Local Observation of Field Polarity Dependent Flux Pinning by Magnetic Dipoles

A scanning Hall probe microscope is used to study flux pinning in a thin superconducting Pb film covering a square array of single-domain Co dots with in-plane magnetization. We show that single flux quanta of opposite sign thread the superconducting film below T(c) at the opposite poles of these dipoles. Depending on the polarity of the applied field, flux lines are attracted to a specific pole of the dipoles, due to the direct interaction with the vortexlike structures induced by the local stray field.     

Expanded fullerides and electron localisation -- lithium-rich ammoniated C(60) phases

The synthesis and characterisation of alkali metal fullerides with complex counterions consisting of ammonia ligands coordinated to lithium cations is discussed. The body-centred cubic packing and accompanying low density of fulleride anions produces localisation of the t(1u) outer electrons of the fulleride anions. This is demonstrated by the electronic behaviour of the x = 3 and 5 members of the series, the latter representing the first detailed study of the electronic behaviour of a localised electron fulleride with five electrons per anion.     

Magnetism and structural chemistry of the n = 1 Ruddlesden-Popper phases La4LiMnO8 and La3SrLiMnO8.

Polycrystalline samples of La4LiMnO8 and La3SrLiMnO8 have been studied by a combination of X-ray diffraction (XRD), neutron diffraction (ND), 6Li MAS NMR, electron microscopy (EM), and magnetometry. Room-temperature XRD and ND measurements suggest that both compounds have the K2NiF4 structure, with a disordered arrangement of Li and Mn over the six-coordinate sites. However, MAS NMR and EM demonstrate the presence of local 1:1 Li:Mn order on these sites, and EM shows that although cation order is well-developed in each xy sheet of corner-sharing octahedra, the sheets are stacked randomly along z. The structures are best described as paracrystalline, and many of the concepts of conventional crystallography are inapplicable. Magnetometry and low-temperature ND experiments show that, despite their paracrystallinity, the two compounds are ordered antiferromagnetically with susceptibility maxima at 26 and 18 K, respectively, and with ordered magnetic moments of 3.61(6) and 2.3(1) muB per Mn cation at 2 K. Anisotropic peak broadening reveals a 2D character in the magnetic behavior of both compounds, and La3SrLiMnO8 is well-modeled as a quadratic layer S = 3/2 Heisenberg antiferromagnet.     

A polar oxide with a large magnetization synthesized at ambient pressure

The perovskite Bi2Mn4/3Ni2/3O6 is polar and combines relative permittivity behavior consistent with ferroelectricity with the magnetic response of a concentrated spin-glass. Bi2Mn4/3Ni2/3O6 is accessible by ambient pressure synthesis despite the instability of the end-members BiMnO3 and BiNiO3 under these conditions.     

Electronic structure, magnetic ordering, and formation pathway of the transition metal oxide hydride LaSrCoO(3)H(0.7)

The role of the hydride anion in controlling the electronic properties of the transition metal oxide hydride LaSrCoO(3)H(0.7) is investigated theoretically by full potential DFT band structure calculation and experimentally by determination of the Neel temperature for three-dimensional magnetic ordering. The mechanism by which hydrogen is introduced into the solid is addressed by in situ X-ray diffraction studies of the formation of the oxide hydride, which reveal both a relationship between the microscopic growth of the observed oxide hydride order and the anisotropic broadening of the diffraction profile, and the existence of a range of intermediate compositions.     

Novel solid-state and solution-phase processes for the modification of high-temperature oxocuprate superconductors: cation dependence of reduction by iodide and by tetrathiafulvalene

Summary The insertion of lithium(I) and zinc(II) into oxocuprates of superconducting compositions was achieved by novel solid-state reactions of LiI and ZnI₂, in which copper is (formally) reduced and I₂ liberated. In comparable insertions from solution, in which the accompanying oxidation of tetrathiafulvalence (yellow red) acts as reaction indicator, magnesium(II), strontium(II), barium(II), manganese(II), zinc(II) and cadmium(II) are shown to be taken up. Nonincorporation of sodium(I) and larger M⁺ into the oxocuprates Y-123 and Bi-2212 (as well as a limitation of zinc(II) uptake into Y-123 cf. lithium(I), from the iodides) are attributable to cationic charge/size effects. XRD changes match those resulting from alternative copper-reduction processes. The relative redox potentials of lattice and complementary redox couple involved are important in governing the extent of cation uptake.