Zr1?x Yb x WMoO8?x/2 (x?=?0, 0.04) ceramics fabricated by in situ synthesis from trigonal polymorph: preparation, sintering process, and negative thermal expansion properties

In this study, negative thermal expansion (NTE) Zr_1−x Yb _x WMoO_8−x/2 (x = 0, 0.04) ceramics were fabricated by in situ synthesis from trigonal polymorphous precursors for the first time. Phase transition was studied by means of powder X-ray diffraction. Study on the sintering process of Zr_1−x Yb _x WMoO_8−x/2 (x = 0, 0.04) ceramics was performed by calcining a series of precursor pellets at 950, 980, and 1000 °C for different times, varying from 1 min to 1 h. The results indicate that the sintering process can be mainly divided into three stages: phase transition from trigonal precursors to cubic Zr_1−x Yb _x WMoO_8−x/2 (c-Zr_1−x Yb _x WMoO_8−x/2, 0–5 min), densification of c-Zr_1−x Yb _x WMoO_8−x/2 (6–30 min), and the final sintering stage with little densification (>30 min). Densification reaches almost the maximum in the duration of ~30 min at assigned temperature. Temperature has little influence on densification of ZrWMoO₈, but improves that of Zr_0.96Yb_0.04WMoO_7.98 evidently. In addition, densification of ZrWMoO₈ can be promoted markedly by introduction of Yb³⁺.     

XRD, Magnetic and M?ssbauer Spectral Studies of?Ag x Ni1?x Fe2O4 Ferrite Nanoparticles

Nanophase Ag _x Ni_1?x Fe₂O₄ (x=0,0.2) ferrites were prepared by glycothermal method. The NiFe₂O₄ (x=0) nanosized sample was also produced by high-energy ball milling for comparison of properties. Structural investigations of the samples were carried out by X-ray diffraction. The experiment reveals that pure Ag-Ni ferrite materials with grain sizes of about 8 nm can be obtained after annealing at relatively low temperature of about 500?°C. The nanosized compounds produced by glycothermal reaction indicate superparamagnetic behavior. A higher value of coercive field (910 Oe) is observed in x=0 milled sample with similar particle size. The zero field cooled (ZFC) and field cooled (FC) magnetization measurements reveals spin glass like behavior of the nanosized compounds.     

Magnetic clusters in Cu1 ? x In1 ? x Fe2x Se2 solid solutions

We have synthesized Cu_{1 ? x} In_{1 ? x} Fe_2x Se₂ solid solutions (CuInSe₂-FeSe join). The ??-FeSe end-member of the solid-solution system is an antiferromagnet and a superconductor with a superconducting transition temperature of 8 K. The magnetic properties of the solid solutions were studied at temperatures from 5 to 300 K in magnetic fields H = 3.98 and 3184 kA/m. By fitting experimental ??(H) magnetization data with Langevin and Curie equations, we determined the magnetic moment, size, and concentration of the clusters and the concentration of noninteracting Fe²⁺ ions. The magnetic moment of the Fe²⁺ ions in the clusters is ??0.8??_B at 2x = 0.06 and 0.08, with a tendency to decrease with increasing iron content. This points to antiferromagnetic exchange interaction, which becomes stronger with increasing cluster size. The number of clusters in the materials increases with iron content up to a critical level 2x _c < 2x = 0.06. In the range 2x = 0.06 to 0.10, the cluster size increases until a limiting composition determined by the boundary of the solid-solution range is reached.     

