Fluorination of perovskite-related phases of composition La1−xSrxFe1−yCoyO3−δ

We report here on the fluorination of the perovskite-related phases La_1−xSr_xFe_1−yCo_yO_3−δ. The introduction of fluorine in place of oxygen is achieved through a low-temperature (400 °C) reaction with poly(vinylidene fluoride). X-ray powder diffraction data show that in all cases the fluorination leads to an expansion in the unit cell, which is consistent with partial replacement of oxygen by fluorine and consequent reduction in the oxidation state of iron and/or cobalt. This reduction in oxidation state is confirmed by X-ray absorption- and Mössbauer-spectroscopy. The Mössbauer spectra show complex magnetically split hyperfine patterns for the fluorinated samples, reflecting the interactions between Fe³⁺ ions, which are not possible in oxides containing Fe⁴⁺.     

Low-temperature synthesis of nanocrystalline Ca1−xHoxMnO3−δ (0?x?0.3) powders

Nanocrystalline Ca_1−xHo_xMnO_3−δ (0?x?0.3) manganites were synthesized as phase-pure by a simple and instantaneous solution autogel combustion method, which is a low temperature initiated synthetic route to obtain fine grain size. All the samples, heated at 800 °C for 18 h, can be produced as phase-pure; the polycrystalline powders are homogeneous and possess ultrafine particle size. The holmium-doped calcium manganites retain the orthorhombic phase of the undoped sample. The scanning electron microscope (SEM) images revealed that the combustion-derived compounds exhibit particle size that decreases with holmium content from 300 to 80 nm. All manganites show two active IR vibrational modes near 400 and 600 cm⁻¹. The high temperature dependence of resistivity was measured using a standard four-probe method in the range 25-600 °C. All the samples exhibit semiconductor behaviour and holmium induces a marked decrease in the electrical resistivity when compared with the parent CaMnO₃. The results can be well attributed to the Mn⁴⁺/Mn³⁺ ratio and to the particle grain size.     

High-temperature electronic transport properties of La1−xCaxMnO3+δ (0.0?x?1.0)

La_1−xCa_xMnO_3+δ (0.0?x?1.0) samples were prepared and their resistivity and Seebeck coefficients were measured in the high-temperature range. Ca doping changes the ratio of Mn³⁺/Mn⁴⁺ and influences the electronic transport behavior markedly. With the increase of Ca concentration, the samples change from a p-type semiconductor to an n-type one and Seebeck coefficient becomes increasingly negative. Low doping (x=0.2) and high doping (x=0.8) induces the drop of the resistivity compared with undoped LaMnO_3+δ and CaMnO_3+δ samples due to the rise of carrier concentration. However, the resistivity of moderate-doped samples (x=0.4, 0.6) is larger than low- and high-doped samples because dopant scattering decreases carrier mobility.     

Anomalous Hall effect in sputter-deposited CoxTi1−xO2–δ films

Hall effect measurements were performed on epitaxial Co_xTi_1−xO_2–δ thin films grown on (0 0 1) LaAlO₃ by reactive RF magnetron co-sputter deposition. Magnetization measurements reveal ferromagnetic behavior in M–H loop at room temperature for Co_xTi_1−xO_2–δ thin films for which x?0.02. An anomalous Hall effect was observed for Co_0.10Ti_0.90O_2−δ films grown with the partial pressure of water P(H₂O)=4×10⁻⁴ Torr or less. These films exhibit a positive ordinary Hall coefficient and a positive magnetoresistance. X-ray diffraction on films grown under these conditions shows evidence for Ti_nO_2n−1 phase due to the deficiency of oxygen. In contrast, Hall measurements taken for undoped and Co-doped TiO₂ thin films grown under more oxidizing conditions show only the ordinary Hall effect with a negative Hall coefficient consistent with n-type conduction. For these films, the magnetoresistance was positive and increased monotonically with increasing magnetic field. The results suggest that Co-doped Ti_nO_2n−1 may be a dilute magnetic semiconducting oxide for which the carriers couple to the spin polarization.     

