Search

The synthesis of phase‐pure ternary solutions of tetradymite‐type materials (BixSb1−x)2Te3 (x=0.25; 0.50; 0.75) in an ionic liquid approach has been carried out. The nanoparticles are characterized by means of energy‐dispersive X‐ray spectroscopy (EDX), powder X‐ray diffraction (PXRD), scanning electron microscopy (SEM), and transmission electron microscopy. In addition, the role of different processing approaches on the thermoelectric properties ‐ Seebeck coefficient as well as electrical and thermal conductivity ‐ is demonstrated.

3D topological insulators (TI) host surface carriers with extremely high mobility. However, their transport properties are typically dominated by bulk carriers that outnumber the surface carriers by orders of magnitude. A strategy is herein presented to overcome the problem of bulk carrier domination by using 3D TI nanoparticles, which are compacted by hot pressing to macroscopic nanograined bulk samples. Bi2 Te3 nanoparticles well known for their excellent thermoelectric and 3D TI properties serve as the model system. As key enabler for this approach, a specific synthesis is applied that creates nanoparticles with a low level of impurities and surface contamination. The compacted nanograined bulk contains a high number of interfaces and grain boundaries. Here it is shown that these samples exhibit metallic-like electrical transport properties and a distinct weak antilocalization. A downward trend in the electrical resistivity at temperatures below 5 K is attributed to an increase in the coherence length by applying the Hikami-Larkin-Nagaoka model. THz time-domain spectroscopy reveals a dominance of the surface transport at low frequencies with a mobility of above 103 cm2 V-1 s-1 even at room temperature. These findings clearly demonstrate that nanograined bulk Bi2 Te3 features surface carrier properties that are of importance for technical applications. © 2021 The Authors. Small published by Wiley-VCH GmbH.

An electrochemical and in situ magnetic study is conducted to identify key parameters for room-temperature electrochemical (magneto-ionic) switching of magnetism by repeated oxidation and reduction of iron species in aqueous KOH. Potentiodynamic routines allow to distinguish the reaction potentials and, in contrast to measurements on bulk Fe electrodes, uncover the importance of dissolution processes in thin film geometry. The pH of the solution affects both dissolution and reduction to metallic iron. The magnetic properties strongly depend on the applied oxidation and reduction potentials. Large magnetic changes are obtained when switching between the potential of reduction to metallic iron and intermediate oxidation potentials, revealing an influence of the structure of the oxide layer. Keywords: Magnetoelectric effect, Magnetic films, Magnetoionic effect, Oxidation, Reduction, Iron

Patterned media for future ultrahigh-density magnetic data storage can be prepared by electrodeposition into ordered arrays of nanostructured templates. Diblock copolymer templates have been proposed as a promising alternative to alumina templates. They possess smaller feature sizes than alumina and thereby allow for higher storage densities. Templates with pore diameters of ∼8 nm have been fabricated by dip-coating a conducting substrate into a solution of polystyrene- block -poly(4-vinylpyridine) (PS-b-P4VP) and 2-(4′-hydroxybenzeneazo) benzoic acid (HABA). A vapor annealing step was interposed for better ordering. The pores are opened by dissolving the HABA from the supramolecular assembly and afterwards filled by electrodeposition. Fe–Pt and Co–Pt alloys are the materials of choice for filling the pores due to their hard magnetic properties like high coercivity and anisotropy. Co-rich Co 80 Pt 20 was chosen for this study because it does not require post annealing as in the case of ordered L1 0 CoPt or FePt. Continuous films have been deposited in order to identify the optimum deposition conditions and the influence of deposition current density on chemical composition, growth morphology, and structural and magnetic properties has been studied systematically. The first results on templates electrochemically filled with Co–Pt are presented together with an analysis of their magnetic properties.

Fe-Pt films with an Fe/Pt ratio close to one can be electrodeposited from an FeSO 4 -H2PtCl 6 -Na 2 SO 4 electrolyte. At the deposition potential, the hydrogen evolution and the reduction of the Pt complex are diffusion limited, and Fe overpotential deposition has not yet set in. The sources of the Fe incorporation are iron hydroxide formation together with Fe underpotential deposition due to Fe-Pt alloy formation. Mossbauer measurements show that the iron in the iron hydroxide is predominantly Fe(III). For stoichiometry reasons, a Pt-rich Fe-Pt phase must be present in addition to the Fe(III)-hydroxide. The Fe 3+ that takes part in the hydroxide formation is produced in the electrolyte by the oxidation of Fe 2+ by the complexed Pt ion. This exchange reaction results in a significantly higher Fe 3+ content in the FeSO 4 -H 2 PtCl 6 -Na 2 SO 4 electrolyte in comparison to the same electrolyte without H 2 PtCl 6 . Fe(lII)-hydroxide formation can be depressed by adding citric acid, that acts as buffering and complexing agent. This leads to a lower iron content of the deposits. The Fe/Pt ratio close to one that is needed for hard magnetic properties can, however, only be achieved with a significant incorporation of iron hydroxide.

