Search

The crystallization process in digital versatile disc (DVD) media was investigated using a time-resolved X-ray diffraction apparatus coupled with in situ photoreflectivity measurement. The time profiles of crystallization were found to be consistent with the changes in photoreflectivity. The phase changes were characterized by the start and end time; 90±1 and 273±1 ns for Ge2Sb2Te5, and 85±1 and 206±1 ns for Ag3.5In3.8Sb75.0Te17.7, respectively. The faster crystallization time in Ag3.5In3.8Sb75.0Te17.7 is ascribed to its characteristic crystallization process; its X-ray diffraction profile shows a significant sharpening during the crystallization process, whereas the peak width of Ge2Sb2Te5 remained unchanged. The present findings suggest that crystal growth control is another key for designing faster phase-change materials.

Based on the maximum entropy method (MEM) charge density, we have visualized the electrostatic potential of a charge-transfer compound, RbMn[Fe(CN) 6 ], which shows the charge-transfer transition from the nominal Mn 2+ –Fe 3+ configuration to the Mn 3+ –Fe 2+ one below 220 K. In the low-temperature phase, the transferred electron spreads over the hexacyanoferrate ([Fe(CN) 6 ]) via the strong Fe t 2 g -CNπ * hybridization. Such an extended charge-transfer state is responsible for the relatively strong ferromagnetic coupling between the neighbouring Mn 3+ spins.

application/pdf, A photoexcitation is one of the promising external fields to control the material phases. Here, the authors have demonstrated that the magnetic and structural properties of a spin-crossover complex, Fe(phen)2(NCS)2 (phen=1,10-phenanthroline), can be reversibly switched by the on-off action of the continuous photoexcitation at the same temperature. The structural data suggest that the density of the high-spin Fe2+ in the photoinduced phase is about 0.88. Suppressed atomic vibrations of the photoinduced phase exclude the conventional heating effect as the origin for the observed optical switching.

We present a contactless method of detecting small changes in the surface temperature of metallic samples over a short time period. The thermometry method incorporates a contactless heater and simplifies sample preparation requirements for calorimetric measurements. We demonstrate that we can measure small temperature shifts (250μK) in 5ms at midrange temperatures (155K). This method does not serve as a conventional thermometer but as an in situ thermometer useful for narrow temperature ranges, such as the region of a phase transition (often exploited in bolometers). The manganite material Nd0.5Sr0.5MnO3 was chosen as a test material for our experiments because it is a well characterized material where resistivity and magnetization [Kuwakara et al., Science 270, 961 (1995)] and thermal conductivity [Kim et al., American Physical Society March Meeting, W24.009 (2004)] have been previously measured and the material undergoes a metal-insulator transition.

application/pdf, The three-dimensional atomic configuration of amorphous Ge2Sb2Te5 and GeTe were derived by reverse Monte Carlo simulation with synchrotron-radiation x-ray diffraction data. The authors found that amorphous Ge2Sb2Te5 can be regarded as "even-numbered ring structure," because the ring statistics is dominated by four- and six-fold rings analogous to the crystal phase. On the other hand, the formation of Ge–Ge homopolar bonds in amorphous GeTe constructs both odd- and even-numbered rings. They believe that the unusual ring statistics of amorphous Ge2Sb2Te5 is the key for the fast crystallization speed of the material.

Effects of hydrostatic pressure on the ferromagnetic phase transition are systematically investigated for doped manganites, R1−xSrxMnO3 (R  La and Nd) by controlling the doping level (x). In both the systems, the positive pressure coefficient (dln Tc|dP) for the Curie temperature increases significantly with the decrease of x (or Tc). We have derive empirical relation between the coefficient and Tc, and will discuss its origin.

