Showing papers on "Transition temperature published in 2003"

Effect of low-temperature annealing on (Ga,Mn)As trilayer structures

Abstract: The effect of low-temperature annealing on (Ga,Mn)As/GaAs/(Ga,Mn)As trilayer structures is studied. Low-temperature annealing significantly increases the ferromagnetic transition temperature TC of top (Ga,Mn)As layers, reaching as high as 160 K, whereas no apparent effect is observed on bottom (Ga,Mn)As layers. The annealing effect on Be-doped trilayers is also presented.

Multiscale origin of the magnetocaloric effect in Ni-Mn-Ga shape-memory alloys

Abstract: We have analyzed magnetization measurements in a series of composition-related Ni-Mn-Ga shape-memory alloys. It is shown that the magnetocaloric effect in the vicinity of the martensitic transition mainly originates from two different contributions: (i) magnetostructural coupling on the mesoscopic scale between the magnetic moments and the martensitic variants, which is also responsible for the magnetic shape-memory effect and (ii) the microscopic spin-phonon coupling which gives rise to the shift of the transition temperature with the applied magnetic field. The relative importance of these two contributions has been shown to vary with composition, which is suitably expressed through the average number of valence electrons per atom $e/a.$ In alloys with a large difference between the Curie and martensitic transition temperatures $(e/a\ensuremath{\simeq}7.5),$ mesoscopic coupling is dominant and a negative giant magnetocaloric effect (increase of temperature by adiabatic demagnetization) is induced at moderate applied fields. In contrast, in alloys when these temperatures are very close to one another $(e/a\ensuremath{\simeq}7.7),$ the microscopic coupling is the most relevant contribution and gives rise to a positive giant effect.

High-Temperature Behavior of CsH2PO4 under Both Ambient and High Pressure Conditions

Abstract: The high-temperature behavior of CsH₂PO₄ has been carefully examined under both ambient and high pressure (1.0 ± 0.2 GPa) conditions. Ambient pressure experiments encompassed thermal analysis, AC impedance spectroscopy, ¹H NMR spectroscopy, and polarized light microscopy. Simultaneous thermogravimetric analysis, differential scanning calorimetry, and evolved gas analysis by mass spectroscopy demonstrated that a structural transition with an enthalpy of 49.0 ± 2.5 J/g occurred at 228 ± 2 °C, just prior to thermal decomposition. The details of the decomposition pathway were highly dependent on sample surface area, however the structural transformation, a superprotonic transition, was not. Polarized light microscopy showed the high temperature phase to be optically isotropic in nature, consistent with earlier suggestions that this phase is cubic. The conductivity of CsH₂PO₄, as revealed by the impedance measurements, exhibited a sharp increase at the transition temperature, from 1.2 × 10⁻⁵ to 9.0 × 10⁻³ Ω¹⁻cm⁻¹, followed by a rapid decline due to dehydration. In addition, chemically adsorbed surface water was shown to increase the conductivity of polycrystalline CsH₂PO₄ over well-dried samples, even at mildly elevated temperatures (>200 °C). At high pressure an apparent irreversible phase transition at 150 °C and a reversible superprotonic phase transition at 260 °C were observed by impedance spectroscopy. At the superprotonic transition, the conductivity increased sharply by ~3 orders of magnitude to 3.5 × 10⁻² Ω¹⁻cm⁻¹ at 275 °C. This high conductivity phase was stable to the highest temperature examined, 375 °C, and exhibited reproducible and highly Arrhenius conduction behavior, with an activation energy for charge transport of 0.35 eV. Upon cooling, CsH₂PO₄ remained in the high-temperature phase to a temperature of 240 °C.

Carbon doping of superconducting magnesium diboride

Abstract: We present details of synthesis optimization and physical properties of nearly single phase carbon doped MgB2 with a nominal stoichiometry of Mg(B0.8C0.2)2 synthesized from magnesium and boron carbide (B4C) as starting materials. The superconducting transition temperature is ≈22 K (≈17 K lower than in pure MgB2). The temperature dependence of the upper critical field is steeper than in pure MgB2 with H c2 (10 K )≈9 T. Temperature dependent specific heat data taken in different applied magnetic fields suggest that the two-gap nature of superconductivity is still preserved for carbon doped MgB2 even with such a heavily suppressed transition temperature. In addition, the anisotropy ratio of the upper critical field for T/Tc≈2/3 is γ≈2. This value is distinct from 1 (isotropic) and also distinct from 6 (the value found for pure MgB2).

