Magnetoelectric coupling at the La1-xSrxMnO3/ionic liquid interface

One of the major quests in today’s microelectronic era is the development of novel low-power magnetic devices for a variety of applications spanning from memory storage and processing to transduction and sensing. Control of magnetism by means of an electric field, based on the phenomenon of magnetoelectric (ME) effect, may be the key alternative to conventional electronics relying on dissipative electrical currents. In the last years various strategies to interconnect electric and magnetic degrees of freedom have been put to test. A promising approach to straightforwardly and precisely master ME coupling is via charge carrier doping of a magnetic material using an external voltage. This can be realized, akin to the working principle of the field effect transistor, by gating a magnetic electrode with an electrically-polarizable solid (e.g. a dielectric or a ferroelectric) or a liquid electrolyte. This dissertation reports on the investigation of ME coupling at solid/liquid interfaces in a prototypical system consisting of a La1-xSrxMnO3 (LSMO) magnetic electrode electrically charged with an ionic liquid (IL) electrolyte. LSMO - a magnetic perovskite manganite - belongs to the celebrated class of strongly-correlated oxides featuring multiple magnetic states, which directly depend on the oxidation state of the magnetically-coupled manganese ions (Mn3+/4+). Upon voltage-driven charge doping the Mn oxidation state is altered, which, in turn, allows to control the balance between double-exchange and superexchange magnetic interactions in LSMO. Furthermore, LSMO possesses a para/ferromagnetic transition slightly above room temperature, which makes it a promising candidate in the perspective of potential applications. Epitaxial thin (≈ 13 nm) and ultrathin (≈ 3 nm) LSMO films were grown onto single-crystalline SrTiO3 substrates via Large-Distance Magnetron Sputtering (LDMS). This deposition technique demonstrated to be an ideal tool for fabrication of LSMO films with highest quality in terms of crystallinity, surface smoothness and magnetic properties. The interfacial ME coupling was investigated by combining in situ Superconducting Quantum Interference Device (SQUID) magnetometry and Cyclic Voltammetry (CV). This experimental configuration allowed to concurrently extract quantitative information about surface charge density and magnetization as a function of different applied voltages and temperatures. The analysis of the interfacial charging/discharging processes revealed that the accumulation/depletion of charge carriers is not only driven by electrostatic (electric double layer capacitance) but also electrochemical (redox pseudocapacitance) doping. The presence of both charging mechanisms indicated that the LSMO/IL system behaves as an archetypal hybrid supercapacitor. Large values of surface charge density up to ≈ 300 μC/cm2 enabled to robustly and flexibly control the magnetic response of LSMO. In case of LSMO thin films with a thickness of ≈ 13 nm a relative magnetic change ΔM/M of up to 33% was reached, whereas for thinner LSMO films of ≈ 3 nm, featuring an enhanced surface-to-volume ratio, ferromagnetism (FM) could be completely suppressed and restored at will. Together with the significant magnitude of the magnetic tuning effect, IL gating provided an outstanding level of reversibility upon cycling, low energy consumption and remarkable switching speed. Additionally, the magnetic signal could be manipulated in-phase and/or anti-phase with respect to the surface charge modulation by appropriately adjusting the applied voltage. The observed interfacial ME coupling can be qualitatively explained with the major features of the bulk magnetoelectronic phase diagram of LSMO. However, in this study a more precise and consistent microscopic model is proposed on the basis of the quantitative values of the ME coupling coefficient |α| = |ΔM/ΔQ| = 3 μB/h+ and the phenomenon of magnetic phase separation. In such scenario competing FM and non-FM domains expand or shrink at the expense of each other upon voltage-induced charge doping. On the whole, this work intends to elucidate the physico-chemical mechanisms behind the ME effect at solid/liquid interfaces with the aim of fostering further studies in the yet unexplored area of ME supercapacitors

Effects of organic additives on calcium hydroxide crystallisation during lime slaking

Organic compounds, often used in cement systems as admixtures, may affect the crystallisation and carbonation kinetics of Ca(OH)2, an important phase of hydrated cement. Here, we investigated changes in Ca(OH)2 morphology in the presence of 3 organic compounds, commonly encountered in cement and lime-based materials: sucrose, pectin and calcium lignosulfonate. The additives were introduced either before or after lime slaking to determine the influence of temperature. Ca(OH)2 crystals and supernatant solutions were characterised at time of slaking and after 6 months of ageing using scanning electron microscopy, X-ray diffraction and optical emission spectroscopy. Our results indicate that the morphology of Ca(OH)2 crystals is modified by the characteristics of the organic molecules which promote formation of Ca(OH)2 with habits that can result in faster carbonation, an effect that is detrimental to cement used in reinforced concrete. These effects are enhanced when the additives are introduced before slaking, likely as a result of thermal degradation

Tailoring epitaxial growth and magnetism in La1-xSrxMnO3 / SrTiO3 heterostructures via temperature-driven defect engineering

