Showing papers on "Transition temperature published in 2000"

Structural transformations of Ge2Sb2Te5 films studied by electrical resistance measurements

Abstract: Temperature dependent measurements of the electrical resistance have been employed to study structural changes in sputtered Ge2Sb2Te5 films. The pronounced changes of film resistance due to structural changes enable a precise determination of transition temperatures and activation energies. Furthermore the technique is sensitive enough to measure the influence of ultrathin capping layers on the transformation kinetics. With increasing temperature the Ge2Sb2Te5 films undergo a structural change from an amorphous to rock salt structure (Fm3m) around 140 °C and finally a hexagonal structure (p3m) around 310 °C. Both structural changes are accompanied by a major drop of resistance. Applying the Kissinger method [Anal. Chem. 29, 1702 (1957)] the activation energy for crystallization to the rock salt structure is determined to be 2.24±0.11 eV, and for the phase transformation to the hexagonal phase to be 3.64±0.19 eV, respectively. A thin capping layer of ZnS–SiO2 leads to an increase of the first transition temperature as well as of the corresponding activation energy (2.7±0.2 eV).

Magnetic entropy change in Ni51.5Mn22.7Ga25.8 alloy

Abstract: A considerable magnetic entropy change has been observed in Ni51.5Mn22.7Ga25.8 alloys under a field of 0.9 T. This change originates from a sharp magnetization jump which is associated with a martensitic to austenitic structure transition. The large low field entropy change and the adjustable martensic–austensic transition temperature indicate a great potential of Ni–Mn–Ga as working materials for magnetic refrigerants in a wide temperature range.

Evolution of sp2 bonding with deposition temperature in tetrahedral amorphous carbon studied by Raman spectroscopy

Abstract: Two transitions in the bonding are found in tetrahedral amorphous carbon (ta-C) films as a function of deposition temperature. The total sp3 fraction shows a sharp decrease at a transition temperature of order 250 °C. In contrast, visible Raman finds that the sp2 sites show a gradual ordering into the graphitic clusters through the sharp bonding transition. The optical gap and resistivity show a similar, gradual transition. This indicates that the sp2 cluster size determines the optical gap, even when the sp2 content does not change. The Raman I(D)/I(G) peak ratio is found to vary inversely with the square of the gap.

Helix Magic. Thermo-Driven Chiroptical Switching and Screw-Sense Inversion of Flexible Rod Helical Polysilylenes

Abstract: An ideal optically active helical chromophoric polymer comprising a flexible rodlike silicon main chain and enantiopure alkyl side chains, poly{(S)-3,7-dimethyloctyl-3-methylbutylsilylene}, underwent a thermo-driven helix−helix transition at −20 °C in isooctane. The transition characteristics, including transition temperature, transition width, the population of right- and left-handed helical motifs, global shape, and screw-pitch, were to be characterized quantitatively by spectroscopically analyzing circular dichroism (CD) and UV absorption characteristics. This is based on the unique property of the rodlike polymer in which the CD band completely matches the corresponding UV band profile at all temperatures. Moreover, fine controlling the contents and chirality of an additional chiral silylene unit incorporated in the copolymers allows free manipulation of the transition temperature in the range from −64 to +79 °C. Molecular mechanics calculation showed remarkable differences in the potential energy cur...

Two-dimensional ferroelectrics

Abstract: The investigation of the finite-size effect in ferroelectric crystals and films has been limited by the experimental conditions. The smallest demonstrated ferroelectric crystals had a diameter of ~200 A and the thinnest ferroelectric films were ~200 A thick, macroscopic sizes on an atomic scale. Langmuir–Blodgett deposition of films one monolayer at a time has produced high quality ferroelectric films as thin as 10 A, made from polyvinylidene fluoride and its copolymers. These ultrathin films permitted the ultimate investigation of finite-size effects on the atomic thickness scale. Langmuir–Blodgett films also revealed the fundamental two-dimensional character of ferroelectricity in these materials by demonstrating that there is no so-called critical thickness; films as thin as two monolayers (1 nm) are ferroelectric, with a transition temperature near that of the bulk material. The films exhibit all the main properties of ferroelectricity with a first-order ferroelectric–paraelectric phase transition: polarization hysteresis (switching); the jump in spontaneous polarization at the phase transition temperature; thermal hysteresis in the polarization; the increase in the transition temperature with applied field; double hysteresis above the phase transition temperature; and the existence of the ferroelectric critical point. The films also exhibit a new phase transition associated with the two-dimensional layers.

