Enhanced magnetization and magnetoelectric coupling in hydrogen treated hexagonal YMnO3

The single phase hexagonal YMnO₃ has been synthesized via sol-gel route by adopting two different sintering conditions. In one case, sintering has been done at ∼700 °C in Ar/H₂ atmosphere and in other case it has been done at ∼1250 °C in air. Magnetic measurements of the samples, synthesized by sintering at relatively lower temperature in Ar/H₂ atmosphere, show the enhanced ferromagnetic behaviour at 10 K. M-H curve shows that the value of saturation magnetization (M_s) at 10 K is 8.04 emu/g for Ar/H₂ sintered sample while it is 2.93 emu/g for the air sintered sample. Moreover, a weak ferromagnetic signal at room temperature has been observed in YMnO₃ compound. Magnetization versus magnetic field (M-H) curves of hydrogen treated samples, measured at room temperature, show small kink in the linear variation near origin, possibly due to presence of weak ferromagnetic interactions in the samples at room temperature. However, the polarization-electric field (P-E) curve shows weak ferroelectric characteristics for the Ar/H₂ sintered samples. It is suggested that the enhanced ferromagnetism in Ar/H₂ sintered sample originates from the presence of oxygen vacancies in the Ar/H₂ sintered samples. Moreover, the magnetoelectric coupling coefficient at room temperature is improved to 106 mV/cm Oe for Ar/H₂ sintered sample as compared to 96 mV/cm Oe for air sintered sample at 40 kHz ac magnetic field frequency.     

Investigation on the low-frequency mechanical properties of La0.6Sr0.4Co1−xFexO3−δ materials

The low-frequency mechanical properties of La_0.6Sr_0.4Co_1−xFe_xO_3−δ (0 ≤ x ≤ 0.8) materials have been measured using a computer-controlled pendulum. For undoped sample, five internal friction peaks (P0, P1, P2, P3 and P4) were observed. However, with the Fe doping, only two peaks (P3 and P4) were found at high temperature. The peaks of P0 and P1 have the feature of phase transition-induced internal friction, while the peaks of P2, P3 and P4 are the relaxation-type. From the analysis, it is suggested that the peak of P0 is due to the phase separation and the peak of P1 is related to the ferromagnetic (FM)-paramagnetic (PM) phase transition. For the peaks of P2, P3 and P4, they were associated with the motion of domain walls. The formation of this kind of domain structure is a consequence of a transformation from the paraelastic cubic phase to ferroelastic rhombohedral phase.     

Structural and thermoelastic properties of CaTiO3 perovskite between 7 K and 400 K

The structural and thermoelastic properties of CaTiO₃ perovskite have been studied using high-resolution powder neutron diffractometry at eighty temperatures in the range 7-400 K. The temperature variation of the unit cell volume, the thermodynamic Grüneisen parameter and the isobaric heat capacity are analysed using a two-term Debye model. Structural parameters are presented as the magnitudes of symmetry-adapted basis-vectors of seven normal modes with wavevectors that lie on the surface of the Brillouin zone of the primitive cubic aristotype phase, and a structural basis for the temperature-dependence of the bond lengths is proposed. A consistency between the vibrational Debye temperatures derived from the atomic displacement parameters and the mean values of the vibrational energies of the three atomic species calculated from the partial phonon densities of states has been found.     

Structural, magnetic and magnetocaloric properties in Pr0.5M0.1Sr0.4MnO3 (M = Eu, Gd and Dy) polycristalline manganites