Local Atomic and Electronic Structure with Magnetism of La0.7Ca0.3Mn1?x Cu x O3 (x=0, 0.03, 0.06, 0.1)

Local atomic and electronic structure with magnetic properties, especially Griffiths phase, of polycrystalline samples La_0.7Ca_0.3Mn_1?x Cu _x O₃ (x=0, 0.03, 0.06, 0.1) have been studied. The X-ray absorption spectra (XAS) of Cu 2p core level prove that the valence state of Cu ions exhibits trivalent state when doping content x≤0.06 and divalent Cu²⁺ ions begin to show for x=0.1. For the valence states of Mn ions, the X-ray photoelectron spectroscopy data show that they are in mixed states of Mn³⁺ and?Mn⁴⁺, and a shift to lower binding energy is observed, which is not attributed to the variation of valence states of Mn ions but the change of crystallographic surroundings, because there is no obvious change detected by X-ray absorption fine structure spectroscopy (XAFS). The Debye-Waller factor (σ ²) of x=0.1 sample is only slightly larger compared to x=0, which may be the origin of enhancement of Griffiths phase observed in the inverse-susceptibility as a function of temperature (H/M～T). The H/M～T curves of Cu doped samples indicate coexistence of FM, AFM and PM phase above Curie temperature?T _C , which may be related to the strong hybridization of O 2p and Mn 3d reflected by O 1s XAS spectra.     

Magnetic Properties of Ru-rich Ru2?x Fe x CrZ (Z=Si, Ge)

The properties of Heusler compounds Ru_2?x Fe _x CrGe are investigated and compared with those of Ru_1.9Fe_0.1CrSi. Ru₂CrGe is confirmed to exhibit an antiferromagnetic transition with Ne??l temperature 13?K by magnetic susceptibility and specific heat measurements. When Fe is substituted for Ru, a peak in the magnetic susceptibility is observed, and in the lower temperature region irreversibility between temperature dependences of magnetization for zero-field-cooling and field-cooling conditions is found. Nevertheless, in specific heat of Ru_1.9Fe_0.1CrGe, there is no anomaly to indicate phase transition. The specific heat is almost identical to that for Ru_1.9Fe_0.1CrSi. The above results demonstrate that the magnetic states in the low temperature region of Fe-substituted samples are spin-glass-like states. Slight substitution of Fe for Ru destroys the long-range-order and lead to peculiar spin-glass-like states.     

Appearance and Disappearance of Superconductivity with?Fe?Site?Co?Substitution in SmFe1?x Co x AsO (x=0.0 to 1.0)

We report synthesis, structural details, and magnetization of SmFe_1?x Co _x AsO with?x ranging from 0.0 to 1.0 at a close interval of 0.10. It is found that Co substitutes fully at the Fe site in SmFeAsO in an iso-structural lattice with a slightly compressed cell. The parent compound exhibited known spin density wave (SDW) character below 150?K. Successive doping of Co at the Fe site suppressed the SDW transition for x=0.05, and later induced superconductivity for x=0.10, 0.15, and 0.20, respectively, at 14, 15.5, and 9?K. The appearance of bulk superconductivity is established by wide open isothermal magnetization M(H) loops. For higher content of Co, i.e. x??0.30, superconductivity is not observed. Clearly, the Co substitution at the Fe site in SmFe_1?x Co _x AsO diminishes the Fe SDW character (x=0.05), introduces bulk superconductivity for?x from 0.10 to 0.20, and finally becomes a non-superconductor. The SmCoAsO also exhibits a secondary AFM-like transition below or around 50?K. The reported AFM ordering of Sm spins is seen from heat capacity C _p(T) at 4.5?K and the same remains invariant with Co doping in SmFe_1?x Co _x AsO. Further, the FM ordering of Co-spins (seen in magnetization measurements) is not evident in C _p(T) studies, suggesting weak correlations between ordered Co-spins in these systems.     

Photoinduced Superconductivity in La2?x Sr x CuO4

Dependences of the superconducting transition temperature (T _c) and chemical potential shift (????) on the hole concentration (n _h ) for La_2?x Sr _x CuO₄ cuprate is obtained taking into account of canonical two-band BCS model containing Fermi surfaces of p and d holes. The shift of chemical potential (??) leads to the curve T _c(n _h ) with a maximum. The dependences of T _c(n _h ) for our system compared with available experimental results. Downward shift (????) of the electron chemical potential (??) with the hole concentration (n _h ) have been found. Self-consistent equations for superconducting order parameters ( $\bar{\Delta}_{p}$ and $\bar{\Delta}_{d}$ ) for both p and d holes are derived using Green??s function and equation of motion method. The temperature dependences of superconducting gaps and specific heat based on this model are also calculated. The enhancement of T _c due to doping is observed.     