The order of the magnetic phase transitions in RCo2 (R = rare earth) intermetallic compounds

Abstract: It has been found experimentally that the order of the magnetic phase transitions in RCo2 compounds (R standing for rare-earth metals) at Tc changes from second order for the light-rare-earth series up to TbCo2 to first order for the heavier-rare-earth compounds DyCo2, HoCo2 and ErCo2. On the basis of results of fixed-spin-moment band-structure calculations for the isostructural compound YCo2 at different lattice constants, we propose an explanation for this behaviour. In contrast to the widely accepted Inoue-Shimizu theory for this class of compounds, our explanation also includes Pr, Nd which were thought to behave differently due to the influence of crystal-field effects. We show that an itinerant-electron metamagnetic transition in these compounds can occur only over a certain range of lattice constants and that the possibility of a first-order phase transition is connected to features of the electronic structure rather than to the magnitude of the transition temperature as conjectured earlier. The influence of the latter is only important if the transition takes place at elevated temperatures, where effects of spin fluctuations can suppress a first-order transition.

Vortex-matter phase diagram in YBa2Cu3Oy

Abstract: Due to an error during the printing process, many of the figures in this paper were printed incorrectly, despite appearing correctly in the proofs that were checked by the authors and passed for press. All the figures from this paper are reprinted correctly below. Figure 1. The temperature dependence of the resistivity for untwinned YBa2Cu3Oy at various magnetic fields (H||c axis). Magnetic field increases from right to left. Inset, the field dependence of the resistivity around Hm(T) with hysteresis subloop. Figure 2. Magnetization as a function of temperature in ZFC and FCC modes at 5 T for untwinned YBa2Cu3Oy. The inset is an expanded view of the magnetization and the jump in the magnetization μ0(M - Ms) near the vortex-lattice melting temperature. Figure 3. The inverse logarithmic derivative of the resistivity near the vortex-glass transition temperature Tg for untwinned YBa2Cu3Oy. Inset, the temperature dependence of the resistivity above (broken curves) and below (full curves) the critical point. Figure 4. Magnetization curves at various temperatures for untwinned YBa2Cu3Oy. Inset, the field derivative dM/dH shows a sharp peak at a characteristic field H*. Figure 5. The magnetic-field dependence of the critical current density Jc for untwinned YBa2Cu3Oy at several temperatures. Figure 6. The magnetic-field dependence of the critical current density Jc for both untwinned and heavy-twinned YBa2Cu3Oy at 58, 63, and 70 K. Figure 7. The vortex-matter phase diagram in untwinned YBa2Cu3Oy. The transition lines Tm(H), Tg(H), and H*(T) terminate at the critical point and divide into three different phases of the vortex liquid, the vortex glass, and the Bragg glass. The full curve is a fit to the field-driven transition line Bdis(T). Figure 8. Magnetization curves of untwinned YBa2Cu3Oy before and after electron irradiation with different irradiation doses for different temperatures: (a) T Tcp. Figure 9. The magnetic-field dependence of the critical current density Jc for untwinned YBa2Cu3Oy before and after electron irradiation. Figure 10. (a) Normalized resistivity against temperature in untwinned YBa2Cu3Oy before and after electron irradiation. (b) The inverse logarithmic derivative of the resistivity at 13 T in the vicinity of the vortex-glass transition temperature Tg. Inset, the reduced temperature dependence of the activation energy at 13 T. Figure 11. The vortex-matter phase diagram in untwinned YBa2Cu3Oy before and after electron irradiation. The three different phases, i.e., the vortex liquid, the vortex glass, and the Bragg glass, are divided by transition lines Hm(T), Hg(T), and H*(T). The full curves are fits to the field-driven transition line Bdis(T).

