We describe a way of generating films (<2 mm2; <40 μm thick) of aligned Fe-filled carbon nanotubes. These Fe nanowires are usually composed of single Fe crystals, and have dimensions from 5–40 nm outer diameter and <10 μm in length. The carbon tubes, which coat the wires, have external diameters of ∼20–70 nm and are <40 μm in length. High-resolution electron energy loss spectroscopy, x-ray powder diffraction, and elemental mapping of the tubular structures reveal only characteristic metallic signals and the effective absence of oxygen (or any other nonmetallic element) within the wires. The material exhibits coercivities in the 430–1070 Oe range, i.e., greater than those reported for Ni and Co nanowires.

We demonstrate that one of the most commonly used criteria to ascertain that tunneling is the dominant conduction mechanism in magnetic tunnel junctions-fits of current-voltage (I-V) data-is far from reliable. Using a superconducting electrode and measuring the differential conductance below T c , we divide samples into junctions with an integral barrier and junctions having metallic shorts through the barrier. Despite the clear difference in barrier quality, equally reasonable fits to the I-V data are obtained above T c. Our results further suggest that the temperature dependence of the zero-bias resistance is a more solid criterion, which could therefore be used to rule out possible pinholes in the barrier.

Point contact and tunnelling experiments performed at low temperatures were used to study the electronic behaviour of the icosahedral quasicrystalline alloys AlPdRe, AlCuFe, and AlPdMn. With samples of high quality we observed at low temperatures a zero-bias anomaly that we related to the decrease of the electronic density of states (DOS) due to the electron-electron interaction. This interaction tends to diminish the DOS at the Fermi level and can be seen as the energy pseudogap of the alloy. Our experiments indicate that the DOS is strongly modified near and consists of a spiky feature in a broad pseudogap, with the width of the feature of the order of 100 meV or even larger for the AlPdRe, whereas it is as small as 20-22 meV for Al-Cu-Fe and 17-20 meV for Al-Pd-Mn. The broad pseudogap has widths larger than 400 meV for AlPdRe, whereas for AlCuFe it is about 80-90 meV and for AlPdMn it is of the order of 110-122 meV. The studies were performed on three samples of the compositions , , and . The junctions were of the types alloy-Au(In, Al) and alloy-insulator-Au(In, Al), and were studied at different temperatures between that of liquid nitrogen and 2 K, and even to 400 mK for the AlCuFe alloy.

No abstract

No abstract

The effect of boron excess in the structure and superconducting properties of NbB 2 is reported. Rietveld refinements of the x-ray diffraction patterns indicate that boron excess induces significant changes in the Nb-B bond length, increasing the c-axis. In contrast, the B-B bond length remains essentially constant. Magnetization behaviour was studied in the temperature range from 2 to 15 K. We found that for (B/Nb) exp 2.20(2) of boron excess samples display superconductivity with a maximum T C of about 9.8 K at (B/Nb) exp = 2.34(1). High pressure measurements in samples with two different boron contents reveal that T C decreases at different ratios, dT C /d P. Superconducting parameters were determined, indicating that NbB 2+x is a type II superconductor. We correlated the change of T C with the evolution of the structural parameters and found that it coincides with theoretical predictions of band structure.

Interest in magnetic-tunnel junctions has prompted a re-examination of tunneling measurements through thin insulating films. In any study of metal-insulator-metal trilayers, one tries to eliminate the possibility of pinholes (small areas over which the thickness of the insulator goes to zero so that the upper and lower metals of the trilayer make direct contact). Recently, we have presented experimental evidence that ferromagnet-insulator-normal trilayers that appear from current-voltage plots to be pinhole-free may nonetheless in some cases harbor pinholes. Here, we show how pinholes may arise in a simple but realistic model of film deposition and that purely classical conduction through pinholes may mimic one aspect of tunneling, the exponential decay in current with insulating thickness.

The 5d electron-based Sr2−xLaxIrO4 system (0≤x≤0.2) has been synthesized by a solid-state route. The x = 0 composition is a nonmetallic weak ferromagnet with a Curie temperature at about 240 K. The crystal structure behaviour and magnetic properties exhibited by this Sr2−xLaxIrO4 system can be explained on the basis of the extended character of the 5d electrons of the Ir cation and its valence states. Rietveld analysis of x-ray powder diffraction on Sr2IrO4 agrees well with previous structural neutron experiments. Furthermore, density functional theory (DFT) calculations predict that I41/acd represents a more stable crystal structure than K2NiF4 (I4/mmm). Electrical resistivity and magnetic properties of Sr2−xLaxIrO4 are strongly dependent on the Ir3+ content. The Sr2−xLaxIrO4 magnetic behaviour in the range of 2–240 K can be ascribed to a weak ferromagnet, produced by an array of canted antiferromagnetically ordered Ir4+ magnetic moments.