Polarization-induced electro resistance and magneto resistance in La0.67Ca0.33MnO3/BaTiO3 composite film

La_0.67Ca_0.33MnO₃/BaTiO₃ composite films have been grown on Nb-doped SrTiO₃ substrates by the sol–gel method. The magnetic and ferroelectric properties in the composite films are investigated. A three-state memory is formed by applying a vertical electric field across the La_0.67Ca_0.33MnO₃/BaTiO₃ heterostructure, this behavior is attributed to the polarization-mediated resistive switching effect. In addition, the transport properties of La_0.67Ca_0.33MnO₃ thin film can be modulated by an external magnetic field, a 10.3 K shift of the metal insulator transition temperature is obtained with the change of applied magnetic field from 0 T to 6 T. Consequently, in La_0.67Ca_0.33MnO₃/BaTiO₃ heterostructure, the resistance behavior can be modulated by piezoelectric effect, ferroelectric polarization and magnetic field simultaneously.     

La0.6Ca0.4CoO3, La0.1Ca0.9MnO3 and LaNiO3 as bifunctional oxygen electrodes

A series of perovskite catalysts was investigated for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline electrolyte and at room temperature, supplied by oxygen or air. A meniscus cell was used to screen-test candidate catalysts for their bifunctionality and assess their activity for ORR at 3 mm depth of immersion (DOI) in the electrolyte. Based on the meniscus data LaNiO₃, La_0.1Ca_0.9MnO₃ and La_0.6Ca_0.4CoO₃ were selected for further assessment in microelectrode and half-cell studies. Activity tests for the ORR and OER, Tafel slopes at high current densities and apparent activation energies for the ORR were determined using a microelectrode technique on samples of the selected perovskites, La_0.1Ca_0.9MnO₃, La_0.6Ca_0.4CoO₃ and LaNiO₃ with and without graphite support. Tafel slopes of ca. 120 mV per decade and apparent activation energies of approximately 18 kcal mol⁻¹ were measured at high cathodic current densities. Cycle-life and performance of La_0.1Ca_0.9MnO₃, La_0.6Ca_0.4CoO₃ and LaNiO₃-based gas-diffusion electrodes in half-cell configurations were tested at a constant current density of 25 mA cm⁻² with subsequent and intermittent polarizations. Similar activities resulted in the ORR, while increased numbers of cycles were observed for the La_0.1Ca_0.9MnO₃-based electrode. Furthermore, electrode material compositions, especially PTFE contents were optimized to conform to the establishment of the three phase interactions of the electrode structure. Transmission Electron microscopy (TEM) and BET-surface area analyses were carried out in order to find out the morphological and surface properties of the perovskite materials.     

Large increase of Curie temperature in (110)-oriented La0.7Sr0.3MnO3 films

From the perspectives of scientific researches and practical applications, it is desirable to explore high operating temperature ferromagnetic films. The effect of biaxial strain on magnetic properties of (110)-oriented La_0.7Sr_0.3MnO₃ films was studied. High quality La_0.7Sr_0.3MnO₃ films were grown on (110)-oriented perovskite single crystal substrates using pulsed laser deposition, varying substrate-induced misfit strains from ??2.27–0.75%. A remarkable enhancement of Curie temperature has been achieved for (110)-oriented La_0.7Sr_0.3MnO₃ films clamped with small misfit strains (i.e., grown on LAST (110)). The enhanced Curie temperature of (110)-oriented La_0.7Sr_0.3MnO₃ films could be attributed to the misfit strain between the films and the underlying substrates and may have technological implication for applications at high temperature environments.     