Monodisperse Oligofluorenes Forming Glassy-Nematic Films for Polarized Blue Emission

Abstract: The first series of monodisperse glass-forming, nematic oligofluorenes was synthesized following a divergent−convergent approach. Both chain length and pendant structure were found to affect solid morphology and phase transition temperatures. With optimized structural parameters, a Tg close to 150 °C and a Tc beyond 375 °C were achieved. This material class is characterized by its ability to form monodomain glassy nematic films via spin-coating, thermal annealing at 10 to 20 °C above Tg for 15−30 min, and cooling to room temperature without encountering crystallization. The absorption and emission dichroic ratios of 80−90-nm-thick films increase from 7 to 17 with an increasing molecular aspect ratio, while the photoluminescence quantum yield varies erratically from 43 to 60%. Superior stability of emissive color and of emission dichroic ratio and that against thermally activated crystallization were also demonstrated through prolonged heating of a film at 10 °C above Tg under argon.

Large magnetic entropy change in the metallic antiperovskite Mn3GaC

Abstract: A large positive magnetic entropy change ΔSM is observed in the metallic antiperovskite Mn3GaC near its first-order metamagnetic transition temperature 159 K where the stoichiometric compound transforms from an antiferromagnetic to a canted ferromagnetic state accompanied by a discontinuous volume change of −0.46% without change of symmetry. The unusual field dependence of the ΔSM of Mn3GaC shows a very rapid linear increase from zero to a saturation value within a field interval smaller than 1 T. The broadening of the peak of ΔSM to low temperature with increasing field change creates an ΔSM plateau in the temperature dependence of the MCE, which is of significance for practical application of these materials in the Ericsson-cycle magnetic refrigerator.

Ferroelectricity in free niobium clusters.

Abstract: Electric deflections of gas-phase, cryogenically cooled, neutral niobium clusters [NbN; number of atoms (N) = 2 to 150, temperature (T) = 20to 300kelvin], measured in molecular beams, show that cold clusters may attain an anomalous component with very large electric dipole moments. In contrast, room-temperature measurements show normal metallic polarizabilities. Characteristic energies kBTG(N) [Boltzmann constant kB times a transition temperature TG(N)] are identified, below which the ferroelectric-like state develops. Generally, TG decreases [110 > TG(N) > 10K] as N increases, with pronounced even-odd alternations for N > 38. This new state of metallic matter may be related to bulk superconductivity.

Normal ferroelectric to relaxor behavior in laser ablated Ca-doped barium titanate thin films

Abstract: Ba1–xCaxTiO3 thin films (x = 0.05 to 0.17) were deposited on Pt-coated Si substrates using a pulsed excimer laser ablation technique. X-ray diffraction and scanning electron microscope studies of the Ba1–xCaxTiO3 targets exhibit a polycrystalline nature and thin films also show the same but with a significant orientation along the (111) direction. Secondary ion mass spectrometer analysis reveals the presence of a sharper interface existing at the thin film substrate. The dielectric phase transition temperature of (Ba1–xCax)TiO3 targets were sharp and the transition temperature was found to decrease from 140 °C to 110 °C with an increase in the values of x (x>0.05 at. %). The laser ablated Ca-doped BaTiO3 thin films deposited at 100 mTorr exhibited a higher dielectric constant, lower dielectric loss, and an anomalous decrease in phase transition was observed. The anomalous phase transition decrease was ascribed to the occupancy of the Ca2+ in the Ti4+ site. There was a cross over from the sharp to diffused phase transition for a higher composition of Ca (>9 at. %) in BaTiO3 thin films. The diffuse transition behavior might be due to the larger number of the Ca2+ ions occupying the Ti4+ site, eventually introducing larger compositional and structural disorder and this occupancy leads to the generation of oxygen vacancies. The activation energy obtained from impedance spectroscopy was 1.05 eV, and was attributed to the oxygen vacancy motion.

Competing orders and disorder-induced insulator to metal transition in manganites.