Syntheses and Magnetic-Properties of Zn-Diluted Sr-Based Perovskite Cobalt Oxides, Sr1?x Zn x CoO3; 0.05≤x≤0.3

In order to check the solubility of Zn in the (Sr_1?x Zn _x )CoO₃ perovskite structure and their research findings, several polycrystalline samples have been prepared under wide extreme synthesis conditions at 6?GPa/1300?C1650?°C. While 0.05??x??0.3 compositions revealed single phased cubic structure materials, x>0.3 showed multi-phased materials for (Sr_1?x Zn _x )CoO₃ system. Like other substituted perovskite cobalt oxide systems (Ca, Y, Ho and Ce), the transport properties of the present materials show rather sizable changes with respect to ??x??, although there are insignificant variations in lattice parameter and in Curie temperature, T _c . All the present samples show soft ferromagnetism with T _c in the range of 272?C285?K for 0.05??x??0.3. The effective paramagnetic moment, P _eff determined from the paramagnetic region decreases upon the substitution of Zn for Sr-site. These P _eff (3.3?C2.8???_B/Co) values for 0.05??x??0.3 compositions seem to suggest that the Co⁴⁺ lie in intermediate spin (IS) state for the present (Sr_1?x Zn _x )CoO₃ series, although they are slightly smaller than those expected for IS-Co⁴⁺; P _eff=3.87???_B/Co. The electrical resistivity is found to increase with increase of ??x?? for the investigated samples. The temperature and field dependence of both positive and negative magnetoresistance (MR) are noted for the Zn substituted samples. About 5% of ?CMR is observed for x=0.05 sample around the transition temperature (280?K) under the field strength difference, ??H=90?kOe. The present research findings are compared with our previous results on different perovskite cobalt oxides.     

Synthesis, Crystal Growth, and Acoustophotovoltaic Effect of TlIn1 – x Pr x Se2 and TlIn1 – x Pr x Te2

Procedures were developed for growing perfect TlIn_{1 – x} Pr _x Se₂ and TlIn_{1 – x} Pr _x Te₂ (0 x 0.08) single crystals. The growth charges were synthesized by a process involving partial cooling of a tilted, rotating tube. The crystals grown by the floating zone technique were found to readily cleave in two, mutually perpendicular mirror planes to give rectangular parallelepipeds. The crystals were shown to exhibit a sizeable acoustophotovoltaic effect: a combined action of radiation and sound waves produced an emf between the electrodes or a short-circuit current.     

Structural, transport, magnetic, and dielectric properties of La1−x Te x MnO3 (x = 0.10 and 0.15)

In this study, we have investigated the structure, temperature-dependent resistivity, magnetization, and dielectric properties of La_1?x Te _x MnO_3±δ (x = 0.10 and 0.15). X-ray diffraction analysis confirms the rhombohedral crystal symmetry with space group R $ \overline{3} $ c. For both the samples, the temperature dependence of magnetization plots show paramagnetic-to-ferromagnetic phase transition. The Curie temperature (T _c) and magnitude of magnetization increase with the Te concentration. Field-dependent magnetization produces the asymmetric hysteresis loop that has been attributed to the magneto crystalline anisotropy induced by lattice distortion and the rare earth spin coupling at room temperature. Temperature-dependent resistivity plots exhibit metal–insulator transition (MIT) and charge-ordering state. These plots have been fitted using variable range hopping model, and the density of states [N(E_F)] has been estimated. Magnetoresistance is measured as a function of temperature in the field of 1T, 5T, and 8T. The dielectric constant shows an anomaly near MIT. The dielectric constant exhibits a peaking behavior with the applied frequency and the temperature dependence of dielectric constant attains colossal values at high temperatures.     