We have performed neutron powder diffraction and Rietveld analysis of RbMn[Fe(CN)6] at 300K and 40K, using the Kinken powder diffractometer for high efficiency and high resolution measurements (HERMES) installed at the JRR-3M reactor at the Japan Atomic Energy Research Institute, Tokai, Japan. The room temperature structure (see Table I) determined by neutron diffraction is reasonable, because the Mn–N bond distance (1⁄4 2:195ð3Þ A) and the Fe– C bond distance (1⁄4 1:922ð4Þ A) are comparable with the values of the other transition metal cyanides. The atomic coordinates of the high-temperature phase reported in our previous paper were seriously deviate from the values. Here, note that the neutron structural analysis has clear advantage over the X-ray structural analysis if the neighboring atomic species in the periodic table should be distinguish, like RbMn[Fe(CN)6]. So, we concluded that the previous Rietveld analysis on the X-ray diffraction pattern was trapped a wrong local minimum. Actually, we found a structural solution close to those shown in Table I. We show in Fig. 1 the recalculated lattice constants and bond distances of RbMn[Fe(CN)6] in the warming-run. We confirmed that the bond distances of the low-temperature phase are the same within the experimental errors between the structures determined by the neutron powder diffraction and that reported previously. The cubic-tetragonal transition at 290K accompanies a significant Jahn–Teller distortion of the MnN6 octahedra, and is considered to be driven by the charge-transfer from the Mn2þ site to the Fe3þ site.

2 p , c, and of Tb1 xCaxMnO3 (0:5 x 1:0) against Ca concentration x at 10K. The powder patterns for the lower and higher x-range can be well reproduced by the single orthorhombic (Pnma: open symbols) model. In the concentration range of 0:65 x 0:85, however, the powder patterns were reproduced by the twophase model with orthorhombic and monoclinic (P21=m: filled symbols) phases. Note that the room temperature structure was the single-phase even in this x region. The lattice constants, a, b= ffiffiffi

Heat treatment effects on the physical properties have been investigated for Sr2FeMoO6 single crystals with double perovskite structure. We have evaluated the occupancy g of the Fe3+ ion on one sublattice of the perovskite-B site by Rietveld refinement of the synchrotron-radiation X-ray powder patterns. Both the saturation magnetization M s and critical temperature T c for the ferrimagnetic transition decrease linearly with g in the large-g region (g ≥∼ 0.84), but begin to deviate from the empirical relations for g ≤∼ 0.84. We interpreted this behavior in terms of variation of the correlation length of the spin ordering.

Photoinduced resistivity change (ΔR/R) in a La0.6Sr0.4MnO3 film has been investigated with variation of the excitation energy E exc (1.88–3.20 eV) and temperature (290–400 K). The magnitude of the photoinduced resistivity change steeply decreases with increasing temperature beyond T c∼315 K. In addition, we have found that the photoinduced signal is enhanced when the down-spin e g carriers are excited. Based on these experimental findings, we have ascribed the positive ΔR/R signal to the photoinduced spin disorder in the ferromagnetic metallic (FM) state.

We have observed a transient transmission change (ΔT) induced by photoirradiation in La0.6Sr0.4MnO3 film at room temperature. The magnitude of ΔT is enhanced with a decrease of the probe photon energy \hbarω, suggesting a resonance feature to the “Drude-like” component. We interpreted the photoswitching effect in terms of the photoinduced suppression of the spin correlation in the half-metallic double-exchange system.

We have observed room temperature magnetoresistance (MR) in Fe-site-substituted Sr2FeMoO6 crystal with a double perovskite structure. These melt-grown crystals have few grain boundaries and show high conductivity (∼10-3 Ω·cm at 300 K), which is advantageous for a practical application. We will discuss the possible origin on the basis of the substituent concentration dependence and temperature dependence of the magnitude of the MR.

Anisotropic magnetic and transport properties as well as magnetic structures have been investigated for single crystals of bilayer manganites (La1-zNdz)2-2xSr1+2xMn2O7 as a function of doping level x and averaged ionic radius z of the rare-earth ion. We have derived a global magnetic phase diagram as a function of x and z. The ferromagnetic metallic state observed in La1·2Sr1·8Mn2O7 (x = 0· 4 and z = 0· 0) is replaced by a layered antiferromagnetic state beyond x ≥ 0·45. The phase diagram thus obtained is compared with that for the cubic manganites (La1-zNdz)1-xSrxMnO3.