Structural, magnetic and magnetocaloric properties of Pr_0.5M_0.1Sr_0.4MnO₃ (M = Eu, Gd and Dy) powder samples have been investigated by X-ray diffraction (XRD) and magnetic measurements. Our samples have been synthesized using the solid state reaction method at high temperature. Rietveld refinements of the X-ray diffraction patterns show that all our samples are single phase and crystallize in the distorted orthorhombic system with Pbnm space group. Magnetization measurements versus temperature in a magnetic applied field of 50 mT show that all our samples exhibit a paramagnetic-ferromagnetic transition with decreasing temperature. The Curie temperature T_C is found to be 270 K, 258 K and 248 K for M = Eu, Gd and Dy, respectively. Arrott plots show that all our samples exhibit a second order magnetic phase transition. From the measured magnetization data of Pr_0.5M_0.1Sr_0.4MnO₃ (M = Eu, Gd and Dy) samples as a function of magnetic applied field, the associated magnetic entropy change |ΔS_M| and the relative cooling power RCP have been determined. In the vicinity of T_C, |ΔS_M| reached, in a magnetic applied field of 1 T, maximum values of 1.37 J/kg K, 1.23 J/kg K and 1.18 J/kg K for M = Eu, Gd and Dy, respectively.     

The Mn-doping effect on the charge ordering state of La1/3Sr2/3FeO3

The longitudinal ultrasonic velocity (V_l), as well as resistivity has been measured in single-phase polycrystalline La_1/3Sr_2/3Fe_1−xMn_xO₃ (x = 0, 0.025, 0.05) at a frequency of 10 MHz, from 20 K to 300 K. It is found that with increasing Mn-doping level, the resistivity increases and the charge ordering transition temperature T_CO shifts to lower temperature. For all samples, upon cooling down from 300 K, a substantial softening in V_l above T_CO and dramatic stiffening below T_CO are observed. This abnormal elastic softening above T_CO can be described well by the mean-field theory, which indicates that this feature is due to the electron-phonon coupling via the Jahn-Teller effect and this coupling is enhanced with the Mn doping. Below T_CO, another softening in V_l is observed for x = 0, and weakens with the increasing of Mn content. This character is attributed to the breathing-type distortion of Fe-O octahedron and suggests that the charge disproportionation (CD) transition is suppressed by the Mn substitution.     

Electrical characterization of a-Si:H(n)/c-Si(p) structure

In this study, n-type hydrogenated amorphous silicon (a-Si:H) was fabricated on p-type crystalline silicon (c-Si) substrates to obtain heterojunction diodes. The amorphous films were obtained by the Plasma Enhanced Chemical Vapor Deposition (PECVD) technique. Temperature dependent current-voltage (I-V-T) measurements and investigation of the dc current injection mechanism of a-Si:H(n)/c-Si(p) device structure have been performed. The series resistance (4.6-8.2 Ω) values displayed nearly temperature independent behavior and the ideality factor varied between 2.7 and 1.6 in the temperature range 100-320 K. The forward bias I-V-T characteristics of c-Si/a-Si:H heterojunctions are found to behave like the Schottky junctions where carrier injection is especially influenced by the carrier generation-recombination in the junction interface formed on the amorphous side. The temperature dependent ideality factor behavior shows that tunneling enhanced recombination is valid rather than thermionic emission theory. In the frame of this model, characteristic tunneling energy and characteristic temperature are found to be 9 meV and 1900 K, respectively. It is concluded that fabricate n-type hydrogenated amorphous silicon is a preferable semiconductor material layer with low interface state density because the temperature dependent interface state density calculations give values of the order of 10¹⁴ eV⁻¹ cm⁻².     

Dependence of magnetic properties on the film thickness and Pt atomic fraction of post-annealed Fe1−xPtx-C granular films on MgO(1 0 0) and SiO2/Si(1 0 0) substrates

The Fe_1−xPt_x-C granular films with different Pt atomic fractions (0.09 ≤ x ≤ 0.52) and film thicknesses (5 nm ≤ t ≤ 100 nm) were deposited on MgO(1 0 0) and SiO₂/Si(1 0 0) substrates by facing-target sputtering and post-annealing. With the increasing x, the ordered L1₀ FePt grains form. All of the films are ferromagnetic, and the easy axis is in the film plane. With the decrease of t, the films turn from hard ferromagnetic to soft ferromagnetic. The maximum coercivity of the 100-nm thick Fe_1−xPt_x-C granular films measured at a 10-kOe field is 3.7 kOe at x = 0.48. The coercivity of the Fe_0.56Pt_0.44-C granular films increases, and the magnetization measured at a 10-kOe field decreases with the increasing t. The reversal mechanism of the 100-nm thick Fe_1−xPt_x-C granular films turns from the domain wall motion to the Stoner-Wohlfarth rotation mode as x increases. However, the reversal mechanism of the Fe₅₆Pt₄₄-C granular films with different t approaches the Stoner-Wohlfarth rotation mode, and is film-thickness independent.     

