X-ray study of structural phase transitions in nanocrystalline LaMnO3+δ perovskite

The nanocrystalline LaMnO_3+δ perovskite was synthesized by the microwave-assisted glycothermal method and calcined at several temperatures up to 1500°C. The prepared samples were examined by the X-ray powder diffraction with the aim to show that LaMnO_3+δ exhibits the size-induced structural phase transitions. The as-received nanocrystals of LaMnO_3+δ are of tetragonal, pseudo-cubic symmetry not known for bulk material. The samples calcined at temperatures 750–1100°C have trigonal symmetry known from the high-temperature phase of LaMnO₃ single crystal. The samples calcined from 1200°C to 1500°C have two phases: trigonal and orthorhombic. Thus, the observed phase sequence is inverted with respect to that of the bulk material, which is the characteristic of the size-induced mechanism of phase transitions in the nanocrystals. The critical crystallite sizes for both structural transitions were evaluated as 20 and 100?nm.     

Thermoelectric properties of Ca3−xYxCo4O9+δ ceramics

The Ca_3?xY_xCo₄O_9+δ (x=0, 0.15, 0.3) ceramics were prepared by combining the polyacrylamide gel method and the spark plasma sinter (SPS) technology in order to improve the thermoelectric properties of Ca₃Co₄O_9+δ ceramics. The Seebeck coefficients and the resistivities of the Y-doped samples were obviously enhanced due to the decrease of carrier concentration, and their thermal conductivities were decreased due to the impurity scattering effect. The thermoelectric properties were improved at high temperature by Y-doping according to the power factor analysis and the thermoelectric figure of merit (ZT) data. The optimized figure of merit ZT=0.22 at 973 K was obtained for Ca_2.7Y_0.3Co₄O_9+δ.     

Cu-EPR in YBa2Cu3O6+δ single crystals

We report on the observation of Cu-EPR signals in single crystalline material of YBa₂Cu₃O₆₊ in the narrow oxygen concentration range 0.7<<0.9 and for temperatures 80 K<T<200 K. We provide evidence that the signal results from Cu²⁺ ions located in Cu(1)OCu(1) chain fragments.     

Effect of fuel on the formation structure,transport and magnetic properties of LaMnO3+ δ nanopowders

Single-step low-temperature solution combustion (LCS) synthesis was adopted for the preparation of LaMnO_{3+ δ} (LM) nanopowders. The powders were well characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), surface area and Fourier transform infrared spectroscopy (FTIR). The PXRD of as-formed LM showed a cubic phase but, upon calcination (900°C, 6 h), it transformed into a rhombohedral phase. The effect of fuel on the formation of LM was examined, and its structure and magnetoresistance properties were investigated. Magnetoresistance (MR) measurements on LM were carried out at 0, 1, 4 and 7 T between 300 and 10 K. LM (fuel-to-oxidizer ratio; ψ = 1) showed an MR of 17% at 1 T, whereas, for 4 and 7 T, it exhibited an MR of 45 and 55%, respectively, near the T _M-I. Metallic resistivity data below T _M-I showed that the double exchange interaction played a major role in this compound. It was interesting to observe that the sample calcined at 1200°C for 3 h exhibited insulator behavior.     

Transverse resistance of YBa2Cu3O7−δ single crystals

Measured is the transverse electrical resistance of YBa₂Cu₃O_7−δ single crystals with different oxygen deficiency values (δ) in the temperature range T_c − 300 K. The experimental data are approximated by an empiric expression accounting for the fluctuation conductivity near T_c and the semiconductor-like resistance regime. Our analysis of the concentration dependences of the fitting parameters, in particular, reveals that the resistance temperature dependence is largely affected by the sample's non-homogeneity. The latter, in turn, causes a T_c anisotropy and variable-range hopping conductivity between different phases. The deduced maximal values of the basal-plane coherence length, ξ_xy(0), are comparable with those for low-temperature superconductors.     