Non-adiabatic polaron hopping conduction in CaMn1−xCrxO3 (0?x?0.3)

DC electrical conductivity (σ_dc) of electron-doped antiferromagnetic CaMn_1−xCr_xO₃ (0?x?0.3) has been discussed elaborately in the light of polaron hopping conduction. The increase in Cr doping concentration increases the conductivity and decreases the activation energy. Non-adiabatic polaron hopping conduction is observed in all the manganites at high temperatures. The analysis of σ_dc data shows that small polarons are formed at lower concentrations (?5%) of Cr doping and undoped samples. However, large polarons are materialized at higher doping (?10%) concentrations. This is consistent with the fact that doped Cr³⁺ has larger ionic size compared to that of Mn⁴⁺. Again, strong electron-phonon (e-ph) interaction is perceived in undoped and 5% Cr-doped samples but not in manganites with larger doping concentration. This also confirms the formation of larger polarons with the increase of x. Mott's variable range hopping (VRH) model can elucidate the dc conductivity at very low temperatures. It has been detected that single phonon-assisted hopping is responsible for the dc conduction in the Cr-doped CaMnO₃ manganites.     

La2−xSrxCuO4−δ superconducting samples prepared by the wet-chemical method

In this work, we report on the physical properties of good-quality polycrystalline superconducting samples of La_2−xSr_xCu_1−yZn_yO_4−δ (y=0, 0.02) prepared by a wet-chemical method, focusing on the temperature dependence of the critical current. Using the wet-chemical method, we were able to produce samples with improved homogeneity compared to the solid-state method. A complete set of samples with several carrier concentrations, ranging from the underdoped (strontium concentration x≈0.05) to the highly overdoped (x≈0.25) region, were prepared and investigated. The X-ray diffraction analysis, zero-field cooling magnetization and electrical resistivity measurements were reported on earlier. The structural parameters of the prepared samples seem to be slightly modified by the preparation method and their critical temperatures were lower than reported in the literature. The temperature dependence of the critical current was explained by a theoretical model which took the granular structure of the samples into account.     

Valence changes of manganese and praseodymium in Pr1−xSrxMn1−yInyO3−δ perovskites upon cation substitution as determined with XANES and ELNES

Systematic valence changes in Pr_1−xSr_xMn_1−yIn_yO_3−δ upon cation substitution with Sr²⁺ and In³⁺ have been found using Mn K-edge and Pr L-edge X-ray absorption, and Mn L_II,III and Pr M_IV,V electron energy-loss spectroscopy. The average valence of the praseodymium ions is close to +3.0 and virtually constant over the sample set when the samples also contained manganese ions. Pr_0.5Sr_0.5InO_3−δ showed a distinct increase in the praseodymium valence state. In contrast, the average valence of the manganese ions changed from the trivalent state to intermediate values between +3.0 and +4.0 and approached the tetravalent state depending on the level of substitution. The knowledge of the valence is required to understand the conduction mechanisms in the material due to the small polaron hopping (electronic conductivity) and motion of oxygen ions along the vacancies (ionic conductivity). Addition of strontium and indium led to the formation of oxygen vacancies. A previously assumed intermediate valence of praseodymium as causal factor for the higher oxygen catalytic activity cannot be confirmed with room temperature measurements.     

Influence of the concentration in Fe-doped BaTiO3 on magnetoelectric couping of layered composites BaTi1−xFexO3–Tb1−yDyyFe2−z

Perovskites BaTi_1−xFe_xO₃ has been synthesized with the concentration x ranging from 0.01 to 0.02. Their transformation point of ferroelectric to paraelectric and the corresponding latent heat of the phase transformation were observed to decrease with increasing the doping level of Fe³⁺. Bonded layered composites BaTi_1−xFe_xO₃–Tb_1−yDy_yFe_2−z have been fabricated and their magnetoelectric effect has been investigated. The sample containing a layer of perovskite BaTi_0.985Fe_0.015O₃ was found to show the maximum transverse ME voltage coefficient, which is about 1422 mV Oe⁻¹ cm⁻¹ under a magnetic field of 1580 Oe, in these bilayers. Analysis shows that the Fe-doped BaTiO₃ with doping level at about 1.5% should have largest piezoelectric coefficient in these ceramics BaTi_1−xFe_xO₃.     

