Magnon mode transition in real space.

Spin excitation of an ilmenite FeTiO3 powder sample is measured by time-of-flight inelastic neutron scattering. The dynamic magnetic pair-density function DM(r, E) is obtained from the dynamic magnetic structure factor SM(Q, E) by the Fourier transformation. The real space spin dynamics exhibit magnon mode transitions in the spin-spin correlation with increasing energy from no-phase-shift to π-phase-shift. The mode transition is well reproduced by a simulation using the reciprocal space magnon dispersions. This analysis provides a novel opportunity to study the local spin dynamics of various magnetic systems.

High-energy spin fluctuation in low-Tc iron-based superconductor LaFePO0.9.

Spin fluctuations are widely believed to play an important role in the superconducting mechanisms of unconventional high temperature superconductors. Spin fluctuations have been observed in iron-based superconductors as well. However, in some iron-based superconductors such as LaFePO0.9, they have not been observed by inelastic neutron scattering (INS). LaFePO0.9 is an iron-based superconductor with a low superconducting transition temperature (Tc = 5 K), where line nodes are observed in the superconducting gap function. The line-node symmetry typically originates from sign reversal of the order parameter in spin-fluctuation-mediated superconductivity. This contradiction has been a long-standing mystery of this superconductor. Herein, spin fluctuations were found at high energies such as 30-50 meV with comparable intensities to an optimally doped LaFeAs(O, F). Based on this finding, the line-node symmetry can be explained naturally as spin-fluctuation-mediated superconductivity.

Crystal Structures and Magnetic Properties of Nickel Chain Compounds PbM2Ni6Te3O18 (M = Mn, Cd).

The synthesis, crystal structures, and magnetic properties of the pentanary oxides PbM2Ni6Te3O18 (M = Mn and Cd) were investigated. These compounds crystallize in a hexagonal structure with space group P63/m, in which the Ni(2+) ions form a zigzag chain along the c axis. From the magnetic susceptibility and specific heat measurements, we found that the PbCd2Ni6Te3O18 behaves as a low-dimensional magnet due to the intrachain antiferromagnetic interaction between Ni(2+) ions. Both compounds show a long-range antiferromagnetic ordering at 25.7 K (M = Cd) and 86.0 K (Mn). The magnetic structure of PbMn2Ni6Te3O18 determined by neutron diffraction measurements is a collinear antiferromagnetic arrangement of Mn(2+) ions in the Mn2O9 dimeric unit and Ni(2+) ions in the zigzag chain.

Magnetic structure and electromagnetic properties of LnCrAsO with a ZrCuSiAs-type structure (Ln = La, Ce, Pr, and Nd).

We report the synthesis, structure, and electromagnetic properties of Cr-based layered oxyarsenides LnCrAsO (Ln = La, Ce, Pr, and Nd) with a ZrCuSiAs-type structure. All LnCrAsO samples showed metallic electronic conduction. Electron doping in LaCrAsO by Mn-substitution for the Cr sites gave rise to a metal-insulator transition. Analysis of powder neutron diffraction data revealed that LaCrAsO had G-type antiferromagnetic (AFM) ordering, i.e., a checkerboard-type AFM ordering in the CrAs plane and antiparallel spin coupling between the adjacent CrAs planes, at 300 K with a large spin moment of 1.57 µB along the c axis. The magnetic susceptibility of LaCrAsO was very small (on the order of 10(-3) emu/mol) and showed a broad hump at ∼550 K. First-principles density functional theory calculations of LaCrAsO explained its crystal structure and metallic nature well, but could not replicate the antiparallel spin coupling between the CrAs layers. The electronic structure of LaCrAsO is discussed with regard to those of related compounds LaFeAsO and LaMnAsO.

Crystal and magnetic structures and properties of BiMnO(3+delta).