Significant enhancement of electrical transport properties of thermoelectric Ca3Co4O9+δ through Yb doping

We report the significant enhancement of the power factor of Ca₃Co₄O_9+δ through Yb doping. The pellets were prepared by pressing under 0.5 GPa and 2 GPa. The highest power factor of 553 μW m^?1 K^?2 due to the significant increase of electrical conductivity was obtained for Ca_2.9Yb_0.1Co₄O_9+δ pressed at 0.5 GPa. This is 2.3 times higher than that of Ca₃Co₄O_9+δ (246 μW m^?1 K^?2). Nanostructure examinations show that the pellets pressed at 0.5 and 2 GPa have different nano-lamella structures. This work suggests that Yb is an effective doping element for enhancing the electrical transport properties of Ca₃Co₄O_9+δ, and the optimum doping level is related to the nanostructure of the bulk pellets.     

Ferro-and antiferromagnetic ordering in LaMnO3+δ

A neutron diffraction study of the crystalline structure and magnetic state of LaMnO_3+δ samples with different deviations from oxygen stoichiometry has been made at 4.2 K. It is shown that annealing at reduced oxygen pressure is accompanied by transformation of the magnetic structure from ferromagnetic, with magnetic moments parallel to the b axis, to antiferromagnetic, with the wave vector k=0 and the moments along the a axis (space group Pnma). A comparison of experimental with expected Mn ion moments suggests that magnetic order does not extend throughout the sample volume. Part of the Mn ions form magnetic clusters ～20 Å in size.     

Magnetotransport properties of La0.67Ca0.33MnO3//La0.67Sr0.33MnO3 bilayers

The perovskite bilayers La0.67Ca0.33MnO3 （LCMO） （100 nm） / La0.67Sr0.33MnO3（LSMO） （100 nm） and LSMO （100 nm） / LCMO （100 nm） are fabricated by a facing-target sputtering technique. Their transport and magnetic properties are investigated. It is found that the transport properties between them are different obviously due to distinguishable structures, and the different lattice strains in both films result in the difference of metal-to-insulator transition. Only single-step magnetization loop appears in our bilayers from 5K to 320K, and the coercive force of LSMO/LCMO varies irregularly with a minimum ～ 2387A/m which is lower than that of LCMO and LSMO single layer films. The behaviour is explained by some magnetic coupling.     

Fluctuation-enhanced conductivity and magnetoconductivity of high-quality YBa2Cu3O7−δ crystals

We present measurements of the in-plane resistivity _ab of YBa₂Cu₃O_7– single crystals withT _c 92 K and _ab (100 K)50 cm. The temperature dependence of the fluctuation conductivity and of the magnetoconductivity aboveT _c is analyzed in terms of direct and indirect fluctuation contributions for layered superconductors. The combination of fluctuation conductivity and magnetoconductivity allows to determine both coherence lengths _ab (0) and _c (0) as well as the phase-relaxation time of the pairs in an unequivocal manner. Evidence for clean limit type-II superconductivity in our crystals is given by large values of the mean free pathl _{ab ab} (0).Dedicated to Prof. Dr. F. Hund on the occasion of his 95th birthday     

Nature of the anomalies of the thermolumenescence properties of dosimetric α-Al2O3 crystals

The results of investigations into the anomalies of the thermoluminescence properties of dosimetric corundum crystals are presented. The decisive role of deep-lying traps in the quenching of luminescence in anion-defect Al₂O₃ monocrystals is shown. The existence of deep-lying traps is demonstrated by the method of direct observations of thermoluminescence (TL) peaks associated with them. Experimental evidence for the influence of the degree of occupation of deep-lying traps on the main features of the TL dosimetric peak at 450 K is given. The results obtained are interpreted for a model of the interactive system of traps, which differs radically from the models described in the literature by a consideration of the temperature dependence of the probability of trapping of charge carriers on deep-lying traps. We believe that the heat quenching of luminescence is due to the thermal ionization of excited F-center states. Ural State Technical University. Translated from Izvestiya Vysshikh Uchebhykh Zavedenii, Fizika, No. 3, pp. 55–65, March, 2000.     

Magnetotransport study of in situ magnetic field textured Dy1Ba2Cu3O7−δ superconductors

The electrical resistivity, thermoelectric power and the Nernst effect have been studied as a function of temperature and magnetic field for a typical superconductor DyBa₂Cu₃O_7– magnetically textured in situ at 1035°C. The three transport coefficients show hybrid microscopic features. In particular, we show the anisotropy with respect to the field direction (H//a,H//c) in all transport coefficients. We verify that Tinkham's law is obeyed for the broadening of the resistive transition in a magnetic field. Similarly we obtainanisotropic broadening exponents for each integrated excess property. From Arrhenius plots we obtain orders of magnitude for the activation energies characterizing each property. They are markedly different from each other.     

