M. Croft

Preparation of Y‐Ba‐Cu oxide superconductor thin films using pulsed laser evaporation from high Tc bulk material

We report the first successful preparation of thin films of Y‐Ba‐Cu‐O superconductors using pulsed excimer laser evaporation of a single bulk material target in vacuum. Rutherford backscattering spectrometry showed the composition of these films to be close to that of the bulk material. Growth rates were typically 0.1 nm per laser shot. After an annealing treatment in oxygen the films exhibited superconductivity with an onset at 95 K and zero resistance at 85 and 75 K on SrTiO3 and Al2O3 substrates, respectively. This new deposition method is relatively simple, very versatile, and does not require the use of ultrahigh vacuum techniques.

Nature of the pulsed laser process for the deposition of high Tc superconducting thin films

The pulsed laser thin‐film deposition process can enable preparation of thin films of complex composition with good control over the film stoichiometry. The film compositions are similar to that of the target pellet and as a consequence this technique appears to be an ideal method for preparing high Tc thin films on a variety of substrates.The factors which contribute to this beneficial phenomenon have been explored by a laser ionization mass spectrometry (LIMS) and a post ablation ionization (PAI) neutral velocity analysis technique in order to determine the mass and velocities of the laser ejected material. In addition, x‐ray absorption measurements on films deposited onto substrates at room temperature were performed in order to identify the presence of short‐range crystalline order in the films. Both of these studies rule out the ejection of stoichiometric clusters of material from the pellet during the laser ablation/deposition process. Instead, binary and ternary suboxides are emitted from the targe...

Quasi two-dimensional electronic properties of the lithium molybdenum bronze, Li0.9Mo6o17

Abstract Four probe electrical resistivity measurements between 0.3K and 300K were made on single crystals of the violet-red bronze Li 0.9 Mo 6 O 17 grown by a temperature gradient flux technique. The temperature variation of the resistivity shows metallic conductivity and highly anisotropic behavior similar to K 0.9 Mo 6 O 17 . The room temperature resistivity, measured in the direction parallel to the plate axis, is 9.5×10 -3 Ωcm and 2.47Ωcm perpendicular to that axis. A phase transition observed at ∼24K is possibly related to the onset of a charge density wave. A transition to the superconducting state is observed at Tc ∼ 1.9K.

Structural, electronic, and dielectric properties of ultrathin zirconia films on silicon

As high-permittivity dielectrics approach use in metal-oxide-semiconductor field-effect transistor production, an atomic level understanding of their dielectric properties and the capacitance of structures made from them is being rigorously pursued. We and others have shown that crystal structure of ZrO2 films have considerable effects on permittivity as well as band gap. The as-deposited films reported here appear amorphous below a critical thickness (∼5.4nm) and transform to a predominantly tetragonal phase upon annealing. At much higher thickness the stable monoclinic phase will be favored. These phase changes may have a significant effect on channel mobility.

Charge ordering and Magnetoresistance of Ca1-xCexMnO3

The two-electron (2e) doped Ca{sub 1{minus}x}Ce{sub x}MnO{sub 3} (0{le}x{le}0.2) system was prepared by sol-gel method and investigated by x-ray absorption spectroscopy (XAS), X-ray diffraction, and temperature and field dependent magnetic and transport measurements. The results are compared in detail to the one-electron (1e)-doped La{sub 1{minus}y}Ca{sub y}MnO{sub 3} system, facilitated by the analogous y=1{minus}2x parametrization as Ca{sub (1+y)/2}Ce{sub (1{minus}y)/2}MnO{sub 3} (0.6{le}y{le}1.0). The XAS results indicate: that the formal valence of cerium is Ce{sup 4+}, thereby validating the charge transfer of 2e per doped Ce to Mn. The XAS also shows separate Mn{sup 3+} and Mn{sup 4+} spectral components, in dramatic contrast to results on 1e-doped systems. Low Ce doping (x=0.025 and 0.05) rapidly stabilizes a robust ferromagnetic component in addition to a persistent antiferromagnetic component in the ordered state. Higher Ce doping (0.075{le}x{le}0.20) induces a charge/orbital ordering transition that increases with composition to near 255 K. In different composition ranges the magnetoresistance manifests inter-grain-tunneling and a field coupling of the local charge/orbital order parameter mechanisms. The latter mechanism appears to induce a large magnetoresistance, as high as {minus}72%, in Ca{sub 0.925}Ce{sub 0.075}MnO{sub 3}.