Abstract: Effects of disorder on the two competing phases, i.e., the ferromagnetic metal and the commensurate charge/lattice ordered insulator, are studied by Monte Carlo simulation. The disorder suppresses the charge/lattice ordering more strongly than the ferromagnetic order, driving the commensurate insulator to the ferromagnetic metal near the phase boundary in the pure case. Above the ferromagnetic transition temperature, on the contrary, the disorder makes the system more insulating, which might cause an enhanced colossal magnetoresistance as observed in the half-doped or Cr-substituted manganites. No indication of the percolation or the cluster formation is found, and there remains the charge/lattice fluctuations instead which are enhanced toward the transition temperature.

Homogeneous nucleation versus glass transition temperature of silicate glasses

Abstract: This paper provides experimental and theoretical evidence for a correlation between the maximum internal nucleation rate, Imax=I(Tmax) [where Tmax is the temperature of maximum nucleation rate] and the reduced glass transition temperature, Tgr, for 51 glass-forming liquids. In addition, it demonstrates an analogous correlation between Tmax, the time-lag of nucleation at Tmax and the reduced glass transition temperature. An explanation is given for these remarkable trends.

Size effect on the ferroelectric phase transition in SrBi2Ta2O9 nanoparticles

Abstract: We report the size effect on the ferroelectric phase transition in SrBi2Ta2O9 nanoparticles. Samples with particle sizes between 11 and 71 nm were prepared by a room-temperature mechanical activation process followed by postannealing at different temperatures. The size of the particles was determined by x-ray diffraction with the aid of Scherrer’s equation. The ferroelectric phase transition in both SrBi2Ta2O9 nanoparticles and bulk sample were investigated by in situ Raman scattering. The results show that the transition temperature (Tc) decreases from its bulk value (605 K) as the size decreases. The size dependence of Tc can be described as Tc=605−1150/(D−2.1) (K), where D (nm) is the particle size. A critical size of 2.6 nm, below which ferroelectricity disappears, was obtained from this empirical expression.

The effect of transformation twins on the seismic-frequency mechanical properties of polycrystalline Ca1-xSrxTiO3 perovskite

Abstract: The low-frequency mechanical properties of polycrystalline Ca1− x Sr x TiO3 (0.5 ≤ x ≤ 0.9) have been investigated as a function of temperature and bulk composition using the technique of dynamical mechanical analysis in three-point bend geometry. Ca1− x Sr x TiO3 forms a cubic solid solution at high temperatures. At lower temperatures it undergoes successive displacive transitions to tetragonal and orthorhombic phases on cooling and increasing CaTiO3 content. The low-frequency mechanical response of the tetragonal phase is dominated by thermally activated displacements of transformation twin domain walls, causing almost a twofold decrease in storage modulus relative to the twin-free cubic phase on cooling through the phase transition (superelastic softening). Below 140 °C the mobility of domain walls decreases and the storage modulus returns to a value close to that of the cubic phase. For x > 0.85, the cubic to tetragonal transition temperature is below 140 °C and domain walls are immobile immediately on formation, greatly reducing the magnitude of mechanical softening. The frequency dependence of the storage modulus is accurately described by a modified Burgers model of anelastic relaxation. Activation energies of 103 and 96 kJ/mol were obtained for samples with x = 0.7 and 0.74, respectively, suggesting that domain walls are strongly pinned by vacancies at the O positions. Superelastic softening is not observed below T c for the tetragonal to orthorhombic phase transition in samples with x = 0.5, 0.55, and 0.6. This is explained by the small value of the spontaneous strain in orthorhombic samples with intermediate compositions, which are distorted from cubic symmetry by less than 1‰. With such small strain contrast between differently oriented twin domains, the effective force on the domain walls due to the external stress is less than the critical unpinning force, preventing displacement of the walls and suppressing superelastic softening.

Colossal magnetoresistance behavior and ESR studies of La1-xTexMnO3 (0.04 <= x <= 0.2)

Abstract: In this paper, we report the magnetic and electrical properties of the Te-doped lanthanum manganite perovskite compound La 1 - x Te x MnO 3 and the related electron spin resonance (ESR) study. This is a material with rhombohedral structure and it shows good colossal magnetoresistance behavior. The magnetoresistance ratio MR=[ρ(0)-ρ(H)]/ρ(0)is about 63% for x=0.04 in an applied magnetic field of 40 kOe. The resistivity of the compounds is affected by temperature and the amount of Te doping. The temperature dependence of the resistivity showed that the conductivity above the Curie temperature was dominated by the hopping of the small polarons, and the resistance was attributed to the electron-phonon and magnon scattering mechanism below the metal-insulator transition temperature. The ESR investigation indicated that the compound was in the paramagnetic phase above 230 K, and in the single ferromagnetic state below 170 K. In the temperature range of 230 to 170 K, the compound showed phase separation phenomena.