Critical behavior near the paramagnetic to ferromagnetic phase transition temperature in La0.6Sr0.4MnO3 ceramic: A comparison between sol-gel and solid state process

Critical behavior near the paramagnetic (PM) to ferromagnetic (FM) phase transition in La_0.6Sr_0.4MnO₃ ceramics prepared by sol-gel and solid state reaction methods has been systematically investigated. The XRD result coupled with the structural Rietveld refinement method exhibited that all samples crystallized in a rhombohedral structure with a space group of R-3C, which indicated the formation of the perovskite structure of La_0.6Sr_0.4MnO₃. The magnetic data obtained from magnetization measurements indicated a second-order phase transition from PM-FM phase around Curie temperature (T_C). Critical exponents, β, γ, and δ values are estimated by various theoretical models such as the modified Arrott plot (MAP), the Kouvel-Fisher plot (KF) and the critical isotherm (CI) techniques. Grain size dependence of the critical behavior of La_0.6Sr_0.4MnO₃ samples with different sizes from nano-scale to mic-scale was detected by fitting the experimental data to the theoretical models.     

The influence of Cu(OH)2 addition on the low-temperature electrochemical performance of La0.75Mg0.25Ni3.5 hydrogen storage alloy

The influence of Cu(OH)₂ addition in 7 mol/L KOH alkaline electrolyte on the electrochemical properties of La_0.75Mg_0.25Ni_3.5 hydrogen storage alloy electrode was investigated in the testing temperature range of from 25 °C to −40 °C in this paper. XRD Rietveld analyses shows that the La_0.75Mg_0.25Ni_3.5 hydrogen storage alloy consists of LaNi₅ phase, (La, Mg)₂Ni₇ phase and (La,Mg)Ni₃ phase. The maximum discharge capacity and the high-rate dischargeability (HRD) of the La_0.75 Mg_0.25Ni_3.5 alloy electrode both decrease with decreasing testing temperature, which mainly due to the slower hydrogen transfer in the bulk of the alloy and the lower electrocatalytic activity at lower temperatures. The scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDS) analyses indicate that, with the addition of Cu(OH)₂ to the KOH alkaline electrolyte, fine particles of metal Cu can be coated on the La_0.75Mg_0.25Ni_3.5 alloy electrode surface during charging process by electrodeposition. The Cu powders deposited on the La_0.75Mg_0.25Ni_3.5 alloy electrode can improve the alloy’s low-temperature discharge properties by decreasing its charge-transfer resistance and increasing its exchange current density. The cyclic stability of the Cu-deposited La_0.75Mg_0.25Ni_3.5 alloy electrode can be also improved to a certain extent by increasing its corrosive resistance in KOH alkaline electrolyte.     

Large temperature coefficient of resistance at near room temperature in Sr-doped La0.72Ca0.28MnO3:Ag0.2 polycrystalline composites

A series of La_0.72Ca_0.28?xSr_xMnO₃:Ag_0.2 (x = 0.00, 0.01, 0.03, 0.05, 0.07, and 0.10) composites was prepared by sol–gel method using methanol as solvent. Sample composition was analyzed by X-Ray diffraction, which revealed that all samples are pseudo-perovskite. Grain size and morphology were characterized by scanning electron microscopy. The results showed that all samples are of high density and large grain size in the range of 50–100 µm. All samples exhibited sharp metal–insulator (M-I) transitions, along with a drastic transition from paramagnetism to ferromagnetism. With increased Sr doping, radius of A-site ion increased, both Curie temperature T_C and M-I transition temperature shift to higher temperature increases, and transition width turns widener which makes a decreased temperature coefficient of resistance (TCR). Larger ionic radius of Sr²⁺, which resulted in enhancement of double exchange, enlarges Mn–O–Mn bond angle, hence improves electrical and magnetic properties. It was found that TCR could reach 24.3% K^?1 by Ag addition at near room temperature, which makes this composite promising potential applications in infrared detection and night vision.     

Synthesis, structural and magnetic properties of nanostructured Ca0.9Gd0.1MnO3 obtained by modified glycine nitrate procedure (MGNP)