Mechanical and phonon properties of the superhard LuB2, LuB4, and LuB12 compounds

We have studied structural, elastic, and lattice dynamical properties of the LuB_2, LuB_4, and LuB₁₂ compounds by using the plane-wave pseudopotential approach to the density-functional theory within the generalized gradient approximation. We have considered three different crystal structures of LuB_x: LuB₂ (P6/mmm), LuB₄ (P4/mbm), and LuB₁₂ (Fm-3m). The most stable structure is found to be tetragonal (P4/mbm) structure. The comparative results on the basic physical parameters such as lattice constants, bulk modulus, bond distances, elastic constants, shear modulus, Young's modulus, and Poison's ratio are reported. Also, we have predicted that LuB₄ and LuB₁₂ compounds are potential superhard materials. Furthermore, the phonon dispersion curves and corresponding phonon density of states (DOS) are computed for considered phases. Our structural and some other results are in agreement with the available experimental and other theoretical data.     

Effects of plasma treatment on bioactivity of TiO2 coatings

In this work, nano-TiO₂ powders were deposited on titanium alloy substrates by atmospheric plasma spraying, followed by plasma immersion ion implantation (PIII) using hydrogen, oxygen and ammonia gases. The bioactivities of PIII-treated TiO₂ coatings were evaluated by the formation of apatite on their surface after soaked in simulated body fluids (SBF) for a period of time. As-sprayed TiO₂ coating is composed of rutile, anatase and TiO_2−x (most of them is Ti₃O₅). After immersion in SBF for two weeks, the hydrogen PIII-treated TiO₂ coating can induce bone-like apatite formation on its surface but apatite cannot be formed on the surface of as-sprayed and oxygen, ammonia PIII-treated TiO₂ coatings. The results obtained indicated that a hydrogenated surface plays a very important role to induce bioactivity of TiO₂ coatings.     

Fermi surface topology of deuterium-doped vanadium: Compton scattering study

The deuteration-induced effect on the Fermi surface (FS) topology of V was investigated by the synchrotron-based Compton scattering technique with 115 keV X-rays. The three-dimensional occupation number density (OND) of α-VD_0.64 single crystal and V single crystal was reconstructed by the directional Compton profiles along 13-18 directions. The observed OND shows that the FS topology of α-VD_0.64.differs from that of V at Γ, N, and H points in the crystal momentum space, which are hole position for V. This indicates that electrons that originate from deuterium modify the upper part of the host metal d bands (3rd and 4th bands) at the Fermi level E_F, and forms the metal-hydrogen bonding states with the lower part of the host metal d band (1st band).     

Simulation of residual stress induced by a laser peening process through inverse optimization of material models

Laser peening (LP) is a surface enhancement technique that induces compressive residual stresses in the surface regions of metallic components to increase fatigue life. Simulation of the LP process is a complex task due to the intensity of the pressure loading (order of GPa) in a very short time period (in nanoseconds). A finite element technique is used to predict the residual stresses induced by the LP process. During the LP process, strain rates could reach as high as 10⁶ s⁻¹, which is very high compared to conventional strain rates. A reliable material model is needed to determine the dynamic response of a material. In this work, an optimization-based approach is developed to obtain the material model constants when there is very little or no experimental data of material behavior available. The approach is presented by comparing the residual stress prediction from simulation with available experimental results for Ti–6Al–4V material. To demonstrate the consistency of the approach, LP experiments have been performed at LSP Technologies on Inconel^®718 with different laser power densities, and the residual stress results are compared with the simulation. The Johnson–Cook, the Zerilli–Armstrong, and the Khan–Huang–Liang material models are used during the simulation procedure. The performance of each model is assessed by comparing the residual stress results between simulation and experiments.     