Thermal Characterization of PMMA Thin Films Using Modulated Differential Scanning Calorimetry

Abstract: Modulated differential scanning calorimetry was used to probe the thermal behavior of poly(methyl methacrylate) (PMMA) adsorbed onto silica from solution and subsequently dried. Changes in the glass transition were studied as a function of the adsorbed amount and solvent system used to deposit the polymer. The glass transition temperature (Tg) of PMMA was found to increase from 108 °C for the bulk polymer to 136 °C for the polymer adsorbed on silica from toluene (1.0Am). For the polymer adsorbed at approximately half that amount (0.6Am), the glass transition temperature increased to 158 °C. The breadth of the transition also increased from 11 °C for bulk to 61 and 58 °C for the 1.0Am and 0.6Am samples, respectively. When PMMA was adsorbed from a mixed solvent system (5:1 benzene/acetonitrile solution), the transition temperature and its breadth increased even further. For a sample with 0.5Am adsorbed amount from the mixed solvent system, the Tg was increased to 167 °C and the width of the transition was i...

Ordering, martensitic and ferromagnetic transformations in Ni-Al-Mn heusler shape memory alloys

Abstract: Order-disorder, martensitic and magnetic transformations in the Ni 2 AlMn alloys have been investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. The critical temperatures corresponding to the above transformations have been determined. It is shown that, on aging at low temperature, the parent phase structure changes from that of B2 to L2 1 by an ordering reaction causing the Ms temperature to decrease. The Curie temperature which has the highest value of about T c = 92°C at the Ni 2 AlMn composition decreases steeply on deviating from the stoichiometric composition. Two kinds of new martensite phases have also been detected in the as-aged Ni-21Al-25Mn and Ni-22Al-25Mn alloys.

Strain-dependent vacuum annealing effects in La0.67Ca0.33MnO3−δ films

Abstract: The effects of vacuum annealing on the structural and transport properties of the La067Ca033MnO3−δ films grown on SrTiO3 (LCMO/STO) and NdGaO3 (LCMO/NGO) substrates have been studied A lattice expansion due to oxygen release during the annealing is observed Under the same condition, the change of the out-of-plane lattice parameter in LCMO/STO is two to three times larger than that in LCMO/NGO, indicating a strong tendency for the oxygen in the former to escape Correspondingly, the metal-to-semiconductor transition shifts to lower temperatures, linearly with lattice constant until a critical value, Δd=003 A for LCMO/STO and Δd=005 A for LCMO/NGO, after which a sudden drop of the transition temperature to zero occurs The different lattice strains in both films are presumably responsible for the different critical oxygen contents for the occurrence of the resistive transition

Phase transitions in Ca1 −xSrxTiO3 perovskites: effects of composition and temperature

Abstract: The crystal structures of Ca0.6Sr0.4TiO3 and Ca0.8Sr0.2TiO3 perovskites have been studied at high temperatures by X-ray powder diffraction. The most likely sequence of phase transitions with increasing temperature in Ca0.6Sr0.4TiO3 is the same as that shown by the system CaTiO3–SrTiO3 with increasing Sr content at room temperature. This sequence is: orthorhombic Pbnm, orthorhombic Bmmb, tetragonal I4/mcm and cubic Pmm. The second orthorhombic phase could not be determined in Ca0.8Sr0.2TiO3, either due to the poorer resolution of the diffractometer employed or because the Bmmb phase might not have a stability field at this composition. The transition temperatures decrease on going from Ca0.8Sr0.2TiO3 to Ca0.6Sr0.4TiO3. Calorimetric measurements on 12 samples spanning the system CaTiO3–SrTiO3 then allow one to draw, in combination with the RT XRD data, the temperature vs. composition phase diagram. Phase boundaries show a linear shift toward lower Sr content with the increase of temperature, implying a negligible plateau effect near the CaTiO3 composition, but quantum saturation occurs at low temperatures near the SrTiO3 composition.

