiral spinliquid model. The specific shape of the energy landscape of the system with an extremely flat energy minimum around q=0 and many low lying excited spiral states with q= <111> results in thermal population of this manifold at finite temperatures. The agreement between the experimental results and the spiral spin-liquid model is only qualitative indicating that microstructure effects might be important to achieve quantitative agreement. Application of a magnetic field significantly perturbs the spiral spin-liquid correlations. The magnetic peaks remain broad but acquire more Gaussian lineshapes and increase in intensity. The 1.5 K static magnetic moment increases from 1.58 µB/Co at zero field to 2.08 µB/Co at 10 T. The magnetic excitations appear rather conventional at zero field. Analysis using classical spin wave theory yields values of the nearest and next-nearest neighbor exchange parameters J1=0.92(1) meV and J2=0.101(2) meV and an additional anisotropy term D=-0.0089(2) meV for CoAl2O4. In the presence of a magnetic field, the spin excitations broaden considerably and become nearly featureless at the zone center.