Electrodeposition from a single bath is used to prepare Fe-Pt films and Fe/Pt multilayers for hard magnetic film applications. Electrodeposition of Fe-Pt is not a simple codeposition of Fe and Pt. Large amounts of oxygen are found in as deposited films. In addition, Fe incorporation takes place at potentials positive to the equilibrium reduction potential of Fe2+. The analysis of the electrode processes and of potentiostatically deposited films indicate that both, iron hydroxide incorporation and underpotential deposition of Fe must be considered. The oxygen content could not be reduced in homogeneous films with an Fe-Pt ratio of 1.0 as the depression of iron hydroxide formation also leads to inhibition of iron incorporation. However, the alternating deposition of Fe-rich and Pt-rich layers, both with low oxygen content, leads to a lower integral oxygen content for an Fe/Pt ratio of 1.0. H2 annealing of the multilayered films reduces remaining iron (hydro)oxides and results in high coercivities and remanences.

Composition and structure of electrodeposited Fe–Pt–O films are investigated in dependence on deposition potential. At potentials positive to the hydrogen evolution, a solid solution of Pt with some Fe is present that can be explained by underpotential deposition of Fe. In the potential region where hydrogen evolution by proton reduction is a side reaction, oxygen is detected in the films. The O content scales with the Fe content and at least part of the Fe is oxidized. When applying more cathodic potentials, the fraction of Fe and O increases, grain size is reduced and an amorphous/nanocrystalline phase forms. The Fe content is pH dependent in this potential region. At even more cathodic potentials, overpotential deposition of Fe in the bcc structure sets in, the O content is reduced. The results are discussed with respect to thermodynamics in the Fe–Pt–(O) system. The concepts of underpotential deposition of Fe during Fe-Pt alloy deposition and of hydroxide formation due to a pH rise at the cathode are evaluated. These simple models can only partially describe the deposition process in agreement with the compositional and structural information obtained and a coupled mechanism is proposed.

An electrochemical quartz crystal microbalance study has been performed in order to understand the alloy electrodeposition process and especially the mechanism of oxygen incorporation during Fe–Pt electrodeposition. It reveals that the presence of Pt leads to an enhanced hydrogen evolution reaction. In comparison to the single metal depositions, an additional current and an anomalous mass increase are observed when depositing from the complete electrolyte. The corresponding films exhibit a high O content and a needle-like morphology. These characteristics are altered by nitrogen gas stirring. Fe hydroxide formation due to a pH rise at the cathode and underpotential deposition of Fe deposition due to Fe–Pt alloy formation are discussed to interpret the results.

Oxides with spinel structure have received widespread attention as being suitable electrode materials for rechargeable Li-ion batteries. So far they have mostly been synthesized at relatively high temperatures of between 700 and 800 °C. Here, we describe a new synthesis route to LiMn2O4 and manganese dioxides with spinel structure based on the freeze-drying of aqueous solutions and allowing the preparation of spinels at temperatures of around 400 °C. Undoped and Bi doped samples were prepared and investigated electrochemically in strong alkaline electrolyte (9 M KOH) with regard to their charge/discharge properties. As is known from other modifications of manganese dioxides, the cyclability is improved in the presence of bismuth ions.

We describe a new chemical solution deposition (CSD) route toward inexpensive and environmentally friendly preparation of YBCO coated conductors. This route is based on a fluorine-free polymer and exhibits some advantages compared to the TFA route, where trifluoroacetate precursors are used and aggressive HF is formed during the heat treatment. In this work, the main parameters influencing the structure and properties of YBCO films prepared by means of the polymer method are investigated. For the preparation of the YBCO films, stoichiometric amounts of Y, Ba and Cu nitrates were dissolved in a solution of polyacrylic acid in dimethylformamide. This metal polymer precursor solution was spin coated onto strontium titanate (STO) single crystals. After drying in air, the films were heat treated in an oxygen containing nitrogen atmosphere. The film structure was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). T/sub c/ and J/sub c/ were measured inductively. The preliminary results show that the amount of oxygen during heating and in the dwell time strongly influences the superconducting properties. Good superconducting properties, with T/sub c/ of 89.9 K (/spl Delta/T/sub c/=1.1 K) and J/sub c/ of 10/sup 6/ A/cm/sup 2/ at 77 K, have been obtained on epitaxially grown 250-nm-thick YBCO films using the polymer-based precursors.

Co-rich Co-Pt films have been electrodeposited on Si (100) substrates with a 30 nm thick Cr buffer and a 10 nm Au conducting layer. Films have been deposited on different days after preparing the electrolyte, at current density of 28 mA/cm2, keeping all other deposition parameters constant. It has been observed from pole figures and magnetic measurements that the age of electrolyte has a significant effect on phase formation and magnetic properties of Co-Pt films. This instability in the deposition process with the age of the electrolyte is attributed to the anodic oxidation of ammonia and citrate in the presence of Co2+ ions, which has a significant influence on the deposition of cobalt based alloys. The reduction of Co2+ can be modify by two ways. First, it is possible that the nature of the complexation of Co2+ is changed because of a decomposition of ammonium citrate and formation of new compounds. Secondly, the new molecules formed in the anodic reaction may decrease the rate of reduction of Co2+ by ...