Crystal and magnetic structures of BiMnO(3+delta) (delta = 0.03, 0.08, and 0.14) have been determined by the Rietveld method from neutron diffraction data at 8-10 and 290 K. BiMnO(3.03) (= Bi(0.99)Mn(0.99)O(3)) crystallizes in a monoclinic system (the refinement was performed in space group C2/c; Z = 8; a = 9.5313(3) A, b = 5.57791(17) A, c = 9.7375(4) A, beta = 108.951(2) degrees at 290 K). BiMnO(3.08) (= Bi(0.974)Mn(0.974)O(3)) crystallizes in space group P2(1)/c (Z = 8; a = 9.5565(4) A, b = 5.51823(16) A, c = 9.7051(4) A, beta = 109.442(3) degrees at 290 K). It was found that Mn vacancies are localized mainly in one Mn site (among three sites) in Bi(0.974)Mn(0.974)O(3). Vacancy-ordering and charge-ordering scenarios are suggested as possible reasons for the crystal symmetry change compared with Bi(0.99)Mn(0.99)O(3). BiMnO(3.03) and BiMnO(3.08) are ferromagnetic below T(C) = 82 and 68 K, respectively, with magnetic moments along the monoclinic b axes. Refined magnetic moments at 10 K are 2.88(2)micro(B) in BiMnO(3.03) and 2.33(2)micro(B) in BiMnO(3.08). BiMnO(3.14) (= Bi(0.955)Mn(0.955)O(3)) crystallizes in an orthorhombic system (space group Pnma; Z = 4; a = 5.5136(4) A, b = 7.8069(8) A, and c = 5.5454(5) A at 290 K), and its structure is similar to that of LaMnO(3.11)-LaMnO(3.15). No magnetic reflections were found in BiMnO(3.14) down to 8 K, in agreement with its spin-glass magnetic state. Magnetic and chemical properties of BiMnO(3+delta) (0.02 < or = delta < or = 0.14) have also been investigated and compared with those of LaMnO(3+delta). Systematic changes of magnetic parameters in BiMnO(3+delta) were found to depend on delta.

Origin of the monoclinic-to-monoclinic phase transition and evidence for the centrosymmetric crystal structure of BiMnO3.

Structural properties of polycrystalline single-phased BiMnO3 samples prepared at 6 GPa and 1383 K have been studied by selected area electron diffraction (SAED), convergent beam electron diffraction (CBED), and the Rietveld method using neutron diffraction data measured at 300 and 550 K. The SAED and CBED data showed that BiMnO3 crystallizes in the centrosymmetric space group C2/c at 300 K. The crystallographic data are a = 9.5415(2) A, b = 5.61263(8) A, c = 9.8632(2) A, beta = 110.6584(12) degrees at 300 K and a = 9.5866(3) A, b = 5.59903(15) A, c = 9.7427(3) A, beta = 108.601(2) degrees at 550 K, Z = 8, space group C2/c. The analysis of Mn-O bond lengths suggested that the orbital order present in BiMnO3 at 300 K melts above TOO = 474 K. The phase transition at 474 K is of the first order and accompanied by a jump of magnetization and small changes of the effective magnetic moment and Weiss temperature, mueff = 4.69 microB and theta = 138.0 K at 300-450 K and mueff = 4.79 microB and theta = 132.6 K at 480-600 K.

Finite size effects of nanoparticles on the atomic pair distribution functions.

The finite size effects of nanoparticles on the atomic pair distribution functions (PDF) are discussed by calculating the radial distribution functions (RDF) on nanoparticles with various shapes, such as sheet, belt, rod, tube and sphere, assuming continua. Their characteristics are shown depending on the shapes and the sizes of the nanoparticles. The formulas of a PDF analysis which take account of such effects are presented and are found to reproduce the experimental data.

BiScO3: centrosymmetric BiMnO3-type oxide.

With neutron powder diffraction, electron diffraction, and second-harmonic generation, we have shown that BiScO3 has a structure closely related to that of multiferroic BiMnO3, but BiScO3 crystallizes in the centrosymmetric space group of C2/c. These results bring up a question about the origin of ferroelectricity in BiMnO3. BiScO3 may serve as a model system to understand the role of Mn3+ ions in the ferroelectricity of BiMnO3.