Magnetic characterization and thermoelectric power of Ni1−yZny Cu0.3Fe1.7 O4; 0.0⩽y⩽0.6

Samples of Ni_1−yZn_yCu_0.3Fe_1.7O₄; 0.0?y?0.6 were prepared by the solid state reaction method. X-ray investigations were carried out in order to assure the formation of the samples in single spinel phase. The analysis of X-ray data shows that the unit cell parameter increases with increasing Zn concentration and ascribed to the variation of the predicted cation distribution. Seebeck coefficient measurements were performed to know the type of charge carriers participating in the conduction mechanism. The magnetic susceptibility for the prepared samples was measured using Faraday^’s method at different temperatures as a function of the magnetic field intensity. The magnetic parameters were calculated from the magnetic susceptibility data, in the temperature range (300–800 K) at three different magnetic field intensities of (1280, 1733 and 2160 Oe). The effective magnetic moment (μ_eff) showed that, the critical Zn content was y=0.2$y=0.2$.     

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⁺.     

Structural properties and electrical behaviour in the polycrystalline lanthanum-deficiency La1−x□xMnO3 manganites

Electrical conductance and X-ray diffraction (XRD) measurements of lanthanum-deficiency La_1−x□_xMnO₃ (x=0.05, 0.10 and 0.20) polycrystalline samples were performed to examine the effect of the internal pressure at B-site on the conduction mechanism. The structural study reveals that all samples crystallize in the rhombohedral system. The electronic conduction appears to be thermally activated at high temperature, which indicates the presence of semiconductor behaviour. The increase of the x content converts 3x Mn³⁺ to 3x Mn⁴⁺ ions with smaller ionic radius, which reduces the internal pressure and leads to the increase of the one-electron bandwidth W. This increase causes the appearance of metallic behaviour at low temperature for x=0.10 and 0.20 content.     

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.     

Magnetic,transport and microstructural properties of polycrystalline samples with nominal composition of La0.7Ca0.3Mn1−xVxO3 (0⩽x⩽0.2)

Polycrystalline samples with nominal composition of La_0.7Ca_0.3Mn_1−xV_xO₃ (0?x?0.2) sintered in air were investigated by ac susceptibility, dc magnetization, magnetoresistance (MR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and differential thermal analysis (DTA) measurements. It is found that V could not substitute for Mn to form La_0.7Ca_0.3Mn_1−xV_xO₃ phase when the samples are sintered in air. The obtained samples contain several phases such as (La, Ca)Mn_1−δO₃, LaCa₃V₃O₁₂, Mn₃O₄ phases, etc. and constitute multi-phase composites. The Curie temperature T_C and spontaneous magnetization of the composites decrease, and the resistance of the composites increases as the V content increases. The addition of V may effectively improve the low-field MR response at low temperatures due to the variation in the microstructure of the composites.     

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.     

Heterogeneous magnetic and electronic states in Ca1−xLaxMnO3−δ (x⩽0.12) single crystals with electron doping

The magnetic, transport, and optical properties of electron-doped Ca_1−xLa_xMnO_3−δ single crystals with x  ?0.12 were studied. The magnetic measurements show that in single crystals with x=0$x=0$ and 0.05 the G-type AFM phase with weak FM component is realized and in crystals with x=0.10$x=0.10$ and 0.12 the G- and C-type AFM phases coexist. The C-type magnetic structure arises at less concentration of La than in polycrystalline samples as a result of oxygen vacancies being additional source of electrons. Under magnetic transitions in the G- and C-type phases, resistivity and magnetoresistance of the doped single crystals have anomalies. Optical absorption in IR range indicates formation of a charge gap in crystals with x=0.10$x=0.10$ and 0.12 at appearance of the C-AFM and monoclinic phase with orbital/charge ordering. By comparing optical and transport properties, heterogeneous electronic state and its relation with heterogeneous magnetic state are shown.     

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.     

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.     

Magnetic and transport properties of bilayered manganites La1.2Sr1.8Mn2-xGaxO7（x=0,0.08）

The magnetic and electrical properties of nonmagnetic Ga +3 ion substitution for Mn site are investigated in the bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7.When the Mn is substituted by Ga,the ferromagnetic property obviously weakens,the magnetic transition temperature decreases and a spin-glass behaviour occurs at low temperature.Meanwhile,doping causes the resistivity to dramatically increase,the metal-insulator transition temperature to disappear,and a greater magneto-resistance effect to occur at low temperature.These effects result from the fact that Ga substitution dilutes the magnetic active Mn-O-Mn network and weakens the double exchange interaction,and further suppresses ferromagnetic ordering and metallic conduction.     