Defect chemistry and electrical properties of La1−xSrxCoO3−δ IV. Electrical properties

This paper considers electrical properties of La_1−xSr_xCoO_3−δ in terms of defect models, such as random defect model and the cluster model. It is shown that the experimental data of the electrical conductivity may be explained in terms of the random defect model rather than the cluster model.     

Visible luminescence properties of Er3+–Dy3+ codoped tellurite glasses

Er³⁺ and Dy³⁺ codoped tellurite glasses have been synthesized. Five emission bands in the PL spectrum under 325 nm pumping were observed. Three of them correspond to Er³⁺ and the other two correspond to Dy³⁺, respectively. The PL spectrum revealed that the intensity of Dy³⁺ characteristic emission was enhanced as Er³⁺ concentration increased while keeping Dy³⁺ concentration constant. Due to small mismatch between the energy level of Er³⁺:⁴F_7/2 and Dy³⁺:⁴F_9/2 resonant energy was possibly transferred between them. This process can give rise to an enhancement of the PL intensity of 484 and 574 nm from Dy³⁺. The PL spectra of these glasses cover the blue, green and red wavelength range and the intensities of those emission bands could be controlled by adjusting the concentration of relevant rare-earth ions. These glasses with the controllable CIE coordinates might be a potential candidate for the widely realistic application such as solid-state white lighting and multicolor display.     

Comparison of spectroscopic properties of Yb:YAP and YbiYAG crystals

The Yb:YAG and Yb:YAP crystals have been grown by Czochralski method. The absorption spectra and the fluorescence spectra of Yb:YAG and Yb:YAP crystals have been investigated. It is shown that the Yb:YAG crystal has better laser properties and smaller threshold power than Yb:YAP crystal. In addition, the absorption cross-section of the Yb:YAP crystal is 2.16 times of that of the Yb:YAG crystal, so laser diode pumped Yb:YAG lasing can be easily realized. Because YAP single crystal is anisotropic, it is provided with polarization characteristics.     

Critical properties of XY model on two-layer Villain－ferromagnetic lattice

We investigate phase transitions of the XY model on a two-layer square lattice which consists of a Villain plane (J) and a ferromagnetic plane (I), using Monte Carlo simulations and a histogram method. Depending on the values of interaction parameters (I,J), the system presents three phases: namely, a Kosterlitz－Thouless (KT) phase in which the two planes are critical for I predominant over J, a chiral phase in which the two planes have a chiral order for J predominant over I and a new phase in which only the Villain plane has a chiral order and the ferromagnetic plane is paramagnetic with a small value of chirality. We clarify the nature of phase transitions by using a finite size scaling method. We find three different kinds of transitions according to the values of (I,J): the KT transition, the Ising transition and an XY-Ising transition with ν=0.849(3). It turns out that the Ising or XY-Ising transition is associated with the disappearance of the chiral order in the Villain plane.     

Magnetic properties of nano-sized 5 at.%Fe–Al systems

High energy ball milling is a promising materials processing technique that is widely used to produce nanocrystalline structures. However, when stainless steel or hardened steel containers and balls are used for milling, contamination from the milling medium can influence the material properties of the final nanostructured products due to intercalation of iron (Fe) as an impurity. This study reports the effect of iron contamination on nanocrystalline aluminum (Al) powder. ⁵⁷Fe Mössbauer spectroscopy and bulk magnetization studies using a vibrating sample magnetometer show that pure Al powder milled in hard steel media is strongly ferromagnetic at room temperature due to Fe contamination of about 5 at.% from the milling medium. TEM studies indicate that the system consists mainly of nano-sized Fe interspersed in Al with average crystallite sizes of ~2 and ~5 nm for Fe and Al, respectively. A comparative study of this system made with a mechanically alloyed Fe–Al system with the same percentage of Fe mixed with pure Al and mechanically alloyed using tungsten carbide vials and balls shows that the saturation magnetization, coercivity, Curie temperature, and low temperature behavior (field cooled–zero field cooled) are very different in the two cases. The different magnetic properties of the two systems can be attributed to the presence of magnetic and non-magnetic phases present.