8 research outputs found
Large magnetoresistances and non-Ohmic conductivity in EuWO[1+x]N[2-x]
The magnetic field and voltage dependent electronic transport properties of EuWO[1+x]N[2-x] ceramics are reported. Large negative magnetoresistances are observed at low temperatures, up to 70% in the least doped (x=0.09) material. Non-Ohmic conduction emerges below the 12 K Curie transition. This is attributed to a microstructure of ferromagnetic conducting and antiferromagnetic insulating regions resulting from small spatial fluctuations in the chemical doping
ChemInform Abstract: Electronic Tuning of Two Metals and Colossal Magnetoresistances in EuWO1+xN2-x Perovskites.
This article is closed access.A remarkable electronic flexibility and colossal magnetoresistance effects have been discovered in the perovskite oxynitrides EuWO1+xN 2-x. Ammonolysis of Eu2W2O9 yields scheelite-type intermediates EuWO4-yNy with a very small degree of nitride substitution (y = 0.04) and then EuWO1+xN 2-x perovskites that show a wide range of compositions -0.16 < x < 0.46. The cubic lattice parameter varies linearly with x, but electron microscopy reveals a tetragonal superstructure. The previously unobserved x < 0 regime corresponds to oxidation of Eu (hole doping of the Eu:4f band), whereas x > 0 materials have chemical reduction of W (electron doping of the W:5d band). Hence, both the Eu and W oxidation states and the hole/electron doping are tuned by varying the O/N ratio. EuWO1+xN2-x phases order ferromagnetically at 12 K, and colossal magnetoresistances (CMR) are observed in the least doped (x = -0.04) sample. Distinct mechanisms for the hole and electron magnetotransport regimes are identified. © 2010 American Chemical Society
Electronic tuning of two metals and colossal magnetoresistances in EuWO1+ xN2- x perovskites
A remarkable electronic flexibility and colossal magnetoresistance effects have been discovered in the perovskite oxynitrides EuWO1+xN 2-x. Ammonolysis of Eu2W2O9 yields scheelite-type intermediates EuWO4-yNy with a very small degree of nitride substitution (y = 0.04) and then EuWO1+xN 2-x perovskites that show a wide range of compositions -0.16 0 materials have chemical reduction of W (electron doping of the W:5d band). Hence, both the Eu and W oxidation states and the hole/electron doping are tuned by varying the O/N ratio. EuWO1+xN2-x phases order ferromagnetically at 12 K, and colossal magnetoresistances (CMR) are observed in the least doped (x = -0.04) sample. Distinct mechanisms for the hole and electron magnetotransport regimes are identified. © 2010 American Chemical Society
The role of steam treatment on the structure, purity and length distribution of multi-walled carbon nanotubes
Purification and shortening of carbon nanotubes have attracted a great deal of attention to increase the biocompatibility and performance of the material in several applications. Steam treatment has been employed to afford both purification and shortening of multi-walled carbon nanotubes (MWCNTs). Steam removes the amorphous carbon and the graphitic particles that sheath catalytic nanoparticles, facilitating their removal by a subsequent acidic wash. The amount of metal impurities can be reduced in this manner below 0.01 wt.%. The length distribution of MWCNTs after different steam treatment times (from 1 h to 15 h) was assessed by box plot analysis of the electron microscopy data. Samples with a median length of 0.57 mu m have been prepared with the reported methodology while preserving the integrity of the tubular wall structure. (C) 2015 Elsevier Ltd. All rights reserved
The role of steam treatment on the structure, purity and length distribution of multi-walled carbon nanotubes
Purification and shortening of carbon nanotubes have attracted a great deal of attention to increase the biocompatibility and performance of the material in several applications. Steam treatment has been employed to afford both purification and shortening of multi-walled carbon nanotubes (MWCNTs). Steam removes the amorphous carbon and the graphitic particles that sheath catalytic nanoparticles, facilitating their removal by a subsequent acidic wash. The amount of metal impurities can be reduced in this manner below 0.01 wt.%. The length distribution of MWCNTs after different steam treatment times (from 1 h to 15 h) was assessed by box plot analysis of the electron microscopy data. Samples with a median length of 0.57 mu m have been prepared with the reported methodology while preserving the integrity of the tubular wall structure. (C) 2015 Elsevier Ltd. All rights reserved
Ag2CuMnO4 : A new silver copper oxide with delafossite structure
The use of hydrothermal methods has allowed the synthesis of a new silver copper mixed oxide, Ag2CuMnO4, the first example of a quaternary oxide containing both elements. It crystallizes with the delafossite 3R structure, thus being the first delafossite to contain both Ag and Cu. Synthesis conditions affect the final particle size (30-500 nm). Powder X-ray diffraction Rietveld refinement indicates a trigonal structure (R (3) over barm) and cell parameters a = 2.99991 angstrom and c = 18.428 angstrom, where Cu and Mn are disordered within the octahedral B positions in the plane and linearly coordinated Ag occupies de A position between layers. X-ray absorption near edge spectroscopy (XANES) for copper and manganese, and XPS for silver evidence + 2, + 4, and + 1 oxidation states. The microstructure consists of layered particles that may form large twins showing 5 nm nanodomains. Finally, magnetic measurements reveal the existence of ferromagnetic coupling yielding in-plane moments that align antiferromagnetically at lower temperatures. The singularity of the new phase resides on the fact that is an example of a bidimensional arrangement of silver and copper in an oxide that also shows clear bidimensionality in its physical properties. That is of special relevance to the field of high T. superconducting oxides, while the ferromagnetic coupling in a bidimensional system deserves itself special attention. (c) 2006 Elsevier Inc. All rights reserved
Direct Monolithic Integration of Vertical Single Crystalline Octahedral Molecular Sieve Nanowires on Silicon
We developed an original strategy
to produce vertical epitaxial
single crystalline manganese oxide octahedral molecular sieve (OMS)
nanowires with tunable pore sizes and compositions on silicon substrates
by using a chemical solution deposition approach. The nanowire growth
mechanism involves the use of track-etched nanoporous polymer templates
combined with the controlled growth of quartz thin films at the silicon
surface, which allowed OMS nanowires to stabilize and crystallize.
α-quartz thin films were obtained after thermal activated crystallization
of the native amorphous silica surface layer assisted by Sr<sup>2+</sup>- or Ba<sup>2+</sup>-mediated heterogeneous catalysis in the air
at 800 °C. These α-quartz thin films work as a selective
template for the epitaxial growth of randomly oriented vertical OMS
nanowires. Therefore, the combination of soft chemistry and epitaxial
growth opens new opportunities for the effective integration of novel
technological functional tunneled complex oxides nanomaterials on
Si substrates