Magnetization Density Distribution of Sr_{2}IrO_{4}: Deviation from a Local j_{eff}=1/2 Picture.

5d iridium oxides are of huge interest due to the potential for new quantum states driven by strong spin-orbit coupling. The strontium iridate Sr_{2}IrO_{4} is particularly in the spotlight because of the so-called j_{eff}=1/2 state consisting of a quantum superposition of the three local t_{2g} orbitals with, in its simplest version, nearly equal populations, which stabilizes an unconventional Mott insulating state. Here, we report an anisotropic and aspherical magnetization density distribution measured by polarized neutron diffraction in a magnetic field up to 5 T at 4 K, which strongly deviates from a local j_{eff}=1/2 picture even when distortion-induced deviations from the equal weights of the orbital populations are taken into account. Once reconstructed by the maximum entropy method and multipole expansion model refinement, the magnetization density shows four cross-shaped positive lobes along the crystallographic tetragonal axes with a large spatial extent, showing that the xy orbital contribution is dominant. The analogy to the superconducting copper oxide systems might then be weaker than commonly thought.

Crystal Structure of Magnetoelectric Ba2MnGe2O7 at Room and Low Temperatures by Neutron Diffraction.

For a symmetry-consistent theoretical description of the ferroelectric phase of Ba2MnGe2O7 melilite compound, a precise knowledge of its crystal structure is a prerequisite. Here we report results of single-crystal neutron diffraction experiments on Ba2MnGe2O7 at room (300 K) and low (10 K) temperatures. The structural model based on the tetragonal space group P4̅21 m describes the Ba2MnGe2O7 symmetry both at room and low temperatures. We found reflections forbidden in the typical P4̅21 m melilite-type structure. A comparison of the experimental data collected by means of both thermal and cold neutrons with simulated multiple diffraction patterns allows us to unambiguously demonstrate that forbidden peaks originate from multiple diffraction (Renninger effect) rather than from real symmetry lowering. The precise structural parameters at 300 and 10 K are presented for the first time and compared with those of other magnetoelectric melilite-type germanates.

The low-temperature crystal structure of the multiferroic melilite Ca2CoSi2O7.

In the antiferromagnetic ground state, below TN ≃ 5.7 K, Ca2CoSi2O7 exhibits strong magnetoelectric coupling. For a symmetry-consistent theoretical description of this multiferroic phase, precise knowledge of its crystal structure is a prerequisite. Here we report the results of single-crystal neutron diffraction on Ca2CoSi2O7 at temperatures between 10 and 250 K. The low-temperature structure at 10 K was refined assuming twinning in the orthorhombic space group P2(1)2(1)2 with a 3 × 3 × 1 supercell [a = 23.52 (1), b = 23.52 (1), c = 5.030 (3) Å] compared with the high-temperature normal state [tetragonal space group P42(1)m, a = b ≃ 7.86, c ≃ 5.03 Å]. The precise structural parameters of Ca2CoSi2O7 at 10 K are presented and compared with the literature X-ray diffraction results at 130 and 170 K (low-temperature commensurate phase), as well as at ∼ 500 K (high-temperature normal phase).

Magnetic behaviour of synthetic Co(2)SiO(4).

Synthetic Co(2)SiO(4) crystallizes in the olivine structure (space group Pnma) with two crystallographically non-equivalent Co positions and shows antiferromagnetic ordering below 50 K. We have investigated the temperature variation of the Co(2)SiO(4) magnetic structure by means of non-polarized and polarized neutron diffraction for single crystals. Measurements with non-polarized neutrons were made at 2.5 K (below T(N)), whereas polarized neutron diffraction experiments were carried out at 70 and 150 K (above T(N)) in an external magnetic field of 7 T parallel to the b axis. Additional accurate non-polarized powder diffraction studies were performed in a broad temperature range from 5 to 500 K with small temperature increments. Detailed symmetry analysis of the Co(2)SiO(4) magnetic structure shows that it corresponds to the magnetic (Shubnikov) group Pnma, which allows the antiferromagnetic configuration (G(x), C(y), A(z)) for the 4a site with inversion symmetry 1 (Co1 position) and (0,C(y),0) for the 4c site with mirror symmetry m (Co2 position). The temperature dependence of the Co1 and Co2 magnetic moments obtained from neutron diffraction experiments was fitted in a modified molecular-field model. The polarized neutron study of the magnetization induced by an applied field shows a non-negligible amount of magnetic moment on the oxygen positions, indicating a delocalization of the magnetic moment from Co towards neighbouring O owing to superexchange coupling. The relative strength of the exchange interactions is discussed based on the non-polarized and polarized neutron data.

Structural behaviour of synthetic Co2SiO4 at low temperatures.

Synthetic Co2SiO4 has an olivine structure with isolated SiO4 groups (space group Pnma) and shows magnetic ordering below 50 K. Single-crystal neutron diffraction was applied to determine precise crystal structure parameters at low temperatures. No structural phase transition was revealed in the temperature range 2.5-300 K. Lattice parameters were determined by high-resolution X-ray powder diffraction between 15 and 300 K. There is a clear evidence of an anomalous thermal expansion related to the magnetic phase transition which can be attributed to magnetostriction.