Controlled switching dynamics in Si-SiO<Subscript>2</Subscript>-VO<Subscript>2</Subscript> structures

The dynamic current-voltage characteristics of a Si-SiO₂-VO₂ structure exhibiting the switching effect have been studied. It is demonstrated that the switching dynamics in this system can be controlled either by applying a bias voltage to the silicon substrate or by illuminating the vanadium dioxide surface. The latter method is based on the light-induced variation of the capacitance of the space charge region. The possibility of creating new semiconductor devices employing these effects is considered.     

Numerical modeling of the electrical properties of Si-SiO<Subscript>2</Subscript>-VO<Subscript>2</Subscript> structures

We have numerically modeled the ac current passage through a Si-SiO₂-VO₂ structure, which is known to exhibit switching with an S-like characteristic due to a metal-semiconductor phase transition in vanadium dioxide. It is shown that the dynamics of switching at high frequencies (10⁵–10⁹ Hz) can be effectively controlled, which makes such structures promising elements for use in high-frequency microelectronics as analogs of thyristors and photothyristors.     

Magneto-optical properties of polycrystalline CoCrFeO<Subscript>4</Subscript> films

A methods of obtaining polycrystalline CoCrFeO₄ films based on solid-phase reactions in Cr₂O₃/Co/Fe layer structures in the regimes of isothermal annealing and self-propagating high-temperature synthesis (SHS) is described. The magneto-optical properties of the obtained films have been studied. It is established that the spectra of magneto-optical characteristics depend on the solid-phase reaction regime and synthesis temperature. The Faraday rotation angle exhibits a local maximum (2θ_F = 5 deg/μm) at a wave-length of 630 nm.     

Rietveld analysis of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> thin films obtained by RF-sputtering

Calcium copper titanate, CaCu₃Ti₄O₁₂, CCTO, thin films with polycrystalline nature have been deposited by RF sputtering on Pt/Ti/SiO₂/Si (100) substrates at a room temperature followed by annealing at 600 °C for 2 h in a conventional furnace. The crystalline structure and the surface morphology of the films were markedly affected by the growth conditions. Rietveld analysis reveal a CCTO film with 100 % pure perovskite belonging to a space group Im3 and pseudo-cubic structure. The XPS spectroscopy reveal that the in a reducing N₂ atmosphere a lower Cu/Ca and Ti/Ca ratio were detected, while the O₂ treatment led to an excess of Cu, due to Cu segregation of the surface forming copper oxide crystals. The film present frequency -independent dielectric properties in the temperature range evaluated, which is similar to those properties obtained in single-crystal or epitaxial thin films. The room temperature dielectric constant of the 600-nm-thick CCTO films annealed at 600 °C at 1 kHz was found to be 70. The leakage current of the MFS capacitor structure was governed by the Schottky barrier conduction mechanism and the leakage current density was lower than 10^?7 A/cm² at a 1.0 V. The current–voltage measurements on MFS capacitors established good switching characteristics.     

Elastic properties of Li<Subscript>2</Subscript>Zn<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The complete elastic modulus matrix of Li₂Zn₂(MoO₄)₃ single crystals has been measured for the first time. The sound velocity has been measured in different directions of the crystals by a pulse-phase method. The measurement results have been used to calculate elastic moduli. The sound velocity has been calculated in the three main crystallographic planes of the crystals.     

Mechanical and electrical switching of local ferroelectric domains of K<Subscript>0.5</Subscript>Bi<Subscript>4.5</Subscript>Ti<Subscript>4</Subscript>O<Subscript>15</Subscript> film

Ferroelectric/Piezoelectric K_0.5Bi_4.5Ti₄O₁₅ (KBT) film was fabricated by pulsed laser deposition method and confirmed by ferroelectric, dielectric measurements and local butterfly-type piezoresponse hysteresis loops. Importantly, ferroelectric domain switching by both electrical field and mechanical force in KBT film was demonstrated. The dark and bright contrast represents the PFM response of the up and down polarized domains, which can be written by a dc bias of ±12 V or a mechanical force of 40–50 nN. The successful demonstration of mechanical force switching of ferroelectric domain in KBT film other than electric field provides a novel mean for information storage and sensors.     