An X-ray diffraction and MAS NMR study of the thermal expansion properties of calcined siliceous ferrierite.

Abstract: Powder and single-crystal X-ray diffraction, combined with MAS NMR measurements, has been used to study the thermal expansion of siliceous zeolite ferrierite as it approaches a second-order displacive phase transition from a low-symmetry (Pnnm) to a high-symmetry (Immm) structure. Below the transition temperature, ferrierite exhibits positive thermal expansivity. However, above the transition temperature a significant change in thermal behavior is seen, and ferrierite becomes a negative thermal expansion material. Accurate variable-temperature single-crystal X-ray diffraction measurements confirm the transition temperature and allow the changes in average atomic position to be followed with temperature. The results from the single-crystal X-ray diffraction study can be correlated with 29Si MAS NMR chemical shifts for the low-temperature phase. At low temperatures the results show that the positive thermal expansivity is driven by an overall increase in Si−Si distances related to an increase in Si−O−Si bon...

The influence of atomic order on the magnetic and structural properties of the ferromagnetic shape memory compound Ni2MnGa

Abstract: The effect of atomic order on the martensitic phase transition and magnetic properties of stoichiometric Ni2MnGa has been investigated in a sample quenched from 1000 °C. Magnetization, resistivity and x-ray diffraction measurements indicate that the structural phase transition occurs at ~ 103 K, substantially lower than the value reported for samples quenched from 800 °C and ordered in the Heusler L21 structure. A small reduction in the ferromagnetic moment was also observed, although the Curie temperature remained largely unaffected. The electronic Sommerfeld coefficient obtained from heat capacity measurements is enhanced but smaller than that observed for the 800 °C quenched sample. The results are consistent with band structure calculations and the electronic changes brought about by atomic disorder.

Nanometer-resolved interfacial fluidity.

Abstract: Confined liquids can have properties that are poorly predicted from bulk parameters. We resolve with 0.5 nm resolution the nanoscale perturbations that interfaces cause on fluidity, in thin 3-methylpentane (3MP) films. The films of glassy 3MP are much less viscous at the vacuum-liquid interface and much more viscous at the 3MP-metal interface, compared to the bulk of the film. We find that the viscosity at the interfaces continuously returns to the bulk value over about a 3 nm distance. The amorphous 3MP films are constructed using molecular beam epitaxy on a Pt(111) substrate at low temperatures (<30 K). Ions are gently inserted at specific distances from the substrate with a 1 eV hydronium (D3O+) or Cs+ ion beam. The voltage across the film, which is directly proportional to the position of the ions within the film, is monitored electrostatically as the film is heated at a rate of 0.2 K/s. Above the bulk glass transition temperature (Tg) of 3MP (77 K), the ions are expected to begin to move down through...

Photo-Cross-Linkable PNIPAAm Copolymers. 4. Effects of Copolymerization and Cross-Linking on the Volume-Phase Transition in Constrained Hydrogel Layers

Abstract: Photo-cross-linkable temperature and pH-responsive polymers based on N-isopropylacrylamide and 2-(dimethylmaleimido)-N-ethyl-acrylamide were synthesized and spin-coated to produce uniform thin films of cross-linked, responsive hydrogels. Surface plasmon resonance and optical waveguide spectroscopy were used to determine the effect on the volume-phase transition when these materials are confined to a thin film. The film thicknesses ranged from 10 nm to 2.5 μm, and the volume-phase transition temperature and swelling ratio of the films fell into two overlapping regimes separated by at least one critical thickness. In the thick-film regime, greater than 100−760 nm, depending on the cross-linking density, ionizable comonomer concentration, and reference state, the films exhibited two transition temperatures. This can be explained by the stress imposed on the hydrogel as it swells perpendicular to the substrate. In the thin-film regime, less than 270−440 nm, depending on cross-linking density, the presence of ...