New Perspectives for Cuprate Research: (Ca x La1?x )(Ba1.75?x La0.25+x )Cu3O y Single Crystals

We report the successful growth of a large (Ca _x La_1?x )(Ba_1.75?x La_0.25+x )Cu₃O _y (CLBLCO) single crystal. In this material, x controls the maximum of T _c ( $T_{c}^{\max }$ ), with minimal structural changes. Therefore, it allows a search for correlations between material properties and $T_{c}^{\max }$ . We demonstrate that the crystals are good enough for neutron scattering and cleave well enough for Raman scattering. These results open new possibilities for cuprate research.     

Magnetic Moments in SmCo5 and SmCo5?x Cu x Films

SmCo₅ is an emerging perpendicular magnetic material for super-high density magnetic recording, due to its large magnetic anisotropy energy. In this paper, the magnetic moments of SmCo_5?x Cu _x have been studied using first principles calculation based on density-functional theory (DFT). Calculations are performed using the pseudopotential plane wave DFT code Vienna ab initio simulation package (VASP) with the projector augmented wave (PAW) method. The local density approximation LDA+U method is used for the calculation of the exchange correlation energy of Sm. The calculation results show that the average Co magnetic moment of SmCo_5?x Cu _x decreases with the increase of Cu doping concentrations, and the influence of the Cu doping on the spin state of Co is greater than that of Sm. The magnetic anisotropy energy of SmCo₅ is analyzed. The electronic density of states and the differential in spin densities of atoms show that the spatial distribution of 4f electron and the 4f?C3d coupling are the controlling factors of the magnetic anisotropy energy of SmCo₅.     

Synthesis, Characterization and Magnetoresistance Studies of Tl0.5Pb0.5Sr1.6Ba0.4CaCu2?x Ru x O7?δ Superconductor

Bulk superconducting samples of type Tl_0.5Pb_0.5Sr_1.6Ba_0.4CaCu_2−x Ru _x O_7−δ , (Tl,Pb)/Sr-1212, with 0.0≤x≤0.525 were synthesized by the conventional one-step solid-state reaction technique via a short time, using SrO₂ instead of SrCO₃. X-ray powder diffraction XRD, scanning electron microscope SEM and the electrical resistivity measurements at different applied dc magnetic fields (0.0–4.4 kG) were performed in order to investigate the effect of Ru⁴⁺ ions substitution on (Tl,Pb)/Sr-1212 phase. Enhancement of the phase formation, the superconducting transition temperature T _c and the hole carriers concentration P was observed up to x=0.075, and they decreased for further increase in x. The magnetoresistance data of these samples were analyzed within the thermally activated flux creep TAFC model and the Ambegaokar and Halperin AH theory. The derived flux pinning energy U, the critical current density J _c (0) and the upper critical magnetic field B _c2(0) enhanced up to x=0.075, beyond which they decreased for further increase in Ru-content. The coherence length at 0 K ζ(0) decreased up to x=0.075 and increased with x>0.075. In addition, we reported a comparison between the effect of Ru substitution on the physical properties for Gd-123, (Bi,Pb)-2223 and (Tl,Pb)/Sr-1212 phases.     

Conduction in nanostructured La1?x Sr x FeO3 (0 ≤ x ≤ 1)

We investigated electrical properties of nanostructured La_1?x Sr _x FeO₃ (0 ?? x ?? 1) from 300 K?C400 K. The nanostructured La_1?x Sr _x FeO₃ (0 ?? x ?? 1) was synthesized by citrate gel method requiring no pH control. X-ray diffraction pattern showed that single phase LaFeO₃ with an orthorhombic structure was formed. The structure changed into rhombohedral for x = 0.5 and it became cubic for x = 1.0. For x ?? 0.5, our material showed non-linear current-voltage characteristics and for x > 0.5 it showed linear current-voltage characteristics. Poole Frenkel type conduction mechanism was found to be operative in LaFeO₃ from 300 K?C400 K. The experimental values of field-lowering coefficient were by 2.56?C6.41 times higher than the predicted value and were attributed to the presence of localized fields. The increase in conductance with Sr content was due to formation of Fe⁴⁺ ions in addition to Fe³⁺ with the increase in Sr content. Impedance spectroscopy and ac conductivity analysis of La_1?x Sr _x FeO₃ (0 ?? x ?? 1) was also carried out in the temperature range from 300 K?C400 K and frequency was varied from 20 Hz - 2 MHz. The ac conduction followed the correlated barrier hopping model in La_0.9Sr_0.1FeO₃.     