Interface-dependent resistance switching in Nd<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> ceramics

Interface-dependent electric-pulse-induced resistance switching effect (EPIR) in Nd_0.7Sr_0.3MnO₃ ceramics was studied. The results reveal that the EPIR effect originates from the interface between the electrodes and the bulk, and the EPIR ratio as well as the high and low resistance states can be strongly influenced by applying a large electrical field on the sample for different intervals. Also, the pulse parameters have great effect on the stability of EPIR and the optimal pulse width, pulse amplitude and read bias are obtained. Based on the space charge limited current mechanism together with the theory of interfacial charge-trapped state, the interface-dependent resistance switching effect is discussed.     

Nanostructured Pt-γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CeO<Subscript>2</Subscript> catalytic materials and coatings

A new process is described for the suspension synthesis of polycrystalline Pt-γ-Al₂O₃-CeO₂ catalytic materials and coatings, with maltose, C₁₂H₂₂O₁₁, as a reductant and structure former. The process parameters have been optimized in terms of the dispersion medium composition and the way in which Pt is introduced. The coatings are highly uniform in chemical and phase composition, as evidenced by quantitative analysis, optical microscopy, and x-ray microanalysis results. Characteristically, the coatings have a highly porous structure and good adhesive properties. Catalysts prepared by the proposed process show high activity for the oxygen oxidation of CO. The process can be used to fabricate Pt-Pd-Rh catalysts on block supports for the purification of vehicle exhaust gases and industrial off-gases. It has the advantage that high-porosity multicomponent catalytic coatings can be produced on cordierite block supports in a single step.     

Cu doping effect on the resistive switching behaviors of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> thin films

Spin-coated Cu_xCo_1?xFe₂O₄ (x = 0, 0.2, 0.4, 0.6, and 0.8) thin films were prepared on Pt/TiO₂/SiO₂/Si substrates. Pt/Cu_xCo_1?xFe₂O₄/Pt structures were fabricated to investigate the effect of Cu doping concentration on the resistive switching behaviors. Structural and morphology characterizations revealed that Cu doping improved the crystallization of the thin films as compared to undoped CoFe₂O₄. Current–voltage characterization showed that all Cu_xCo_1?xFe₂O₄ thin films showed unipolar resistance switching, but the distribution range of the set voltage, reset voltage, and resistances were much reduced by Cu doping. Clear improvement in the stability of these parameters started to appear with x = 0.4, and the optimized performance was observed in the Pt/Cu_0.6Co_0.4Fe₂O₄/Pt structure. The improved stability of the switching parameters was attributed to the enhancement of hopping process between the Fe ions and the Cu ions in the spinel lattice. Our results indicated that appropriate adjustment of the doping elements in oxides can be a feasible approach in achieving stable resistance switching memory devices.     

Relaxation of optical excitations in crystals of the Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system with radiation defects

The photoluminescence and short-time persistence (afterglow) kinetics in pure and doped Y₂O₃-Al₂O₃ crystals excited with UV laser pulses (12 ns, 337 nm) were studied using samples irradiated with gamma quanta from a ⁶⁰Co source to a dose from 10⁴ to 10⁷ Gy. The relaxation time of the samples studied increases, with decreasing symmetry of the crystal lattice, in the following order: garnet—orthoaluminate—ruby—yttria. The afterglow duration and intensity significantly decrease in gamma-irradiated crystals, which is explained by the predominant recombination of close electron-hole pairs. Garnet-neodymium crystals are characterized by high radiation stability and fast relaxation kinetics.     

