Markus Valkeapää

Oxygen Ordering and Mobility in YBaCo4O7+δ

Extraordinary oxygen ordering and mobility have been found in an oxygen-nonstoichiometric mixed-valence cobalt oxide, YBaCo(4)O(8.5). The excess oxygen atoms appear to be incorporated in an orderly way into the YBaCo(4)O(7) parent lattice with different configurations as the oxygen content varies. Intense electron-beam irradiation was used to observe recurrent oxygen migration in the lattice, causing reversible structural modulation transitions. The oxygen migration can be attributed mainly to the electron-beam heating effect. This study highlights the high degree of freedom of surplus oxygen within the investigated structure and advances our understanding of the oxygen diffusion process in related transition-metal oxide systems.

Josephson effects in magnesium diboride based Josephson junctions

We report on Josephson effects in Josephson junctions fabricated from magnesium diboride (MgB2) thin films using a focused ion beam (FIB) milling technique. The films were deposited on SrTiO3(100) and substrates at room temperature using e-beam evaporation of MgB2 with a post-annealing stage in Mg vapour. Conventional photolithography and Ar-ion-beam milling were used to pattern the films into 4 and 8 µm wide microbridges. A focused ion beam was used to narrow the microbridges to 2 µm. The narrowed microbridges were thinned by making 50 nm wide cuts across them. The depth of each cut was calibrated to remove 75% of the film thickness. A thin MgB2 layer remained on the substrate after this process. The current–voltage (I–V) characteristics of junctions made using this technique show that the junctions carry excess current. The first Shapiro step was observed when one of the junctions was irradiated with a microwave field of frequency f = 8.92 GHz. The Shapiro step appeared at a voltage value V = hf/2e = 18.445 µV.

The Effect of Varying Ca-Content on the Structure of High-Tc Superconductor (CaxLa1-x)(Ba1.75-xLa0.25+x)Cu3O7-δ (x = 0.5, 0.6, and 0.8) Studied by Neutron Powder Diffraction

We have performed neutron powder diffraction (NPD) experiments on polycrystalline powders with nominal compositions (Ca0.5La0.5)(Ba1.25La0.75)Cu3O7- δ , (Ca0.6La0.4)(Ba1.15La0.85)Cu3O7-δ and (Ca0.8La0.2)(Ba0.95La1.05)Cu3O7-δ . The diffraction patterns, analysed by the Rietveld method, show that all samples consist mainly of a tetragonal Y-123 type phase. Unit cell parameters a and c shorten as the calcium content increases: a = 3.8660(2), 3.8634(3), and 3.8624(5) Å; c = 11.6325(11), 11.6143(14), and 11.5822(20) Å for x-values 0.5, 0.6, and 0.8, respectively. For the x = 0.6 and 0.8 samples the Rietveld refinement of calcium occupancies and EDX analysis suggest that the actual composition is closer to x ≈ 0.5. However, since the lattice parameters do change, it is also suggested that at these higher doping levels calcium does enter the Y- site to a larger extent than for the x = 0.5 composition. This is also in accordance with previously reported values for the Tc, which decreases slighty as x changes from 0.5 to 0.6 and has a pronounced change from 80 K to 73 K for x = 0.6 and 0.8, respectively.

The Structure of Na2W2O7·H2O Synthesized under High Pressure and Temperature

A new compound, Na 2 W 2 O 7 .1/2H 2 O, has been found in the Na 2 O-WO 3 -H 2 O system. It is the fourth compound in the Na 2 W 2 O 7 .xH 2 O system and the second phase containing water. The crystals were prepared at a pressure of 1.5 GPa and a temperature of 900°C. Na 2 W 2 O 7 .1/2H 2 O crystallizes in the triclinic space group, P1, Z=2, with lattice constants of a = 8.4902(6) A, b = 7.6636(6) A, c = 5.1501(3) A, α = 91.959(7)°, β=96.521(8)° and γ=111.678(7)°. The structure is built up from double chains of WO 6 octahedra running along c, interconnected by two-dimensional nets of NaO 6 polyhedra. Sodium coordinates to six oxygens forming both octahedra and trigonal prisms.

Phase Transitions and Magnetic Order in La 1−x Sr x MnO 3+δ (x≤ 0.2; 2.85≤ 2−δ≤ 3.00)

The system, La 1 m x Sr x MnO 3 m i (0.0 h x h 0.2; 2.85 h 3 m i h 3.00), has been studied by neutron powder diffraction and Raman scattering. The x= 0.0 and x= 0.1 samples with 3 m i , 3.00, both show orthorhombic lattice symmetry, space group Pnma. La 0.8 Sr 0.2 MnO 3.00 on the other hand show rhombohedral lattice symmetry (space group R-3c) irrespective of temperature. At 130 K and below all the oxidised samples investigated show ferromagnetic order. LaMnO 3 m i does at 10 K show a total magnetic moment of 3.17(4) w B , directed along the c-axes. With an increase in temperature the preferred direction of the magnetic moment become less obvious, and finally, the magnetic order is lost. Quenched samples such as LaMnO 2.85 show monoclinic lattice symmetry. Below , 150 K extra magnetic peaks appear corresponding to an antiferromagnetic order. The doped system, La 0.9 Sr 0.1 MnO 2.85 on the other hand show a sequence of transitions (paramagnetic to ferromagnetic to antiferromagnetic). Raman work on the oxidised samples show an orthorhombic and a rhombohedral phase but it also becomes clear that the rhombohedral phase show a high degree of local distortions.

La 1− x Sr x MnO 3 (0.33 ≤ x ≤ 1.0) Perovskites; A Powder Diffraction and Magnetisation Study

Powders of strontium doped lanthanum manganates (LSM) with a nominal composition La 1 m x Sr x MnO 3 (0.33 h x h 1.0) were prepared by a solid-state reaction. The samples were characterised by x-ray powder diffraction (XRPD), neutron powder diffraction (NPD), and magnetisation measurements performed with a SQUID magnetometer. NPD patterns were further studied with Rietveld analysis. XRPD, NPD and Rietveld analyses show that samples with x up to 0.45 crystallise single phase in a rhombohedral unit cell in space group R-3c (#167). The composition x = 0.55 is best described with a tetragonal unit cell with unit cell parameters a = 3.846 A and c = 3.876 A. From x = 0.60 to x = 0.950 the samples are mixtures of LaMnO 3 and SrMnO 3 phases. Pure SrMnO 3 as well as the composition where x = 0.975 crystallise in hexagonal space group P63/mmc (#194). The bulk resistivity of the as prepared samples shows that increasing strontium content results in higher resistivity in the material.