Luminescent properties of UV excitable blue emitting phosphors MSr4(BO3)3:Ce (M = Li and Na)

A series of Ce³⁺ doped novel borate phosphors MSr₄(BO₃)₃ (M = Li or Na) were successfully synthesized by traditional solid-state reaction. The crystal structures and the phase purities of samples were characterized by powder X-ray diffraction. The optimal concentrations of dopant Ce³⁺ ions in compound MSr₄(BO₃)₃ (M = Li or Na) were determined through the measurements of photoluminescence spectra of phosphors. Ce³⁺ doped phosphors MSr₄(BO₃)₃ (M = Li or Na) show strong broad band absorption in UV spectral region and bright blue emission under the excitation of 345 nm light. In addition, the temperature dependences of emission spectra of M_1+xSr_4−2xCe_x(BO₃)₃ (M = Li or Na) phosphors with optimal composition x = 0.05 for Li and x = 0.09 for Na excited under 355 nm pulse laser were also investigated. The experimental results indicate that the M_1+xSr_4−2xCe_x(BO₃)₃ (M = Li or Na) phosphors are promising blue emitting phosphors pumped by UV light.     

Temperature- and excitation-dependent photoluminescence in TlGaSeS layered crystals

Photoluminescence (PL) spectra of TlGaSeS layered single crystals have been studied in the wavelength region of 695-1010 nm and in the temperature range of 20-56 K. Two PL bands centered at 773 (1.605 eV, A-band) and 989 nm (1.254 eV, B-band) were observed at T = 20 K. Variations of both bands have been investigated as a function of excitation laser intensity in the range from 4.2 to 111.4 mW cm⁻². These bands are attributed to recombination of charge carriers through donor-acceptor pairs located in the band gap of the crystal. Radiative transitions from deep donor levels located at 0.721 and 1.069 eV below the bottom of conduction band to shallow acceptor levels located at 0.008 and 0.011 eV above the top of the valence band are suggested to be responsible for the observed A- and B-bands in the PL spectra, respectively.     

Calculations of thermophysical properties of cubic carbides and nitrides using the Debye–Grüneisen model

The thermal expansivities and heat capacities of MX (M = Ti, Zr, Hf, V, Nb, Ta; X = C, N) carbides and nitrides with NaCl structure were calculated using the Debye–Grüneisen model combined with ab initio calculations. Two different approximations for the Grüneisen parameter γ were used in the Debye–Grüneisen model, i.e. the expressions proposed by Slater and by Dugdale and MacDonald. The thermal electronic contribution was evaluated from ab initio calculations of the electronic density of states. The calculated results were compared with CALPHAD assessments and experimental data. It was found that the calculations using the Dugdale–MacDonald γ can account for most of the experimental data. By fitting experimental heat capacity and thermal expansivity data below the Debye temperatures, an estimation of Poisson’s ratio was obtained and Young’s and shear moduli were evaluated. In order to reach a reasonable agreement with experimental data, it was necessary to use the logarithmic averaged mass of the constituent atoms. The agreements between the calculated and the experimental values for the bulk and Young’s moduli are generally better than the agreement for shear modulus.     

Structural,elastic, electronic,and thermodynamic properties of intermetallic Zr2Cu: A first-principles study

Three competing structures (C11_b, C16 and E9₃) of intermetallic Zr₂Cu have been systematically investigated by first-principles calculations and quasi-harmonic Debye model. Both the calculated equation of states (EOS) and pressure–enthalpy results indicate a structural phase transition from C11_b to C16 phase at around 11–14 GPa. The calculated equilibrium crystal parameters and elastic constants are in consistence with available experimental or theoretical data. All three phases are mechanically stable according to the elastic stability criteria, and ductile according to Pugh's ratio, while the ambient-stable C11_b phase shows a higher elastic anisotropy. Furthermore, differences in the nature of bonding between three competing structures are uncovered by electron density topological analysis. C11_b Zr₂Cu possesses an intriguing pseudo BaFe₂As₂-type structure with the charge density maxima at Zr tetrahedral interstices serving as Fe-position pseudoatoms; C16 Zr₂Cu contains Zr-pair configurations bonded through bifurcated Zr–Zr bonding paths; while the E9₃ phase has only conventional straight bonding. Additionally, through quasi-harmonic Debye model, the pressure and temperature dependences of the bulk modulus, specific heat, Debye temperature, Grüneisen parameter and thermal expansion coefficient for three phases are obtained and discussed.     