Hall effect and sign reversal in Gd(Ba2−xNdx)Cu3O7+δ

We have investigated the Hall measurement of Gd(Ba_2−xNd_x)Cu₃O_7+δ with x=0, 0.05, 0.1, 0.15, and 0.2 in a magnetic field of 0–1 T. The nominal samples has been prepared by the conventional solid-state reaction technique. The iodometric titration experiment was carried out for samples. The Rietveld analysis of the X-ray diffraction patterns indicates that samples are mainly single phase. The normal state Hall coefficient behaves as 1/T in all samples. All samples with nominal compositions show single sign reversal with variation of magnetic field and temperature. The Hall resistivity of the samples with x=0 and 0.1 close to the superconducting transition temperature changes its sign with decreasing temperature and tends to a minimum −Δ_max, and then monotonically goes to zero. The absolute value of −Δ_max decreases with the increase of magnetic field. This can be qualitatively explained by a model calculation based on the time-dependent Ginsburg–Landau theory.     

Photoluminescence of Eu-doped CaMgSi2xO6+2x (1.00?x?1.20) phosphors in UV-VUV region

Alkaline-earth silicate phosphors CaMgSi_2xO_6+2x:Eu²⁺ (1.00?x?1.20) were prepared by traditional solid-state reaction. The phosphors showed an intense blue emission centered around 453 nm, with both 254 and 147 nm excitations. The host absorption below 200 nm in the excitation spectra consisted of two bands around 160 and 190 nm. The band around 160 nm was ascertained to be associated with the SiO₄-tetrahedra and MgO₆-polyhedra, and that around 190 nm was due to the CaO₈-polyhedra or some impurities. The incorporation of excess Si of less than 15% would not lead to formation of impurities and the results indicated that an appropriate Si excess could improve the Photoluminescence (PL) intensity in both ultraviolet (UV) and vacuum ultraviolet (VUV) regions     

Electrical and switching properties of the Se90Te10−xAgx (0?x?6) films

Amorphous Se₉₀Te_10−xAg_x (0?x?6) films are obtained by thermal evaporation technique under vacuum from the synthesized bulk materials on pyrographite and glass substrates. X-ray analysis shows the amorphous nature of the obtained films. The dc electrical conductivity was studied for different thicknesses (165-711 nm) as a function of temperature in the range (298-323 K) below the corresponding T_g for the studied films. The obtained results show that the conduction activation energy has a single value through the investigated range of temperature which can be explained in accordance with Mott and Davis model. The I-V characteristic curves for the film compositions are found to be typical for a memory switch. The mean value of the threshold voltage increases linearly with increasing film thickness (165-711 nm), while it decreases exponentially with increasing temperature in the investigated range for the studied compositions. The results are explained in accordance with the electrothermal model for the switching process. The effect of Ag on the studied parameters is also investigated.     

Spin transistor and quantum spin Hall-effects in CdBxF2−x–p-CdF2–CdBxF2−x sandwich nanostructures

Planar CdB_xF_2−x–p-CdF₂–CdB_xF_2−x sandwich nanostructures prepared on the surface of the n-type CdF₂ bulk crystal are studied to register the spin transistor and quantum spin Hall-effects. The current–voltage characteristics of the ultra-shallow p⁺–n junctions verify the CdF₂ gap, 7.8 eV, and the quantum subbands of the 2D holes in the p-type CdF₂ quantum well confined by the CdB_xF_2−xδ-barriers. The temperature and magnetic field dependencies of the resistance, specific heat and magnetic susceptibility demonstrate the high temperature superconductor properties for the CdB_xF_2−xδ-barriers. The value of the superconductor energy gap, 2Δ = 102.06 meV, determined by the tunneling spectroscopy method appears to be in a good agreement with the relationship between the zero-resistance supercurrent in superconductor state and the conductance in normal state, πΔ/e, at the energies of the 2D hole subbands. The results obtained are evidence of the important role of the multiple Andreev reflections in the creation of the high spin polarization of the 2D holes in the edged channels of the sandwich device. The high spin hole polarization in the edged channels is shown to identify the mechanism of the spin transistor and quantum spin Hall-effects induced by varying the top gate voltage, which is revealed by the first observation of the Hall quantum conductance staircase.     