Ca_0.9Gd_0.1MnO₃ nanopowders with perovskite type crystal structure were synthesized by modified glycine nitrate procedure. Nanopowders were prepared by combining glycine with metal nitrates and/or metal acetates in their appropriate stoichiometric ratios. Modification of the procedure was performed by partial replacement of nitrates by acetates, in order to control the burn-up reaction. Obtained Ca_0.9Gd_0.1MnO₃ powders were calcinated in the temperature interval from 850 °C to 950 °C for 10 min. Properties such as phase evolution, lattice parameters, chemical composition and magnetic properties were monitored by DTA, X-ray diffraction, SEM/EDS and magnetic measurements. Magnetic measurements performed at the sample with the smallest crystallite size showed that a 10% of Gd³⁺ substituted Ca²⁺ ions changes antiferromagnetic properties of CaMnO₃ by the introduction of ferromagnetic interaction due to a double exchange between Mn³⁺ and Mn⁴⁺ ions. Presence of competing interactions and their randomness lead to a formation of a spin glass state below Neel temperature T_N = 110 K. From the high temperature magnetic susceptibility measurements effective magnetic moment of manganese ions is determined which lies between the values for Mn³⁺ and Mn⁴⁺ ions.     

Effect of Fe doping on structure,magnetic and electrical properties La0.7Ca0.3Mn0.5Fe0.5O3 manganite

The crystal and magnetic structures of La_0.7Ca_0.3Mn_0.5Fe_0.5O₃ compound have been studied by neutron powder diffraction in the temperature range of 10–300 К. The magnetization and electrical resistivity measurements have been also performed in the temperature range of 5–300?K in magnetic fields up to 1?T. These experimental results indicate a formation of a complex magnetic state in which the long-range antiferromagnetic G-type phase coexists with the short-range ferromagnetic clusters. The electrical conductivity of La_0.7Ca_0.3Mn_0.5Fe_0.5O₃ demonstrates an anomalous temperature behavior suggesting a switching between different states. The origin of the unconventional magnetic state, the mechanisms of the electrical conductivity, and correlation between magnetic and transport properties in this manganite have been discussed.     

Monodisperse magnetic composite poly(glycidyl methacrylate)/La0.75Sr0.25MnO3 microspheres by the dispersion polymerization

Monodisperse magnetic poly(glycidyl methacrylate) microspheres were prepared by dispersion polymerization of glycidyl methacrylate in cyclohexane in the presence of La_0.75Sr_0.25MnO₃ nanoparticles, surface of which was modified with penta(propylene glycol) methacrylate phosphate (PPGMAP). However, only agglomerates were formed by the dispersion polymerization in toluene. Sterical stabilizer was poly(styrene-block-hydrogenated butadiene-block-styrene) and initiators investigated were 2,2′-azobisisobutyronitrile (AIBN) and 4,4′-azobis(4-cyanovaleric acid) (ACVA). Effects of initiators and other reaction conditions on properties of the composite microspheres were evaluated. The phase composition of the magnetic polymer composite microspheres and the size of magnetic cores were determined by X-ray powder diffraction. The characterization was completed by magnetization measurements, atomic absorption spectroscopy, TEM, SEM and ATR FTIR spectroscopy.     

Percolation like transitions in phase separated manganites La0.5Ca0.5Mn1-xAlxO3-δ

We show that the replacement of Mn with Al strongly affects the magnetization and electrical transport behaviors in La_0.5Ca_0.5Mn_1-xAl_xO_3-δ (x = 0, 0.05, 0.07 and 0.09). Nonmagnetic Al³⁺ ions substitution at Mn sites dilutes Mn³⁺-O^2--Mn⁴⁺ network, thus suppresses the ferromagnetic metallic state in La_0.5Ca_0.5MnO_3-δ and causes a phase separation phenomenon, in which ferromagnetic phase coexists with antiferromagnetic charge ordered phase at low temperatures. With applying sufficiently high magnetic field, step-like metamagnetic transitions were observed in x = 0.05–0.09 systems below helium temperature, in which the antiferromagnetic charge ordered phase collapsed into ferromagnetic phase. Corresponding to the sharp step-like metamagnetic transitions, the resistivity decreases dramatically with increasing magnetic field, exhibiting a percolative insulator-metal transition. The variation of temperature and magnetic field changes the relative fractions of ferromagnetic and charge ordering phases, and percolative insulator-metal transition occurs due to the development of percolation paths between the growing FM domains.     