Fabrication of GdBa2Cu3O7−x films by TFA-MOD process

GdBa₂Cu₃O_7−x (GdBCO) films have been deposited on LaAlO₃ (LAO) (0 0 l) single crystal substrates by trifluoroacetate metal organic deposition (TFA-MOD) method. The effects of oxygen partial pressure and firing temperature on microstructure and critical properties of GdBCO films were discussed. The phase formation, texture and microstructure of films were characterized by X-ray diffraction and scanning electron microscopy. The oxygen partial pressure was considered to play a great role for formation of impurity phase and a-axis oriented grains. The degree of c-axis orientation was also influenced by the firing temperature. The highly c-axis oriented GdBCO film obtained at 815 °C under an oxygen partial pressure of 100 ppm has a high performance critical current density J_c (77 K, self field) = 1.8 MA/cm².     

Magnetic annealing effects on multiferroic BiFeO3/CoFe2O4 bilayered films

In situ magnetic annealing effects on c-axis-preferred multiferroic BiFeO₃/CoFe₂O₄ bilayered by chemical solution deposition route are investigated. It is observed that magnetic annealing can enhance the crystallization quality, texture and densification as well as dielectric properties. In addition, the magnetolosses decrease with increasing the magnetic fields. Moreover, both increase of the polarization and decrease of the leakage current due to magnetic annealing are beneficial for potential applications of BiFeO₃ films.     

Spectroscopic investigation of Dy-doped Li2Gd4(MoO4)7 crystal for potential application in solid-state yellow laser

Dy³⁺:Li₂Gd₄(MoO₄)₇ crystal with dimensions of ∅20 × 25 mm³ has been grown by the Czochralski method. The spectroscopic properties of Dy³⁺:Li₂Gd₄(MoO₄)₇ crystal have been investigated. Based on the analysis of polarized absorption spectra in the framework of the Judd-Ofelt theory, the main spectroscopic characteristics, including the phenomenological intensity parameters, spontaneous transition probabilities, fluorescence branching ratios and radiative lifetimes of Dy³⁺ in the crystal, have been determined. The emission cross-sections for the ⁴F_9/2 → ⁶H_13/2 transition of special interest for laser application have been calculated using the Füchtbauer-Ladenburg (F-L) equation. The fluorescence and radiative lifetimes of ⁴F_9/2 manifold are equal to 126.5 μs and 201.1 μs, respectively, which mean the quantum efficiency is 62.9%. The results propose the possibility of Dy³⁺:Li₂Gd₄(MoO₄)₇ crystal for solid-state yellow laser pumped by commercially available blue laser diodes.     

The structural and electric properties of Li- and K-substituted Bi0.5Na0.5TiO3 ferroelectric ceramics

The structure, dielectric properties and phase transition of lithium and potassium modified Bi_0.5Na_0.5TiO₃ ceramics were investigated widely. The phase transition behavior with respect to changes in composition and temperature was investigated using X-ray diffraction analysis, dielectric and ferroelectric characterizations. The experimental results show that there is a diffusion phase transition in (Na_1−xK_x)_0.5Bi_0.5TiO₃ ceramics at T_m and the diffuseness of the phase transition is more obvious for the samples near the morphotropic phase boundary. In (Na_1−xLi_x)_0.5Bi_0.5TiO₃ system, due to the space charge polarization induced by ions conductivity, the low frequency permittivity increases so remarkably at high temperature that the peak of maximum permittivity vanishes. The hysteresis loops at different temperatures indicate that there is no existence of anti-ferroelectrics in lithium and potassium modified Bi_0.5Na_0.5TiO₃ ceramics above the depolarization temperature T_d. The depolarization reason is that the tetragonal nonpolar phase occurs and leads to the macro-micro domain transformation at about T_d.