Systematics of thin films formed by excimer laser ablation: Results on SmBa2Cu3O7

Excimer laser ablation was used to prepare thin films from a bulk target of SmBa2Cu3O7. The systematic variation in film thickness, stoichiometry, and microstructure, as a function of laser fluence and angle from the target surface normal, was determined. We show that films which are stoichiometric over wide solid angles can be rapidly prepared from this material by use of high laser fluence. Films prepared from various Bi‐Sr‐Ca‐Cu‐O targets at high laser fluence also reproduce bulk stoichiometry. In contrast, films prepared by this technique from a YBa2Cu3O7 target are deficient in Ba and Cu for laser fluences required to produce compositional homogeneity over wide areas.

Strain field and scattered intensity profiling with energy dispersive x-ray scattering

Two powerful synchrotron x-ray scattering techniques for residual strain depth-profiling and tomography-like scatter-intensity profiling of materials are presented. The techniques utilize energy dispersive x-ray scattering, from a fixed microvolume, with microscanning of the specimen being used to profile its interior. The tomography-like profiles exploit scattering-cross-section variations, and can be contrast enhanced by separately monitoring scattering from different crystal structures. The strain profiling technique is shown to finely chronicle the internal strain variation over several mm of steel. Detailed strain profiling for a cantilever spring demonstrates the interplay of residual and external stresses in elastic/plastic deformation. Since surface compression, by shot peening, is a classic method to fortify against fatigue failure, the strain profile for a shot-peened, surface-toughened material is determined and discussed in terms of a simple elastic–plastic stress/strain model. Finally the lattice strains in a WC/Co coated steel composite material are discussed.

Polar and Magnetic Mn2FeMO6 (M = Nb, Ta) with LiNbO3-type Structure - High Pressure Synthesis

Polar oxides are of much interest in materials science and engineering. Their symmetry-dependent properties such as ferroelectricity/multiferroics, piezoelectricity, pyroelectricity, and second-order harmonic generation (SHG) effect are important for technological applications. [1] However, polar crystal design and synthesis is challenging, because multiple effects, such as steric or dipole-dipole interactions, typically combine to form non-polar structures; so the number of known polar materials, especially polar magnetoelectric materials, is still severely restricted. [2] Therefore, it is necessary for the material science community to develop new strategies to create these materials.

Giant Magnetoresistance in the Half-Metallic Double-Perovskite Ferrimagnet Mn2FeReO6

The first transition-metal-only double perovskite compound, Mn(2+) 2 Fe(3+) Re(5+) O6 , with 17 unpaired d electrons displays ferrimagnetic ordering up to 520 K and a giant positive magnetoresistance of up to 220 % at 5 K and 8 T. These properties result from the ferrimagnetically coupled Fe and Re sublattice and are affected by a two-to-one magnetic-structure transition of the Mn sublattice when a magnetic field is applied. Theoretical calculations indicate that the half-metallic state can be mainly attributed to the spin polarization of the Fe and Re sites.

Mössbauer effects and LIII absorption measurements on EuPd2Si2

Abstract Detailed Mossbauer effect and L III absorption spectroscopy experiments on a sample of EuPd 2 Si 2 are reported. A quantitative correlation of these two valence sensitive spectroscopies is made over the range of the thermally induced valence change of 0.51 electrons. The resulting relation between 151 Eu isomer shift and L III determined valence is used to provide a tentative estimate of the valence states of a number of other 1:2:2 Eu compounds and to emphasize the extent to which magnetic order and valence mixing coexists in such compounds.