Magnetotransport characteristics and their nominal hole concentration (x) dependence were investigated in Nd1- x Sr x MnO3 films. Thermal hysteresis in resistivity indicating the occurrence of a first-order phase transition was observed for x=0.35 and 0.50, but essentially not for x=0.40 and 0.45. The x=0.35 and 0.50 films also show hysteresis in magnetoresistance. Once the resistivity is decreased by an order of magnitude by application of a magnetic field, the low-resistivity state is maintained even after removal of the field below a critical temperature. Observed hysteretic behavior suggests the presence of instabilities which compete with double-exchange interaction in specific hole concentration regions, which may be one of the origins of the colossal magnetoresistance.

Optical conductivity spectra and their polarization dependence have been investigated for the layered cupric halides A2CuCl4 (A=C n H2n+1NH3), which form CuCl4-layers analogous to the CuO2-sheets of the cuprate superconductors. In the visible region (2–4 eV), we found two distinct electronic transitions localized on the CuCl4-layer, whose peak energies shift to lower energy (ca. 0.8 eV) by substitution of chlorine with bromine. We ascribed these transitions to charge-transfer (CT) excitations from the ligand (Cl or Br) to metal (Cu) sites.

Effects of hydrostatic pressure on the antiferroelectric phase transitions of proto-typical hydrogen-bonded antiferroelectrics K3D(SO4)2 as well as its hydrogenerated analog (K3H(SO4)2), which shows no phase transition, have been investigated. The critical temperature (TC) for the D-compounds was found to rise with pressure, making a sharp contrast with the other hydrogen-bonded (anti-)ferroelectrics whose TC's usually decrease under pressure. In contrast, no significant pressure effect was observed for the H-compound. The obtained results are discussed in terms of the electrostatic interaction between the electric dipoles in the crystal.

Effects of hydrostatic pressure on the Raman spectra for a prototypical hydrogen-bonded molecular solid, the isotopically mixed crystal of squaric acid ((H1-xDx)2C404; x=0.4), have been investigated. Pressure-enhanced back-and-forth motion of the protons (deuterons) induces a symmetry change of the constituent squaric molecules from the asymmetric (C1h) to centrosymmetric (C4h) form around 3GPa at 300K. Furthermore, the critical pressure (PS) for the symmetry change is found to increase with decreasing temperature. Based on these results, dynamic behaviors of the protons as well as deuterons under pressures are discussed.

We have used high energy-resolution angle-resolved photoemission spectroscopy to study the k-dependent electronic structure of single-crystalline samples of the manganese-based perovskites as a function of doping level, temperature, and layer number. An especially important aspect of the data is a temperature and sample-type dependent pseudogap which drastically depletes the spectral weight at EF. In contrast to the pseudogap observed in the high Tc superconductors, the pseudogap in the manganites is essentially isotropic in k-space, implying that a local effect such as a lattice distortion should be responsible.

A VAST number of compounds are known that exhibit structure transformations in response to changes in temperature, pressure and/or composition. One such example is the family of perovskites, La1-xSrxMnO33 for a limited range of compositions (x), they undergo a structural phase transition from an orthorhombic to a rhombohedral form with increasing temperature1, 2. These compounds are also ferromagnetic, a property that arises from coupling between the charge carriers and localized spin moments of the manganese ions3–7. Here we show that, through careful tuning of the composition, the local spin moments and the charge carriers can in turn be coupled strongly to changes in the structure. For x = 0.170, the crystal structure of the compound can be switched—reversibly or irreversibly, depending on the temperature—by application of an external magnetic field.