The Main Phase Transition of Mica-Supported Phosphatidylcholine Membranes

Abstract: The phase behavior of mica-supported bilayers has been studied with a differential scanning calorimeter, owing to a significant increase of the substrate area resulting from chopping a piece of mica into a large number of microscopic chips. We have found that there are two main phase transitions for supported phosphatidylcholine bilayers, separated by about 2 °C, and that the lower transition temperature is about 2 °C higher than the main phase transition temperature of lamellar bilayers. Therefore, supported bilayers do have the essential feature of free-standing lipid bilayers. However, the double transitions and higher transition temperatures indicate that details of the main phase transition in lipid layers are highly sensitive to their surrounding environment.

A simple FTIR spectroscopic method for the determination of the lower critical solution temperature of N‐isopropylacrylamide copolymers and related hydrogels

Abstract: Linear and crosslinked polymers based on N-isopropylacrylamide (NIPAAm) exhibit unusual thermal properties. Aqueous solutions of poly(N-isopropylacrylamide) (PNIPAAm) phase-separate upon heating above a lower critical solution temperature (LCST), whereas related hydrogels undergo a swelling–shrinking transition at an LCST. A linear copolymer made of NIPAAm/acryloxysuccinimide (98/2 mol/mol) and two hydrogels with different hydrophilicities were prepared. Fourier transform infrared (FTIR) spectroscopy was employed to determine the transition temperature and provide insights into the molecular details of the transition via probing of characteristic bands as a function of temperature. The FTIR spectroscopy method described here allowed the determination of the transition temperature for both the linear and crosslinked polymers. The transition temperatures for PNIPAAm and the gel resulting from the crosslinking with polylysine or N,N′-methylenebisacrylamide (MBA) were in the same range, 30–35 °C. For the gels, the transition temperature increased with the hydrophilicity of the polymer matrix. The spectral changes observed at the LCST were similar for the free chains and the hydrogels, implying a similar molecular reorganization during the transition. The CH stretching region suggests that the N-isopropyl groups and the backbone both underwent conformational changes and became more ordered upon heating above the LCST. An analysis of the amide I band suggests that the amide groups of the linear polymer were mainly involved in hydrogen bonding with water molecules below the LCST, the chain being flexible and disordered in a water solution. During the transition, around 20% of these intermolecular hydrogen bonds between the polymer and water were broken and replaced by intramolecular hydrogen bonds. Similar changes were also observed at the LCST of a gel crosslinked with MBA. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 907–915, 2000

Photocarrier injection effect on double exchange ferromagnetism in (La, Sr)MnO3/SrTiO3 heterostructure

Abstract: We have fabricated a (La1−xSrx) MnO3/SrTiO3 heterostructure and measured the temperature dependence of magnetization and resistivity under white light irradiation. We found that increased resistivity and a decrease of the metal-to-insulator transition temperature by 10 K occur under illumination of light at a wavelength below 400 nm. In particular, an additional photoinduced metal–semiconductor transition is observed below 100 K with decreased magnetization. This behavior can be explained by considering that the hole concentration in the (La1−xSrx) MnO3 layer decreases owing to electron injection following the photoexcitation of SrTiO3. We also confirmed electron injection along a perpendicular direction to the heterostructure using current–voltage measurement.

Grain-boundary effects on the electrical resistivity and the ferromagnetic transition temperature of La0.8Ca0.2MnO3

Abstract: In this letter, grain-boundary effects on the electrical transport and the ferromagnetic transition temperature were investigated in La0.8Ca0.2MnO3. The different ferromagnetic transition temperature (45 K) and the different metal–semiconductor transition temperature (62 K) were observed in the samples with different grain boundaries. Wide grain boundaries can induce large grain-boundary effects on grain, and small grain size also induces large grain-boundary effects on grain due to the increase of grain boundaries. The possible mechanism of grain-boundary effect is explained as the strain effect of grain induced by the distortion at grain boundaries. The strain of grains should enhance the ferromagnetic double exchange interaction.