Among the class of strongly-correlated oxides, La1-xSrxMnO3 $-$ a half metallic ferromagnet with a Curie temperature above room temperature $-$ has sparked a huge interest as a functional building block for memory storage and spintronic applications. In this respect, defect engineering has been in the focus of a long-standing quest for fabricating LSMO thin films with highest quality in terms of both structural and magnetic properties. Here, we discuss the correlation between structural defects, such as oxygen vacancies and impurity islands, and magnetism in La0.74Sr0.26MnO3/SrTiO3 (LSMO/STO) epitaxial heterostructures by systematic control of the growth temperature and post-deposition annealing conditions. Upon increasing the growth temperature within the 500 $-$ 700 $^{\circ}$C range, the epitaxial LSMO films experience a progressive improvement in oxygen stoichiometry, leading to enhanced magnetic characteristics. Concurrently, however, the use of a high growth temperature triggers the diffusion of impurities from the bulk of STO, which cause the creation of off-stoichiometric, dendritic-like SrMoOx islands at the film/substrate interface. As a valuable workaround, post-deposition annealing of the LSMO films grown at a relatively-low temperature of about 500 $^{\circ}$C permits to obtain high-quality epitaxy, atomically-flat surface as well as a sharp magnetic transition above room temperature and robust ferromagnetism. Furthermore, under such optimized fabrication conditions possible scenarios for the formation of the magnetic dead layer as a function of LSMO film thickness are discussed. Our findings offer effective routes to finely tailor the complex interplay between structural and magnetic properties of LSMO thin films via temperature-controlled defect engineering

Proton Conduction in Grain-Boundary-Free Oxygen-Deficient BaFeO2.5+δ_{2.5+δ} Thin Films

Reduction of the operating temperature to an intermediate temperature range between 350 °C and 600 °C is a necessity for Solid Oxide Fuel/Electrolysis Cells (SOFC/SOECs). In this respect the application of proton-conducting oxides has become a broad area of research. Materials that can conduct protons and electrons at the same time, to be used as electrode catalysts on the air electrode, are especially rare. In this article we report on the proton conduction in expitaxially grown BaFeO2.5+δ (BFO) thin films deposited by pulsed laser deposition on Nb:SrTiO3 substrates. By using Electrochemical Impedance Spectroscopy (EIS) measurements under different wet and dry atmospheres, the bulk proton conductivity of BFO (between 200 °C and 300 °C) could be estimated for the first time (3.6 × 10−6 S cm−1 at 300 °C). The influence of oxidizing measurement atmosphere and hydration revealed a strong dependence of the conductivity, most notably at temperatures above 300 °C, which is in good agreement with the hydration behavior of BaFeO2.5 reported previously

An Investigation into the Stability of Graphitic C3N4 as a Photocatalyst for CO2 Reduction

The increasing CO 2 concentration in the atmosphere exerts a significant influence on global warming and climate change. The capture and utilization of CO 2 by conversion to useful products is an area of active research. In this work, the photodriven reduction of CO 2 was investigated using graphitic carbon nitride (g-C 3 N 4 ) as a potential photocatalyst. The photocatalytic reduction of CO 2 was investigated with g-C 3 N 4 powder immobilized on a glass support in a batch gas-phase photoreactor. The experiments were carried out under UV-vis irradiation at 70 \ub0C and an initial pressure of 2.5 bar. The only gas-phase product detected during the irradiation of the g-C 3 N 4 in the presence of CO 2 was CO, and the rate of production was observed to decrease over time. Oxygen-doped g-C 3 N 4 was also tested for CO 2 reduction but had efficiency lower than that of the parent g-C 3 N 4 . Repeated cycles of photocatalytic CO 2 reduction showed a decline in the activity of the g-C 3 N 4 . In the absence of CO 2 some CO generation was also observed. Characterization of used and unused materials, using FTIR and XPS, showed an increase in the oxygen functional groups following UV-vis irradiation or thermal treatment. While others report the use of g-C 3 N 4 as a photocatalyst, this work highlights the important need for replicates and control testing to determine material stability

Giant voltage-induced modification of magnetism in micron-scale ferromagnetic metals by hydrogen charging

Owing to electric-field screening, the modification of magnetic properties in ferromagnetic metals by applying small voltages is restricted to a few atomic layers at the surface of metals. Bulk metallic systems usually do not exhibit any magneto-electric effect. Here, we report that the magnetic properties of micron-scale ferromagnetic metals can be modulated substantially through electrochemically-controlled insertion and extraction of hydrogen atoms in metal structure. By applying voltages of only ~ 1 V, we show that the coercivity of micrometer-sized SmCo5, as a bulk model material, can be reversibly adjusted by ~ 1 T, two orders of magnitudes larger than previously reported. Moreover, voltage-assisted magnetization reversal is demonstrated at room temperature. Our study opens up a way to control the magnetic properties in ferromagnetic metals beyond the electric-field screening length, paving its way towards practical use in magneto-electric actuation and voltage-assisted magnetic storage

Hybrid supercapacitors for reversible control of magnetism

Electric field tuning of magnetism is one of the most intensely pursued research topics of recent times aiming at the development of new-generation low-power spintronics and microelectronics. However, a reversible magnetoelectric effect with an on/off ratio suitable for easy and precise device operation is yet to be achieved. Here we propose a novel route to robustly tune magnetism via the charging/discharging processes of hybrid supercapacitors, which involve electrostatic (electric-double-layer capacitance) and electrochemical (pseudocapacitance) doping. We use both charging mechanisms—occurring at the La0.74Sr0.26MnO3/ionic liquid interface to control the balance between ferromagnetic and non-ferromagnetic phases of La1−xSrxMnO3 to an unprecedented extent. A magnetic modulation of up to ≈33% is reached above room temperature when applying an external potential of only about 2.0 V. Our case study intends to draw attention to new, reversible physico-chemical phenomena in the rather unexplored area of magnetoelectric supercapacitors