Martensitic phase transformation in single crystal Co<Subscript>5</Subscript>Ni<Subscript>2</Subscript>Ga<Subscript>3</Subscript>

The magnetic, thermal, and transport properties of martensitic phase transformation in single crystal Co₅Ni₂Ga₃ have been investigated. The single crystal Co₅Ni₂Ga₃ shows martensitic transformation at 251 K on cooling and 254 K on warming. Large jumps in the temperature-dependent resistance curve, temperature-dependent magnetization curve, and temperature-dependent thermal conductivity curve are observed at martensitic transformation temperature (T _M). Negative magnetoresistance due to spin disorder scattering was observed in Co₅Ni₂Ga₃ single crystal at all temperature range. The temperature-dependent negative magnetoresistance shows a peak at T _M, which indicates that the spin disorder increases in the process of phase transition. Co₅Ni₂Ga₃ sample exhibits a temperature dependence of thermal conductivity κ(T) (dκ/dT > 0) due to electrons being above temperature 100 K.     

Anomalous magnetic susceptibility in Nd<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> manganite single crystals

Chaotic oscillations of the low-frequency magnetic susceptibility χ in a Nd_0.5Sr_0.5MnO₃ (NSMO) manganite single crystal have been observed in the temperature interval of the coexistence of the ferro- and antiferromagnetic phases of the given composition. It is established that the amplitude of χ oscillations depends on the constant bias magnetic field. The appearance of susceptibility oscillations in NSMO single crystals is attributed to the formation of magnetic transition regions, which can exist at the boundaries between ferro- and antiferromagnetic phases.     

Magnetotransport at the LTO-LTT Structural Transition in La<Subscript>1.48</Subscript>Nd<Subscript>0.4</Subscript>Sr<Subscript>0.12</Subscript>CuO<Subscript>4</Subscript>

Magnetoresistance (MR) was measured in a La_1.48Nd_0.4Sr_0.12CuO₄ single crystal in perpendicular magnetic fields H up to 9 T in the region of the structural transition from the low-temperature orthorhombic (LTO) to low-temperature tetragonal (LTT) phase. The hysteretic MR exhibits discrete jumps or avalanches only when the transition is approached from the LTT phase, and only during the first field sweep. The properties of the hysteresis are found to be independent of the field driving rate. The results are consistent with the presence of magnetostructural domains coupled with the charge degrees of freedom.     

Proton Conductivity and Thermal Properties of Ba(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)<Subscript>2</Subscript>

We have grown single crystals of barium dihydrogen phosphate and studied its thermal transformations during heating to 500°C and its electrotransport properties. Ba(H₂PO₄)₂ (Pccn) has been shown to undergo no phase transitions up to its dehydration temperature. The thermal decomposition of Ba(H₂PO₄)₂, accompanied by dehydration, involves two steps, with maximum rates at ~265 and 370°C, and results in the formation of barium dihydrogen pyrophosphate and barium metaphosphate, respectively. The total enthalpy of the endothermic dehydration events is–244.6 J/g. Using impedance spectroscopy, we have studied in detail the proton conductivity of polycrystalline and single-crystal Ba(H₂PO₄)₂ samples in a controlled atmosphere. Adsorbed water has been shown to have a significant effect on the proton conductivity of Ba(H₂PO₄)₂ up to 130°C. The proton conductivity of the Ba(H₂PO₄)₂ single crystals has been shown to be anisotropic. The conductivity anisotropy correlates with specific structural features of the salt. Higher conductivity values, 3 × 10^–9 to 2 × 10^–7 S/cm in the range 60–160°C, have been observed in the [100] crystallographic direction, exceeding the conductivity along [010] by an order of magnitude. The activation energy for proton conduction is 0.80 eV.     

Room temperature magnetic exchange coupling in multiferroic BaTiO<Subscript>3</Subscript>/CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> magnetoelectric superlattice

Multiferroic BaTiO₃/CoFe₂O₄ superlattice films are deposited by laser molecular beam epitaxy. The film growth modes are studied by in situ reflection high energy electron diffraction and the film structures are revealed by high resolution transmission electron microscopy study. Ferroelectric switching behavior was studied by piezoresponse force microscopy, and it shows that good ferroelectricity was retained in the superlattice. Such a multiferroic superlattice also shows a magnetic exchange coupling under room temperature. Detailed analysis reveals that different growth modes and the substrate strain effect may be responsible for the magnetic exchange coupling.     

