Epitaxial ferroelectric memristors integrated with silicon

Neuromorphic computing requires the development of solid-state units able to electrically mimic the behavior of biological neurons and synapses. This can be achieved by developing memristive systems based on ferroelectric oxides. In this work we fabricate and characterize high quality epitaxial BaTiO3-based memristors integrated with silicon. After proving the ferroelectric character of BaTiO3 we tested the memristive response of LaNiO3/BaTiO3/Pt microstructures and found a complex behavior which includes the co-existence of volatile and non-volatile effects, arising from the modulation of the BaTiO3/Pt Schottky interface by the direction of the polarization coupled to oxygen vacancy electromigration to/from the interface. This produces remanent resistance loops with tunable ON/OFF ratio and asymmetric resistance relaxations. These properties might be harnessed for the development of neuromorphic hardware compatible with existing silicon-based technology

Thermoelectric properties of CaMnO3 films obtained by soft chemistry synthesis

Polycrystalline randomly oriented CaMnO3 films were successfully deposited on sapphire substrates by soft chemistry methods. The precursor solutions were obtained from a mixture of metal acetates dissolved in acids. The Seebeck coefficient and the electrical resistivity were measured in the temperature range of 300 K < T < 1000 K. Modifications of thermal annealing procedures during the deposition of precursor layers resulted in different power factor values. Thermal annealing of CaMnO3 films at 900 °C for 48 h after four-layer depositions (route A) resulted in a pure perovskite phase with higher power factor and electrical resistivity than four-layer depositions of films annealed layer by layer at 900 °C for 48 h (route B). The studied films have negative Seebeck coefficients indicative of n-type conduction and electrical resistivities showing semiconducting behavio

Characteristic length scale of the magnon accumulation in Fe3O4/Pt bilayer structures by incoherent thermal excitation

The dependence of Spin Seebeck effect (SSE) with the thickness of the magnetic materials is studied by means of incoherent thermal excitation. The SSE voltage signal in Fe3O4/Pt bilayer structure increases with the magnetic material thickness up to 100 nm, approximately, showing signs of saturation for larger thickness. This dependence is well described in terms of a spin current pumped in the platinum film by the magnon accumulation in the magnetic material. The spin current is generated by a gradient of temperature in the system and detected by the Pt top contact by means of inverse spin Hall effect. Calculations in the frame of the linear response theory adjust with a high degree of accuracy the experimental data, giving a thermal length scale of the magnon accumulation (Λ) of 17 ± 3 nm at 300 K and Λ = 40 ± 10 nm at 70 K.This work was supported by the Spanish Ministry of Science (through Project No. MAT2014-51982-C2-R, including European social fund), the Aragon Regional government (Project No. E26), and Thermo-Spintronic Marie Curie CIG Project (Grant Agreement No. 304043).Peer Reviewe

Pulsed current-voltage electrodeposition of stoichiometric Bi2Te3 nanowires and their crystallographic characterization by transmission electron backscatter diffraction

Bi2Te3 nanowires with diameters ranging from 25 to 270 nm, ultra-high aspect ratio, and uniform growth front were fabricated by electrodeposition, pulsing between zero current density during the off time and constant potential during the on time (pulsed-current-voltage method, p-IV). The use of zero current density during the off time is to ensure no electrodeposition is carried out and the system is totally relaxed. By this procedure, stoichiometric nanowires oriented perpendicular to the c-axis is obtained for the different diameters of porous alumina templates. In addition, the samples show a uniform growth front with ultra-high aspect ratio single crystal nanowires. The high degree of crystallinity was verified by transmission electron backscatter diffraction. This characterization revealed that the nanowires present both large single crystalline areas and areas with alternating twin configurations

Tuning the interfacial charge, orbital, and spin polarization properties in La0.67Sr0.33MnO3/La1-xSrxMnO3 bilayers

The possibility of controlling the interfacial properties of artificial oxide heterostructures is still attracting researchers in the field of materials engineering. Here, we used surface sensitive techniques and high-resolution transmission electron microscopy to investigate the evolution of the surface spin-polarization and lattice strains across the interfaces between La0.66Sr0.33MnO3 thin films and low-doped manganites as capping layers. We have been able to fine tune the interfacial spin-polarization by changing the capping layer thickness and composition. The spin-polarization was found to be the highest at a critical capping thickness that depends on the Sr doping. We explain the non-trivial magnetic profile by the combined effect of two mechanisms: On the one hand, the extra carriers supplied by the low-doped manganites that tend to compensate the overdoped interface, favouring locally a ferromagnetic double-exchange coupling. On the other hand, the evolution from a tensile-strained structure of the inner layers to a compressed structure at the surface that changes gradually the orbital occupation and hybridization of the 3d-Mn orbitals, being detrimental for the spin polarization. The finding of an intrinsic spin-polarization at the A-site cation observed in x-ray magnetic circular dichroism (XMCD) measurements also reveals the existence of a complex magnetic configuration at the interface, different from the magnetic phases observed at the inner layers

Onion-like Fe3O4/MgO/CoFe2O4 magnetic nanoparticles: new ways to control magnetic coupling between soft/hard phases