Y1−xCaxBa2Cu3O7−δ thin films: From phase fluctuations to a complex order parameter

This report presents a study of the temperature dependence of the penetration depth in Y_1−xCa_xBa₂Cu₃O_7−δ thin films, which shows that a contribution to the order parameter from phase fluctuations in optimally doped samples cannot be excluded, and demonstrates the appearance of an imaginary component in the order parameter in overdoped samples. Measurements were performed using two complementary techniques: the parallel plate resonator (PPR), operated at 10 GHz, and far infrared (FIR) transmission, where the absolute value of the penetration depth can be obtained.     

Structure and dehydration of the pyrochlore system NaW2−yMoyO6+δ·nH2−zO between 10 and 675 K

The temperature dependence of the structure of the pyrochlore NaW_2−yMo_yO_6+δ·nH_2−zO has been investigated using a variety of diffraction and spectroscopic methods. The positions of OH⁻/H₂O molecules in the structure have been determined. Increases in temperature induce small lattice parameter changes, which are thought to result from movement of the H₂O molecules in the pyrochlore lattice.     

Critical RF losses in fine particles of La1−xAgyMnO3+δ: Prospects for temperature-controlled hyperthermia

Temperature and frequency dependencies of the real (χ′) and imaginary (χ″) parts of the dynamic magnetic susceptibility were studied experimentally in fine particles of La-Ag manganites prepared by various methods. The samples under study have the Curie points in the range T_C=42-48 °C, which is a medical hyperthermia range of interest. When approaching T_C from below, a critical peak of χ″ was revealed, followed by a steep drop while passing to the paramagnetic phase. The experiments on the magnetic radio-frequency (RF) heating of the manganite aqueous suspensions demonstrated good autostabilization of the temperature near T_C. Peculiar instability is found in the heating kinetics, caused by the observed critical behavior of the RF losses. The prospects of the La-Ag manganites as candidates for application in the temperature-controlled hyperthermia are discussed.     

Glass transition behavior of binary GaxSe100−x (0?x?10) glass systems

The glass transition behavior of glassy Ga_xSe_100−x (x=0, 2.5, 5, 7.5 and 10) systems were investigated using differential scanning calorimetry (DSC). The variation of glass transition temperature, T_g, with Ga concentration has been studied. The value of activation energy of glass transition, E_g, has been found to increase with increase in Ga content. This increase in E_g has been explained in terms of the average heat of atomization for these glasses.     

Effect of K doping on the physical properties of La0.65Ca0.35−xKxMnO3 (0?x?0.2) perovskite manganites

The effects of K doping in the A-site on the structural, magnetic and magnetocaloric properties in La_0.65Ca_0.35−xK_xMnO₃ (0?x?0.2) powder samples have been investigated. Our samples have been synthesized using the solid-state reaction method at high temperature. The parent compound La_0.65Ca_0.35MnO₃ is an orthorhombic (Pbnm space group) ferromagnet with a Curie temperature T_C of 248 K. X-ray diffraction analysis using the Rietveld refinement show that all our synthesized samples are single phase and crystallize in the orthorhombic structure with Pbnm space group for x?0.1 and in the rhombohedral system with R3¯c space group for x=0.2 while La_0.65Ca_0.2K_0.15MnO₃ sample exhibits both phases with different proportions. Magnetization measurements versus temperature in a magnetic applied field of 50 mT indicate that all our investigated samples display a paramagnetic-ferromagnetic transition with decreasing temperature. Potassium doping leads to an enhancement in the strength of the ferromagnetic double-exchange interaction between Mn ions, and makes the system ferromagnetic at room temperature. Arrott plots show that all our samples exhibit a second-order magnetic-phase transition. The value of the critical exponent, associated with the spontaneous magnetization, decreases from 0.37 for x=0.05 to 0.3 for x=0.2. A large magnetocaloric effect (MCE) has been observed in all samples, the value of the maximum entropy change, |ΔS_m|_max, increases from 1.8 J/kg K for x=0.05 to 3.18 J/kg K for x=0.2 under a magnetic field change of 2 T. For x=0.15, the temperature dependence of |ΔS_m| presents two maxima which may arise from structural inhomogeneity.     