We have investigated effects of random substitution of proton sites with deuterons on the temperature‐ and pressure‐induced phase transitions in squaric acid crystal, which consists of ferroelectrically ordered molecular sheets of [C4O4]2−(SQ2−) skeletons and intervening protons or deuterons. In the isotopically mixed crystals (H1−xDx )2SQ, the critical temperature Tc for the dielectric phase transition was found to increase linearly with the deuterium concentration x, while the critical pressure Pc increases nonlinearly with x at 300 K. Around the pressure ∼20 kbar above Pc, the constituent SQ2− skeletons in the paraelectric phase show a symmetry change from the asymmetric (C1h ) to centrosymmetric (C4h ) form, which is likely caused by the deformation of the correlated proton (deuteron) potential.

Infrared (IR) and visible absorbance spectra were measured as a function of hydrostatic pressure for five kinds of organic crystals composed of hydrogen‐bonded molecular chains. An unconventional vibrational band is observed in the IR spectra around 1800 cm− 1 commonly in these compounds and its intensity rapidly increases with pressure. We have ascribed the observed band to a proton vibration localized around kink‐type defects or solitons in the hydrogen‐bonded molecular chains. In accordance with the pressure‐induced growth of the IR band, a new electronic absorption band appears in the visible region below the ordinary molecular excitation, suggesting a change in the electronic structures within the molecules around the soliton.

Charge and orbital ordering states of manganites have been observed directly by using synchrotron x-ray diffraction techniques. The alternating pattern of Mn3+ and Mn4+ in charge ordering state was elucidated by the anomalous dispersion method. The orbital ordering at Mn3+ sites is presented based on resonant x-ray scattering near Mn K-absorption edge. The scattering mechanism has been developed by considering the splitting of the Mn 4p levels due to the orbital ordering. The coupling among the charge, spin and orbital degrees of freedom is discussed.

Temperature variation of lattice structure has been investigated for ferromagnetic $\mathrm{La}({\mathrm{Sr}}_{0.8}{\mathrm{Ca}}_{0.2}{)}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ and layered-type (A-type) antiferromagnetic ${\mathrm{NdSr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}.$ We have found that distortion of the ${\mathrm{MnO}}_{6}$ octahedra decreases in the spin-ordered phase for both the systems. We further estimated the relative stability of the ${e}_{g}$ orbitals by means of Madelung potential calculation, and discussed the orbital state in the spin-ordered phase.

We have investigated the charge-density distribution of an antiferromagnetic bi-layered manganite, NdSr 2 Mn 2 O 7 , by the maximum entropy method (MEM) using the synchrotron X-ray powder data. The magnetic structure below T N =150 K is known to be a layered-type (A-type). We have found a significant change of the electron distribution around the Mn site below T N . Our observation indicates d x 2 - y 2 -orbital polarization in the A-type antiferromagnetic state.

We have investigated the electronic structures of perovskite manganese oxides, Nd0.5Sr0.5MnO3, (Nd0.062Sm0.938)0.5Sr0.5MnO3, La0.84Sr0.16MnO3 and La0.7Ca0.3MnO3, by X-ray photoemission (XPS) and X-ray bremsstrahlung isochromat spectroscopy in addition to vacuum ultraviolet inverse photoemission. It is found that the VUV-IPES spectra seem to be well explained by the band-structure calculation. Mn 3p-3d resonance photoemission measurements have also been performed, where the contributions of the occupied Mn 3d states are extracted. We observed hybridization between the Mn 3d and O 2p states from the Mn 2p and 3p spectra of all samples, and the Mn spin state from the Mn 3s XPS spectra of Nd0.5Sr0.5MnO3 and (Nd0.062Sm0.938)0.5Sr0.5MnO3.