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.     

Effect of multielement doping on low-field magnetotransport in La0.7−xMmxCa0.3MnO3 (0.0?x?0.45) manganite

We report the synthesis, structure and low-field magnetotransport properties of Mischmetal (Mm)-doped La_0.7−xMm_xCa_0.3MnO₃ (0?x?0.45) manganite. Mischmetal—Mm—is a natural mixture of rare earth elements La, Ce, Pr and Nd with ∼28%, 50%, 6% and 16% composition, respectively. All the samples crystallize in orthorhombic structure. Increasing x (Mm), corresponding to decreasing the La-site average ionic radii (〈r_A〉) hence increasing the size mismatch (i.e. variance σ²), results in strong suppression of ferromagnetism (T_C) and the associated metallicity (T_IM). It may be pointed out that Mm (La, Ce, Pr and Nd) substitution has been done to create two effects. First, creation of multivalence of Mn (2+, 3+ and 4+) via Ce substitution and second to create higher degree of disorder due to size difference brought in not only by Ce but also by Pr and Nd. Evidences and arguments based on XPS analysis suggest that multivalent ions La, Mm and Ca, and the resulting presence of Mn²⁺, Mn³⁺ and Mn⁴⁺, causes the simultaneous operation of ferromagnetism-double exchange (Mn²⁺/Mn³⁺ and Mn³⁺/Mn⁴⁺) and antiferromagnetic-superexchange (Mn³⁺/Mn³⁺ and Mn²⁺/Mn²⁺) interaction. In addition, Mm doping also creates inhomogenities at La—as well as Mn—site due to size and valency difference. A curiously huge magnetoresistance as high as ∼63% for x=0.35, under a moderate magnetic field of ∼10 kOe has been observed and even at low magnetic field of ∼3 kOe MR is ∼30%. The competing double exchange and superexchange coupled with inhomogenities are the most likely cause for the occurrence of large ∼63% CMR in the Mm-doped LCMO.     

Structure and magnetic properties of magnetostrictive compounds Tb0.36Dy0.64(Fe0.85Co0.15)2−xBx (0⩽x⩽0.15)

Crystal structure and magnetic properties of magnetostrictive compounds Tb_0.36Dy_0.64(Fe_0.85Co_0.15)_2−xB_x (0?x?0.15) have been investigated at room temperature. The matrix of these compounds keeps a cubic MgCu₂-type structure. Lattice parameter a of the Laves phase decreases to reach a minimum at x=0.10, then increases with increasing boron content. Through analyzing the Mössbauer spectra, the easy magnetization direction (EMD) for all samples is confirmed to lie along 〈111〉 direction at room temperature, suggesting the presence of the giant magnetostriction. The mean hyperfine field H_hf and the deduced iron moment μ_Fe increase with increasing boron content, resulting in the enhancement of both Curie temperature T_C and spin reorientation temperature T_r. Although the addition of B enlarges the magnetocrystalline anisotropy constant K₁, the composition dependence of the ratio λ/K₁ for Tb_0.36Dy_0.64(Fe_0.85Co_0.15)_2−xB_x, however, reaches a maximum value at x=0. 05 under high magnetic fields.     

Double M–I transitions and low-temperature resistivity minimum of La2/3Ca1/3Mn1−xCoxO3(0⩽x⩽0.15) manganite

A series of Mn-site Co-doping samples La_2/3Ca_1/3Mn_1−xCo_xO₃ (0?x?0.15) have been prepared. The structure, magnetic and transport properties of this system have been systematically investigated. All the samples showed good single phase, and the lattice parameters decreased with the increase of doping concentration x. Only one paramagnetic–ferromagnetic transition was observed. The Curie temperature T_C decreases gradually and the transition width becomes wider with the increase of x. The abnormal transport properties were induced by Co doping, characterized by the double metal–insulator transitions and low-temperature minimum behavior. The present results are discussed and possible explanations were given based on the related theory and previous reported results.     