The Nodal SDW Gap and the Superconducting Gap in?BaFe2?x Co x As2

The spin density wave (SDW) transition in BaFe₂As₂ and the superconducting transition in BaFe_1.84Co_0.16As₂ were investigated by Raman scattering. The symmetries of the nodal SDW gap at 400?cm^?1 and the superconducting gap at 75?cm^?1 are both?B _2g. The superconducting coherent peak energy is smaller than the gap energy of the hole pocket, indicating that the peak is the resonant peak in the S _?? superconductor. The superconducting symmetry is given by B _2g in the orbital combination and A _1g (S _??) in the momentum space. The exchange interaction energies are estimated from the two-magnon peak.     

Dielectric and optical properties of TlGa1 − x Er x S2 (x = 0, 0.001, 0.005, 0.01) single crystals

TlGa_{1 ? x} Er _x S₂ (x = 0, 0.001, 0.005, 0.01) solid solutions, based on the layered compound TlGaS₂, have been prepared by direct elemental synthesis. The effect of Er concentration on the dielectric and optical properties of the TlGa_{1 ? x} Er _x S₂ solid solutions has been studied. The results demonstrate that increasing the Er content of the TlGa_{1 ? x} Er _x S₂ solid solutions decreases the real part of their complex dielectric permittivity and increases their dielectric loss tangent. The conductivity (σ) of the TlGa_{1 ? x} Er _x S₂ solid solutions in the frequency range f = 1 to 35 MHz exhibits σ ～ f ^0.8 behavior, indicative of hopping charge transport through their band gap. We have evaluated the key parameters of this charge transport mechanism. We have studied temperature-dependent optical properties of the TlGa_{1 ? x} Er _x S₂ solid solutions. At temperatures in the range T = 77–200 K, the TlGa_0.999Er_0.001S₂ solid solution has an absorption band near its fundamental absorption edge, which is due to transitions to a direct exciton state.     

Magnetic Transport Properties in GdBa2Cu3?x Ru x O7?δ Superconducting Phase

Bulk superconducting samples of type GdBa₂Cu_3?x Ru _x O_7?δ phase, Gd-123, with?x ranging from 0.0 to 0.15 were prepared by the conventional solid-state reaction technique. X-ray powder diffraction (XRD) and the electrical resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution on Gd-123 phase. Enhancement of the phase formation and the superconducting transition temperature T _c for GdBa₂Cu_3?x Ru _x O_7?δ phase up to x=0.05 was observed. The effect of magnetic field up to 4.4?kG on the electrical resistivity behavior of the prepared samples was studied to investigate the flux motion of this phase. The derived flux pinning energy?U, based on the thermally activated flux creep TAFC model, decreased with increasing the magnetic field?B. The flux pinning energy followed the exponent behavior as U(B)～B ^?β . The superconducting transition width ΔT increased as the magnetic field increased, showing the scaling relation as ΔT～B ⁿ . Using Ambegaokar and Halperin AH theory, the magnetic field and temperature dependence of U was found to be U(B,T)～ΔTB ^?η , η=β+n. The critical current density J _c (0) enhanced up to x=0.05, beyond which it decreased with further increase in Ru-content.     

Investigating the Structure of Ce1?x La x CoIn5 Using NQR

We report NQR data on the heavy-fermion compound Ce _x La_1?x CoIn₅. The NQR spectrum of the In(1) site is composed of five different peaks, which are described by a binomial distribution of the nearest-neighbor La atoms. Our results suggest that an analysis of the NQR spectra yields a more accurate doping level than energy dispersive X-ray spectroscopy (EDS) results.