Beatriz Rivas-Murias

Highly transparent and conductive films of densely aligned ultrathin Au nanowire monolayers.

The combination of low electrical resistance and high optical transparency in a single material is very uncommon. Developing these systems is a scientific challenge and a technological need, to replace ITO in flexible electronic components and other highly demanding applications. Here we report a facile method to prepare single layers of densely aligned ultrathin Au-nanowires, homogeneous over cm(2) areas. The as-deposited films show an electrical/optical performance competitive with ITO and graphene-based electrodes. Moreover, the Au-films show a good stability under ambient conditions, and the large aspect ratio of the ultrathin nanowires makes them perfect for deposition in flexible substrates.

Dielectric response of the charge-ordered two-dimensional nickelate La1.5Sr0.5NiO4

We report the dielectric response of La1.5Sr0.5NiO4, a system that experiences charge ordering above room temperature (TCO=480K) and a rearrangement of its charge-order pattern in the temperature region 160–200K. A careful analysis of the role of the electrical contacts used, sample thickness, and grain size on the experimental data allows us to determine that this material exhibits a high intrinsic dielectric constant. In addition, the temperature dependence of the dielectric constant, that shows a maximum in the region of the rearrangement of the charge-order pattern, points to a link between the two phenomena.

Effect of cation disorder on structural, magnetic and dielectric properties of La2MnCoO6 double perovskite.

The origin of dielectric anomalies and magnetodielectric response of La(2)MnCoO(6) has been investigated by means of ultra-high resolution synchrotron x-ray powder diffraction, neutron powder diffraction, resistivity, magnetization and dielectric measurements. The study has been performed on two different bulk samples presenting different degrees of Mn/Co order: 95 and 74%. Beside the well-known influence on magnetic properties, our results show that the main effect of disorder lies on the electrical resistivity. Bond distances clearly show Mn(4+)/Co(2+) valence states in the well-ordered sample, while for the disordered one this picture still holds. AC resistivity data show dielectric anomalies and a small magnetodielectric effect, but impedance complex plane analyses prove that these phenomena appear at the frequency-temperature region where extrinsic effects dominate the dielectric response.

Magnetic-field-dependent dielectric constant in La2∕3Ca1∕3MnO3

We report a rather high dependence of the dielectric permittivity on the magnetic field in La2∕3Ca1∕3MnO3. The variation is maximum at around 270K, little above the Curie temperature TC, and it reaches a 35% under only 0.5T. We attribute this phenomenon to the space-charge or interfacial polarization produced between the insulator and the metallic regions segregated intrinsically in the material above TC.

Dielectric properties of the charge-ordered mixed oxide CaMn7O12

We present in this paper a detailed investigation of the dielectric properties of the mixed oxide CaMn7O12 that shows a charge-ordering transition at TCO = 440 K. Surprisingly, this compound presents a high dielectric constant at room temperature. Data taken at several frequencies and temperatures point to relaxor dielectric behaviour, that can be attributed to the electronic inhomogeneities present in the material. Extrinsic Maxwell–Wagner effects make a significant contribution to this dielectric response, as revealed by the studies made on samples of different particle size and using different types of contact. The intrinsic dielectric constant of this material is e �(intrinsic) ∼ 30 at room temperature, a value that is relatively high for this type of compound and that we relate to the presence of the electronic process of charge ordering in this material.

Room-Temperature Ferromagnetism in Thin Films of LaMnO3 Deposited by a Chemical Method Over Large Areas

Hole-doping into the Mott insulator LaMnO3 results in a very rich magneto-electric phase diagram, including colossal magnetoresistance and different types of charge and orbital ordering. On the other hand, LaMnO3 presents an important catalytic activity for oxygen reduction, which is fundamental for increasing the efficiency of solid-oxide fuel cells and other energy-conversion devices. In this work, we report the chemical solution (water-based) synthesis of high-quality epitaxial thin films of LaMnO3, free of defects at square-centimeter scales, and compatible with standard microfabrication techniques. The films show a robust ferromagnetic moment and large magnetoresistance at room temperature. Through a comparison with films grown by pulsed laser deposition, we show that the quasi-equilibrium growth conditions characteristic of this chemical process can be exploited to tune new functionalities of the material.

Dielectric properties of Bi1-xSrxMnO3 (x=0.40,0.50) manganites: Influence of room temperature charge order

The great tendency of Bi–Sr manganites to present charge order (CO) at very high temperatures and partial hybridization of the 6s2 lone pair with O p-orbitals makes these materials interesting as potential candidates for obtaining high dielectric constants with very small frequency dependence at room temperature. We have studied the dielectric properties of Bi1−xSrxMnO3 (x=0.40,0.50) perovskites that present CO well above room temperature (TCO∼525K). Thick (∼4mm) samples exhibit, at room and higher temperatures, high values (above 106) of the dielectric constant along a huge range of frequencies in which er′ is nearly unvarying. The dependence on the thickness of the dielectric constant, its evolution with temperature, and the Cole–Cole diagrams evidence the influence of extrinsic effects. To properly compare the results in the two compounds, we have performed a detailed structural study using neutron and synchrotron diffraction. CO is more extended in x=0.40. The results in the two compositions allowed u...

High quality thin films of thermoelectric misfit cobalt oxides prepared by a chemical solution method

Misfit cobaltates ([Bi/Ba/Sr/Ca/CoO]nRS[CoO2]q) constitute the most promising family of thermoelectric oxides for high temperature energy harvesting. However, their complex structure and chemical composition makes extremely challenging their deposition by high-vacuum physical techniques. Therefore, many of them have not been prepared as thin films until now. Here we report the synthesis of high-quality epitaxial thin films of the most representative members of this family of compounds by a water-based chemical solution deposition method. The films show an exceptional crystalline quality, with an electrical conductivity and thermopower comparable to single crystals. These properties are linked to the epitaxial matching of the rock-salt layers of the structure to the substrate, producing clean interfaces free of amorphous phases. This is an important step forward for the integration of these materials with complementary n-type thermoelectric oxides in multilayer nanostructures.

Tunnel Conduction in Epitaxial Bilayers of Ferromagnetic LaCoO3/La2/3Sr1/3MnO3 Deposited by a Chemical Solution Method

We report magnetic and electronic transport measurements across epitaxial bilayers of ferromagnetic insulator LaCoO3 and half-metallic ferromagnet La2/3Sr1/3MnO3 (LCO/LSMO: 3.5 nm/20 nm) fabricated by a chemical solution method. The I-V curves at room temperature and 4K measured with conducting atomic force microscopy (CAFM) on well-defined patterned areas exhibit the typical features of a tunneling process. The curves have been fitted to the Simmons model to determine the height (φ) and width (s) of the insulating LCO barrier. The results yield φ = 0.40 ± 0.05 eV (0.50 ± 0.01 eV) at room temperature (4K) and s = 3 nm, in good agreement with the structural analysis. Our results demonstrate that this chemical method is able to produce epitaxial heterostructures with the quality required for this type of fundamental studies and applications.

High dielectric constant in charge-ordered Ca1.75Pr0.25MnO4

We report the dielectric response of Ca2?xPrxMnO4 (x = 0 and 0.25). Specifically, their intrinsic dielectric constants are calculated by impedance spectroscopy from the values of the obtained bulk capacitances. For Ca2MnO4, that does not show charge-order, the measured intrinsic dielectric constant is 14. The Pr doping induces charge-order in the system, and also a noteworthy increase in , that reaches a value of ~220 for x = 0.25, unusual in these kind of oxides. This result provides a new link between both phenomena.