Magnetoresistance resulting in a drop in resistivity of more than three orders of magnitude that is strongly coupled to lattice striction has been observed under a relatively low magnetic field (0.4 tesla at 115 kelvin) for a single crystal of perovskite-type manganese oxide with finely controlled ionic radii of the A sites, (Nd,Sm) 1/2 Sr 1/2 MnO 3 . The colossal magnetoresistance phenomena are viewed as a first-order insulator-to-metal phase transition induced by a magnetic field, which accompanies a metamagnetic (antiferromagnetic-to-ferromagnetic) transition and a structural change in the lattice. Clear hystereses and abrupt changes in magnetization, striction, and resistivity were observed in increasing and decreasing magnetic fields at temperatures (113 to 150 kelvin) just above the Curie temperature.

We have measured the spinwave dispersion throughout the Brillouin zone of the doubleexchange ferromagnet La0.7Pb0.3MnO3 by using inelastic neutron scattering. An unexpectedly simple Heisenberg Hamiltonian with solely a nearestneighbour coupling accounts for the entire dispersion relation. The value of the Curie temperature Tc calculated from the derived coupling constant agrees with the experimental value to within 15%. The results, including damping of the zone boundary spin waves near Tc, can be fully understood in the framework of the doubleexchange Hamiltonian. In the paramagnetic phase of the twodimensional analogue La1.2Sr1.8Mn2O7, we have observed longlived antiferromagnetic clusters coexisting with ferromagnetic fluctuations. This effect must be included in theories that attempt to explain giant magnetoresistance in manganites, at least in two dimensions.

MANGANESE oxides with the cubic perovskite structure (typified by LaMnO3) have stimulated considerable interest because of their magnetoresistive properties1–9; they exhibit extremely large changes in electrical resistance in response to applied magnetic fields, a property that is of technological relevance for the development of magnetic memory and switching devices. But for such applications to be viable, great improvements will be needed in both the sensitivity and temperature dependence of the magnetoresistive response. One approach under consideration for optimizing these properties is chemical substitution10. Here we demonstrate an alternative strategy, in which we synthesize layered variants of the cubic perovskite parent compounds that have a controlled number of MnO2 sheets per unit cell. This strategy is structurally analogous to that employed for the systematic exploration of the high-transition-temperature copper oxide superconductors11. We find that the magneto-resistive properties of these materials depend sensitively on the dimensionality of the manganese oxide lattice. Although the properties of our materials are still far from optimal, further exploration of this series of layered perovskites may prove fruitful.

Lattice structural effects on the spin ordering of bilayer manganites have been investigated by molecular orbital calculation on the MnO 6 cluster, based on the reported atomic coordinates. We found that necessary conditions for the purely ferromagnetic ordering are (1) small energy splitting A (≤0.15 eV) between the up-spin b 1 orbital (d x 2 - y 2-like) and the up-spin a 1 (d 3 z 2 - r 2-like) orbital and (2) a large coefficient (C a 1 đ O 1 2 p ≥0.30) of the 2p orbital of inner apical oxygen O1 in the a 1 orbital.

Dielectric properties have been investigated for proton-deuteron mixed crystals of 5-bromo-9-hydroxyphenalenone with strong intramolecular hydrogen bonds. The fully deuterated compound shows two successive structural transitions, i.e. , incommensurate (IC) transition ( T IC ∼ 37 K) and lock-in transition ( T C ∼20 K). With increasing proton concentration ( x ), both of the critical temperatures decrease and the transitions disappear at x IC ∼ 0.65 and x C ∼ 0.25, respectively. This observation suggests that intramolecular tunneling motion of the protons significantly influences the structural phase transitions via the molecular tautomerization process.

The crystal of squaric acid (SQ) shows dielectric property caused by tautomerization coupled with intermolecular proton transfer. We prepared a squaric acid derivative, 1,4-phenylenebis(squaric acid) (PBSQ), and determined its crystal structure. In the unit cell one PBSQ molecule and two water molecules are incorporated. There are one dimensional hydrogen-bonded chains PBSQ molecules. The distance between hydrogen-bonded O···O is very short (2.48A). The result suggests that PBSA exists as mono deprotonated species accompanied by a counter cation of protonated 2H2O molecules (O···O distance of 2.45A).