Phase Transition of the PLGA-g-PEG Copolymer Aqueous Solutions

Abstract: The aqueous solution of poly(lactic acid-co-glycolic acid)-g-poly(ethylene glycol) becomes a gel as the temperature increases. The sol-to-gel transition temperature can be controlled from 15 to 45 °C by varying the number of poly(ethylene glycol) grafts and the composition of the polymer. In addition, hysteresis between heating and cooling cycles could be controlled by adding poly(ethylene glycol) with different molecular weights as an additive. To prove the hypothesis of micellar aggregation for the sol-to-gel transition and the change in hydration status for the gel-to-sol transition, several experiments were performed. Small-angle neutron scattering and Raman spectroscopy sensitively detected the sol-to-gel transition, because it involves aggregation of the scattering particle of micelles. IR and 13C NMR showed that little change in hydration status is involved during the sol-to-gel transition, whereas significant change in hydration status is involved in the gel-to-sol transition. The intrinsic viscos...

Dielectric properties of Na1-xKxNbO3 in orthorhombic phase

Abstract: Pellets of ceramic Na1−xKxNbO3 (x = 0, 0.2 and 0.5), were prepared by conventional solid-state reaction method. Prepared samples were characterized using XRD and SEM. The frequency and temperature variation of dielectric constant, loss tangent and dielectric conductivity of prepared samples were measured in the frequency range from 10 KHz-1 MHz, and in the temperature range from 50–250°C for x = 0.2 and 0.5, and between 50 and 480°C for x = 0 compositions. It was observed that the dielectric constant and loss tangent decrease, and conductivity increases with increasing frequency. Near the transition temperature the material shows anomalous behaviour for the observed properties, and the peaks of dielectric constant and loss tangent were observed shifting towards lower temperature with increasing frequency.

Pressure tuning of electron-phonon coupling: the insulator to metal transition in manganites.

Abstract: An extended temperature and pressure-dependent investigation is carried out on a La0.75Ca0.25MnO3 sample exploiting the infrared absorption technique coupled to a diamond anvil cell. The pressure dependence of the insulator to metal transition temperature T(IM) is determined for the first time up to 11.2 GPa. The T(IM)(P) curve we propose to model the present data has an exponential-like behavior with an associated characteristic pressure P* playing the role of a decay constant. It is found that the equivalence between an external and an internal (chemical) pressure holds over a limited range of pressure, namely, P< or =2P*. Moreover, a certain universality character is associated with the proposed model curve in its ability to account for a large class of low-disorder manganites characterized by intermediate electron-phonon coupling.

Molecular dynamics study of the effects of voids and pressure in defect-nucleated melting simulations

Abstract: The defect-nucleated melting of Ar has been simulated by the gradual heating of lattices that contain voids using isobaric molecular dynamics. The criterion given by Solca et al. [Chem. Phys. 224, 253 (1997)] has been used to determine the melting point from the transition temperature versus void size curve. A crystal containing a single void created by the removal of an atom and its nearest (n−1) neighboring atoms was found to give almost the same melting temperature as a crystal containing n randomly distributed single-atom voids. The melting temperature is insensitive to the shape of the void. The critical void size, beyond which there is a sudden drop in the melting temperature, decreases with pressure. At various values of pressures ranging from 0.094 to 531.6 kbar the melting points are found to be in good agreement with the experimental results and with thermodynamic results using the same exp-6 potential. The results are consistent with the Lindemann criterion of melting and in better agreement with the Lindemann criterion than are the thermodynamic results.

Electrical-field control of metal–insulator transition at room temperature in Pb(Zr0.2Ti0.8)O3/La1−xBaxMnO3 field-effect transistor

Abstract: We have constructed field-effect transistor structures that consist of a ferromagnetic (La,Ba)MnO3 channel and a ferroelectric PbZr0.2Ti0.8O3 gate insulator with the aim of controlling the metal–insulator transition at room temperature by applying an electric field. Investigations have revealed that the transition temperature changed from 237.0 K to 242.0 K for the La0.90Ba0.10MnO3 channel layer by ferroelectric remnant polarity (±50 μC/cm2) and from 280.5 K to 283.0 K (±26 μC/cm2) for the La0.85Ba0.15MnO3 channel layer. These shifts, which are linearly proportional to the magnitude of ferroelectric remnant polarization, are induced by the accumulation charge due to the electric field.