Investigation on cation distribution and magnetic properties in self-doped manganites La0.6−xSr0.4MnO3−δ

Self-doped manganites with nominal composition La_0.6−xSr_0.4MnO_3−δ (0≤x≤0.175) have been prepared by the sol–gel method. The X-ray diffraction (XRD) patterns and magnetic measurements indicate that the samples have two phases with the ABO₃ perovskite structure being the dominant phase and Mn₃O₄ being the minor phase when the doping level x≥0.05. On the basis of the thermal equilibrium theory of crystal defects, the contents of various ions in the perovskite phases were estimated, in which there are Mn²⁺ ions and no vacancies at A sites. The ion contents have been corrected by Rietveld fitting of the powder samples' X-ray diffraction data. The change tendency of the Curie temperature T_C vs. the Mn⁴⁺ ion content ratio at the B sites of ABO₃ perovskite phase is in accord with the experimental result of the samples La_1−xSr_xMnO₃.     

Co和Cr掺杂对层状钙钛矿锰氧化物La1.2Sr1.8Mn2O7磁性的影响

研究了层状钙钛矿锰氧化物La_1.2Sr_1.8Mn₂O₇中Mn被Co和Cr替代对磁性能的影响.Co替代Mn后,长程铁磁序被破坏,铁磁性减弱,出现团簇玻璃态和自旋玻璃态,表明Co离子和Mn离子之间不存在双交换作用.而Mn被Cr替代后长程铁磁序仍然保持,证实Cr³⁺和Mn³⁺之间存在铁磁性交换作用. 关键词：     

Structural and magnetic properties in the self-doped perovskite manganites with nominal composition La0.7Sr0.3−xMnO3−δ

Perovskite manganites with nominal composition La_0.7Sr_0.3−xMnO_3−δ (0.00≤x≤0.20) have been prepared by the sol-gel method with the highest heat treatment temperature being 1073 K. The XRD patterns indicate that when the doping level is x≤0.10 the samples have only a single phase, with the R3?c perovskite structure, while for x>0.10, the samples have two phases with the R3?c perovskite being the dominant phase and Mn₃O₄ being the second phase. A quantitative analysis and Rietveld fitting of the X-ray powder diffraction data indicate that on the basis of the thermal equilibrium theory of crystal defects there are Mn²⁺ ions at the A sites and Mn³⁺ plus Mn⁴⁺ ions at the B sites in the ABO₃ perovskite phase. The curves of magnetization versus applied magnetic field at 10 K showed that the magnetic moments of the Mn²⁺ ions at the A sites are antiparallel to those of the Mn³⁺ and Mn⁴⁺ ions at the B sites.     

Cyclic voltammetry characterization of a La0.8Sr0.2Co0.2Fe0.8O3 − δ electrode interfaced to CGO/YSZ

The electrochemical characteristics of a La_0.8Sr_0.2Co_0.2Fe_0.8O_3 − δ cathode electrode interfaced to the CGO layer of a double layer CGO/YSZ electrolyte were studied using cyclic voltammetry, at temperatures of 600 to 850 °C and under oxygen partial pressures ranging from 0.07 to 21 kPa. The aim was to identify the electrochemical processes taking place under cathodic polarization on the basis of differences in the features of the cyclic voltammograms with changing conditions. Depending on temperature, sweep rate and oxygen partial pressure, current peaks appeared both in the forward and backward scans. Furthermore, reversed hysteresis was observed, i.e. higher currents in the backward scan than in the forward scan, with increasing oxygen partial pressure and decreasing temperature. The observed behavior was related to the electrochemical redox of B-sites and concomitant stoichiometry change as well as to the competing reaction of electrochemical oxygen redox, taking also into account the competitive action of chemical reactions occurring in the presence of gaseous oxygen.     

Effect of elevated concentrations of strontium and iron on the structural and dielectric characteristics of Ba(1−x−y)Sr(x)Ti Fe(y)O3 prepared through sol–gel technique

Nano-composite Ba_1−xSr_(x)TiO₃ (BST), where x=0.01–0.50 and doped with different concentrations of iron Ba_(1−x−y)Sr_(x)TiFe _(y)O₃ (BSTF), where x=0.01 and y=0.01–0.05 powders were prepared by sol–gel method.