Chemical waves in self-oscillating gels

Abstract: The behaviors of a poly(N-isopropyl acrylamide) (PNIPA) gel coupled with the Belousov-Zhabotinsky (BZ) reaction has been investigated as a function of temperature and catalyst concentration. In this type of gel, the chemical oscillation in the BZ reaction induces periodic and autonomous swelling-shrinking volume changes of the gel, and conversely a volume change of the PNIPA gel affects the propagation of the chemical wave. Our attention was focused on the effects of mechanical changes on the chemical wave by utilizing the thermally driven volume phase transition of the gel. Both the velocity and the frequency of the chemical wave increased with increasing temperature, and abruptly decreased at the volume transition temperature of the gel, T(c). The diffusion of HBrO2, which is essential for wave propagation, was hindered with increasing temperature. The diffusion of HBrO2 through the gel network in the low temperature region was explained in the same way as a simple diffusion of inactive molecules through a restricted environment.

In-situ study of the R3̅ to R3̅c phase transition in the ilmenite-hematite solid solution using time-of-flight neutron powder diffraction

Abstract: The R 3 to R 3 c cation ordering phase transition in the ilmenite (FeTiO 3 ) – hematite (Fe 2 O 3 ) solid solution has been investigated using in-situ time-of-flight neutron powder diffraction. Four synthetic samples of the solid solution containing 70, 80, 90, and 100% FeTiO 3 (ilm70, ilm80, ilm90, and ilm100, respectively) were heated under vacuum to a maximum of 1350 °C. Powder diffraction patterns were collected at several temperatures on heating and cooling, with a Rietveld refinement performed in each case. Samples ilm80, ilm90, and ilm100 were fully ordered after quenching from the synthesis temperature to room temperature. Sample ilm70 had a higher degree of quenched in disorder, which is the result of chemical heterogeneities produced during quenching and subsequent heating in the neutron experiments. The degree of order in all samples decreased smoothly at high temperatures, with second-order transitions to the R 3 c phase being observed at 1000, 1175, and 1325 °C in ilm70, ilm80, and ilm90, respectively. The transition temperature in ilm100 was higher than the maximum temperature reached in the neutron experiments, and is estimated as ~1400 °C. The character of the transition is typical of that predicted by three-dimensional Ising models and appears to become more first-order in character with increasing Ti-content. The temperature-dependence of the cell parameters reveals that components of the spontaneous strain tensor, e 11 and e 33, are negative and positive, respectively. Little volume strain is associated with long-range ordering. A small negative volume strain due to short-range ordering within the R 3 c phase is identified. The variations in cell parameters and cation-cation distances can be understood in terms of the competing effects of long- and short-range ordering as a function of temperature and composition.

Theoretical study on the structure of Na+-doped helium clusters: Path integral Monte Carlo calculations

Abstract: The microscopic structure of Na+-doped helium clusters (Na+⋅HeN) is investigated by employing the path integral Monte Carlo (PIMC) method. Our primary interest is in determining the nature of superfluidity in these clusters by examining the temperature and size dependences of several physical quantities such as energy and superfluid fraction. Comparison with Boltzmann statistics has also been made to clarify the effects of superfluidity. It is found that clusters of N⩾100 have a triple-layer structure where the first shell is rigid-body and solidlike and the third shell is almost like liquid helium. Between T=1.0 K and T=1.25 K, a superfluid transition is observed in the second shell. This transition temperature shifts to a lower temperature than that observed in bulk liquid helium because the finite system size and strong binding to the cation cause superfluidity to be suppressed. By computing the effective moment of inertia as a function of cluster size, it is found that there exists a specific number o...

Crystallization kinetics of bulk amorphous Se-Te-Sn system

Abstract: In the present work crystallization kinetics of the amorphous Se80−xTe20Snx (0 ≤ x ≤ 9) system have been investigated using Differential Scanning Calorimetry. From the heating rate dependence of the glass transition temperature and the crystallization temperature the activation energy for the glass transition and that for crystallization have been determined using the Kissingers equation and Matusitas equation for non-isothermal crystallization of materials. The effect of addition of Sn to the Se-Te system on the dimentionality of crystal growth has been investigated. An increase in the glass transition temperature with increase in Sn content suggests that Sn plays a role in cross-linking the already existing Se-Te chains which causes an increase in the thermal stability of the material.