Magnetization Studies of K<Subscript>0.8</Subscript>Fe<Subscript>1.7</Subscript>Se<Subscript>2</Subscript> Single Crystals

Magnetization measurements of the magneto-superconducting K_0.8Fe_1.7Se₂ single crystals (T _C =29.6 K) at various applied fields and temperatures have been performed. In the superconducting range, two unusual phenomena are observed. (i) The presence of paramagnetic Meissner effect up to 20 Oe, which is revealed by the positive field-cooled magnetization curves. (ii) A double peak in the hysteresis loop at low temperatures. The isothermal magnetization measured at 35 K clearly indicates the presence of a tiny amount of a ferromagnetic substance (probably pure Fe). The two peculiar phenomena observed are attributed to this impurity.     

Anisotropy of dielectric losses in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and SiO<Subscript>2</Subscript> single crystals

The procedure and results of measurements of the dielectric loss tangent using the dielectric resonator technique on azimuthal modes of the HE (quasi-E) and EH (quasi-H) types are considered. The measurements were performed for uniaxial anisotropic single crystals of Al₂O₃ (at a frequency of 11 GHz) and SiO₂ (at 39 GHz) in a temperature range of 80–373 K and for an isotropic single crystal of Y₃Al₅O₂ (YAG) at room temperature in a frequency range of 9–15 GHz. The proposed method revealed the anisotropy of dielectric losses in Al₂O₃ and SiO₂ single crystals in the temperature range studied. According to this, losses along the optical axis of these crystals are lower than in the transverse plane. In the YAG crystal, the Q values for modes of the two types with the same frequency are close, which corresponds to isotropic losses. The dielectric losses in YAG increase in proportion to the frequency.     

Synthesis and growth of TlInSe<Subscript>2</Subscript> and TlGaSe<Subscript>2</Subscript> single crystals

Methods of synthesis and growth of TlGaSe₂ and TlInSe₂ single crystals are developed. It is shown that the Bridgman-Stockbarger technique is efficient for growth of large TlGaSe₂ single crystals, and zone recrystallization is efficient in the case of TlInSe₂.     

Sequential HAADF-STEM observation of structural changes in Au nanoparticles supported on CeO<Subscript>2</Subscript>

Sequential high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was used to clarify the detailed process of structural changes in Au/CeO₂ catalysts found in our previous TEM observations. The layer-by-layer shrinkage of a Au nanoparticle on CeO₂ during electron beam irradiation and recovery at the same position after switching off the beam were observed with clear resolution of Ce and Au atomic columns. Displaced Au atomic columns at the perimeter of the Au/CeO₂ interface during the electron beam irradiation have been analyzed using first-principles calculations, which indicate that such Au atoms at the perimeter edge are located above oxygen vacancies or Ce-terminated surface regions. The present behavior of Au atoms and nanoparticles provides valuable insights into the mechanisms of both structural changes and catalytic activity of Au/CeO₂ catalysts.     

Reactive Stresses in Ni<Subscript>49</Subscript>Fe<Subscript>18</Subscript>Ga<Subscript>27</Subscript>Co<Subscript>6</Subscript> Shape-Memory-Alloy Single Crystals

The reactive stresses induced in Ni₄₉Fe₁₈Ga₂₇Co₆-alloy single crystals during martensitic transformations with a limited possibility of shape-memory-strain recovery have been experimentally studied. The data on these crystals are compared with the results obtained previously for Cu–Al–Ni, Ni–Ti, and Ni?Fe–Ga crystals. The potential of application of the Ni₄₉Fe₁₈Ga₂₇Co₆ single crystals in designing drives and power motors is demonstrated.