The control of the magnetization inversion dynamics is one of the main challenges driving the design of new nanostructured magnetic materials for magnetoelectronic applications. Nanoparticles with onion-like architecture offer a unique opportunity to expand the possibilities allowing to combine different phases at the nanoscale and also modulate the coupling between magnetic phases by introducing spacers in the same structure. Here we report the fabrication, by a three-step high temperature decomposition method, of Fe3O4/MgO/CoFe2O4 onio-like nanoparticles and their detailed structural analysis, elemental compositional maps and magnetic response. The core/shell/shell nanoparticles present epitaxial growth and cubic shape with overall size of (29+-6) nm. These nanoparticles are formed by cubic iron oxide core of (22+-4) nm covered by two shells, the inner of magnesium oxide and the outer of cobalt ferrite of ~1 and ~2.5 nm of thickness, respectively. The magnetization measurements show a single reversion magnetization curve and the enhancement of the coercivity field, from HC~608 Oe for the Fe3O4/MgO to HC~5890 Oe to the Fe3O4/MgO/CoFe2O4 nanoparticles at T=5 K, ascribed to the coupling between both ferrimagnetic phases with a coupling constant of =2 erg/cm2. The system also exhibits exchange bias effect, where the exchange bias field increases up to HEB~2850 Oe at 5 K accompanied with the broadening of the magnetization loop of HC~6650 Oe. This exchange bias effect originates from the freezing of the surface spins below the freezing temperature TF=32 K that pinned the magnetic moment of the cobalt ferrite shell.Comment: 39 pages, 8 figure

Efficient generation of highly crystalline carbon quantum dots via electrooxidation of ethanol for rapid photodegradation of organic dyes

Achieving versatile routes to generate crystalline carbon-based nanostructures has become a fervent pursuit in photocatalysis-related fields. We demonstrate that the direct electrooxidation of ethanol, performed on Ni foam, yields ultra-small and highly crystalline graphene-like structures named carbon quantum dots (CQDs). We perform simulations of various sp2 and sp3 domains in order to understand the optical properties of CQDs by accounting their contribution as absorbance/luminescent centers in the overall optical response. Experiments and simulations reveal that absorbance bands for as-synthesized CQDs are dominated by small sp2 domains comprised of r7 aromatic-rings. After 48 h synthesis, the dispersion transition from yellow to red, exhibiting new and red shifted absorbance bands. Furthermore, fluorescence emission is governed by medium-sized sp 2 domains (with aromatic ring counts r12) and oxygen-containing groups. These oxygen-rich groups within the CQDs, confirmed by FT-IR and XPS, are responsible for the fast photodegradation of organic dyes, with B90% of methylene blue (MB) being degraded within the first 5 min of light exposure. Our work provides crucial insights about the electrochemical synthesis and overall optical properties of carbon nanostructures, while being effective and reliable toward the degradation of contaminants in water

Co-sputtered PtMnSb thin films and PtMnSb/Pt bilayers for spin-orbit torque investigations

The manipulation of the magnetization by spin-orbit torques (SOTs) has recently been extensively studied due to its potential for efficiently writing information in magnetic memories. Particular attention is paid to non-centrosymmetric systems with space inversion asymmetry, where SOTs emerge even in single-layer materials. The half-metallic half-Heusler PtMnSb is an interesting candidate for studies of this intrinsic SOT. Here, we report on the growth and epitaxial properties of PtMnSb thin films and PtMnSb/Pt bilayers deposited on MgO(001) substrates by dc magnetron co-sputtering at high temperature in ultra-high vacuum. The film properties were investigated by X-ray diffraction, X-ray reflectivity, atomic force microscopy, and electron microscopy. Thin PtMnSb films present a monocrystalline C1b phase with (001) orientation, coexisting at increasing thickness with a polycrystalline phase with (111) texture. Films thinner than about 5 nm grow in islands, whereas thicker films grow ultimately layer-by-layer, forming a perfect MgO/PtMnSb interface. The thin PtMnSb/Pt bilayers also show island growth and a defective transition zone, while thicker films grow layer-by-layer and Pt grows epitaxially on the half-Heusler compound without significant interdiffusion. (C) 2017 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim</p

Thermoelectric oxides: challenges and selected approaches for materials design

Sem resumo disponível.publishe

Exploring tantalum as a potential dopant to promote the thermoelectric performance of zinc oxide

Zinc oxide (ZnO) has being recognised as a potentially interesting thermoelectric material, allowing flexible tuning of the electrical properties by donor doping. This work focuses on the assessment of tantalum doping effects on the relevant structural, microstructural, optical and thermoelectric properties of ZnO. Processing of the samples with a nominal composition Zn1-xTaxO by conventional solid-state route results in limited solubility of Ta in the wurtzite structure. Electronic doping is accompanied by the formation of other defects and dislocations as a compensation mechanism and simultaneous segregation of ZnTa2O6 at the grain boundaries. Highly defective structure and partial blocking of the grain boundaries suppress the electrical transport, while the evolution of Seebeck coefficient and band gap suggest that the charge carrier concentration continuously increases from x = 0 to 0.008. Thermal conductivity is almost not affected by the tantalum content. The highest ZT~0.07 at 1175 K observed for Zn0.998Ta0.002O is mainly provided by high Seebeck coefficient (-464 µV/K) along with a moderate electrical conductivity of ~13 S/cm. The results suggest that tantalum may represent a suitable dopant for thermoelectric zinc oxide, but this requires the application of specific processing methods and compositional design to enhance the solubility of Ta in wurtzite lattice