Magnetic and transport properties in double-doping La(2+4x)/3Sr(1−4x)/3Mn1–xCuxO3 (0⩽x⩽0.20) systems

A series of the double-doping samples La_(2+4x)/3Sr_(1−4x)/3Mn_1–xCu_xO₃(0?x?0.2)$(0?x?0.2)$with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 and the single-doping samples La_2/3Sr_1/3Mn_1–xCu_xO₃(0?x?0.2)$(0?x?0.2)$ have been investigated. For the double-doping samples, though the ratio Mn³⁺/Mn⁴⁺=2:1 has been generally recognized the optimum ratio, the Curie temperature _TC$T_{\mathrm{C}}$ and metallic–insulator transition temperature _Tp1$T_{\mathrm{p}1}$ are more rapidly decreased by Cu substitution than that corresponding to single-doping samples. And the resistivity ρ$\rho$ value for the double doping is larger about two or three orders of magnitude than that corresponding to single doping. At the same time, two resistivity peaks and two magnetoresistance (MR) peaks appear. We suggest that for the double-doping samples the A-site cation size 〈r_A〉 and the A-site mismatch factor σ²${\sigma }^2$ decreases with increasing doping level, which leads to the system microstructural distortion. This microstructural distortion makes the Mn³⁺–O–Mn⁴⁺ cut off more cluster-spin except for the clusters induced by Cu. These cluster interfaces contribute to ρ$\rho$, which exceeds far the contribution of e_g electron decreasing with doping increasing in the single doping. At the same time, such interface scattering also gives rise to the appearance of second peak for the double-doping samples. The experimental results shows that double doping could be also a potential way in tuning colossal MR (CMR), which can give a guide for the adequate selection of CMR materials.     

Synthesis and luminescent properties of a new red-emitting phosphor for solid-state lighting: Eu0.1GdxLa1.9−xTeO6 (0.02?x?0.1)

The photoluminescence (PL) emission and excitation behavior of red-emitting Eu_0.1Gd_xLa_1.9−xTeO₆ (0.02?x?0.1) powder phosphors is reported. Three dominant bands centered at 395, 466 and 534 nm characterized the excitation spectrum. Under the excitation of 395 nm UV light, the emission spectrum exhibits an intense peak centered at 616 nm corresponding to the ⁵D₀→⁷F₂ transition of Eu³⁺. Because the f→f transitions are located in the wavelength range of blue or near-UV range, optimized phosphor, Eu_0.10Gd_0.08La_1.82TeO₆, is a promising material for solid-state lighting based on GaN LEDs applications.     

Structural,magnetic, and transport properties in La(2+4x)/3Sr(1−4x)/3Mn1−xCuxO3 (0⩽x⩽0.20) system

A series of the double-doping samples La_(2+4x)/3Sr_(1−4x)/3Mn_1−xCu_xO₃(0?x?0.2)$(0?x?0.2)$ with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 have been prepared. The structural, magnetic, transport properties and magnetoresistance of the series samples have been investigated. It is found that no apparent crystal structure change is introduced by Cu doping up to x=0.20$x=0.20$. But the Curie temperature _TC$T_{\mathrm{C}}$ and magnetization M   are strongly affected by Cu substitution. A remarkable magnetotransport behavior, characterized by double bumps, is observed, and an obvious low-temperature upturn is found in the range of 0.07?x?0.12$0.07?x?0.12$. As a result, the temperature range of colossal magnetoresistance (CMR) is greatly broadened. Moreover, it is found that the room temperature magnetoresistance (MR) of double-doping samples is obviously larger that the undoped La_2/3Sr_1/3Mn_1−xCu_xO₃ at 300 K, which can give a guide for the adequate selection of the room temperature CMR materials.