P-T-X data on P21/c-clinopyroxenes and their displacive phase transitions

Abstract: The P21/c clinopyroxene kanoite (ideally MnMgSi2O6) was studied as a function of pressure and temperature using powder X-ray diffraction, differential scanning calorimetry (DSC) and optical methods. The temperature of the P21/c to high-temperature (HT) C2/c transition ranges from 425 °C in endmember MnMgSi2O6 to 125 °C in natural samples with an aegirine component. Compiling pigeonite and clinoenstatite–clinoferrosilite literature data, the temperature of the transformation was found to decrease linearly with M2 cation size. A synchrotron powder diffraction study in a heated diamond-anvil cell (DAC) yielded compression and thermal expansion data for low kanoite of composition Mn1.2Mg0.4Fe0.4Si2O6. The high-pressure (HP) phase transition from P21/c to HPC2/c was reversed at 5.8 GPa at 417 °C. The high-temperature phase transition from P21/c to HTC2/c was rather indistinct and occurred at approximately 530 °C and 0.76 GPa. In a separate experiment, the HT transition was observed optically in a hydrothermal DAC between 0.0 and 0.4 GPa. The in-situP-T data of both experiments yielded an increase in transition temperature with increasing pressure (approx. 149 °C/GPa) and suggest a change in character of the transition from first order to continuous with increasing pressure. The data indicate that the HTC2/c and HPC2/c polymorphs are distinct phases with different stability fields. Since the high-temperature and the high-pressure polymorphs of kanoite were shown to be isotypic with other low-Ca clinopyroxenes such as the (Mg,Fe)SiO3 series, the conclusions we draw from this study are valid for all clinopyroxenes with small (<0.88 A) M1 and M2 cation sizes. The petrologic implications of these conclusions for the occurrence of “clinoenstatite” in the Alpe Arami peridotite are discussed.

Spin–lattice interaction in colossal magnetoresistance manganites

Abstract: The metal–insulator transition and underlying spin dynamics in La0.7D0.3MnO3 (D=Ca, Sr) are investigated using optical pump–probe spectroscopy at 1.5 eV. Our measurements, which span the ferromagnetic–paramagnetic transition temperature, reveal that the dynamics of the optically induced spectral weight transfer follow the temperature dependence of the magnetic specific heat. This dependence reflects the intrinsic interdependence between the optical conductivity and magnetism in the manganites allowing for the determination of the spin-lattice coupling magnitude.

Low-field magnetocaloric effect in Pr0.5Sr0.5MnO3

Abstract: The magnetocaloric effect in polycrystalline of Pr1 − xSrxMnO3 (x = 0.3, 0.4, 0.5) was investigated. A large magnetic entropy change (7.1 J/kg K) is discovered in Pr0.5Sr0.5MnO3 under a low magnetic field of 1 T at charge-ordered state transition temperature (161 K). The physical mechanism is related to a drastic magnetization change at a temperature where the field-induced magnetic, electron and structural phase transitions occur (from antiferromagnetic charge-ordered state to ferromagnetic charge-disordered state).

Electrical Properties in Solid Solution Pr1—xSrxMnO3 (0 ≤ x ≤ 0.5)

Abstract: We present the electrical properties of polycrystalline samples Pr 1-x Sr x MnO 3 (0 ≤ x ≤ 0.5). These properties are correlated to the ratio of the Mn 3+ /Mn 4+ cations. The transport properties of the Pr 1-x Sr x MnO 3 samples could be explained on the basis of double exchange mechanism between pairs of Mn 3+ and Mn 4+ cations, with conductivity occurring by electron hopping between manganese ions along Mn-O-Mn bonds. A semiconductor-to-metal transition is observed as a function of temperature for the mixed valence samples Pr 1-x Sr x MnO 3 (0.25 ≤ x ≤ 0.4) with a metallic-like behaviour above a critical temperature T Q. A semiconductor-like one is observed for all the range of temperature (50 to 300 K) for (0 ≤ x ≤ 0.2 and x = 0.5). Below the Curie temperature T c , for the ferromagnetic regimes, the spin disorder scattering process has been observed to play an important role in resistivity behaviour. The evolution of activated energies with the carrier concentration has been investigated.