Electrical and magnetic properties of La1−xSrxMnO3 (0.1 ≤ x≤ 0.25) ceramics prepared by sol–gel technique

La_1−xSr_xMnO₃ (0.1 ≤ x ≤ 0.25) high density ceramics were prepared by sol-gel method using methanol as solvent. X-ray diffraction analysis showed that all samples exhibited single perovskite structure and no second phase was detected. Scanning electron microscopy images exhibited good particle connectivity on the surface of sample, and grain size increased with the increase in Sr doping. Resistivity-temperature curves of samples were measured by standard four-probe method, and curves exhibited significant differences in studied range of Sr doping. Magnetic measurement results indicated that the variation of susceptibility of different samples was quite different, and the Curie temperature of samples increased with the increase in Sr content. For x = 0.2, temperature coefficient of resistance value of the sample was larger, and corresponding peak TCR temperature was 307.1 K, which is very close to room temperature. Thus, La_0.8Sr_0.2MnO₃ ceramics exhibited high TCR value close to room temperature. Combined with its excellent magnetic properties, La_0.8Sr_0.2MnO₃ ceramics may potentially act as effective candidates for uncooled radiation calorimeter and uncooled magnetic sensor. Applications of La_0.8Sr_0.2MnO₃ ceramics in uncooled infrared radiation calorimeter at room temperature will be highly beneficial.     

Effects of Particle Size on Electromagnetic and Microwave Absorption Properties of La0.7Sr0.3MnO3±δ‐Epoxy Composite

La_0.7Sr_0.3MnO_3±δ powders were fabricated by solid‐state reaction method at 1473 K for 4 h. The precursors were prepared by ball‐milling raw materials for 3, 6, 9, and 12 h, respectively. The crystal structures, particle size, and morphologies of precursors and prepared La_0.7Sr_0.3MnO_3±δ were characterized by XRD, laser particle size analyzer and SEM, respectively. It is found that La_0.7Sr_0.3MnO_3±δ possessed large particle size by ball‐milling raw materials for a long time. Results indicated that La_0.7Sr_0.3MnO_3±δ, synthesized by ball‐milling raw materials for 3 h, exhibited the optimal microwave absorption properties. The maximum reflection loss was ?28.8 dB, and the ?6 dB absorption bandwidth was 5.80 GHz.     

Hydrothermal synthesis and magnetocaloric effect of La0.7Ca0.2Sr0.1MnO3

The hydrothermal synthesis and magnetic entropy change for the perovskite manganite La_0.7Ca_0.2Sr_0.1MnO₃ have been studied. The La_0.7Ca_0.2Sr_0.1MnO₃ can be produced as phase-pure, crystalline powder in one step from solutions of metal salts in aqueous potassium hydroxide solution at a temperature of 240 °C in 72 h. Scanning electron microscopy shows that the materials are made up of cuboid-shaped or rod-shaped particles with typical dimension of 4.0 μm. Heat treatment is necessary to improve the magnetocaloric effect for the hydrothermal sample. The maximum magnetic entropy change (∣ΔS_M∣) for the sample annealed at 1200 °C for 6 h is 2.85 J kg^− 1 K^− 1 at the magnetic field change of 2.0 T and 2.23 J kg^− 1 K^− 1 at the magnetic field change of 1.5 T at 315 K. The hydrothermal synthesis method is a feasible route to prepare high quality perovskite material for magnetic refrigeration application.     

Spin entropy enhancement tuned by spin state transition in La0.7−xYxCa0.3CoO3

The effect of spin states on the spin entropy has been studied in La_0.7−xY_xCa_0.3CoO₃ (x=0.0, 0.05, 0.1 and 0.15). Magnetothermopower results reveal that Y-doping improves the spin entropy of La_0.7Ca_0.3CoO₃. The magnetic results indicate that the Co³⁺ ions undergo a transition from an intermediate spin state (t⁵_2ge¹_g; S=1) to a low spin state (t⁶_2ge⁰_g; S=0), which results in the enhancement of the spin entropy. A suitable theoretical model is suggested to explain well the enhancement of the spin entropy induced by spin state transition. This investigation gives strong evidence that the enhanced spin entropy contribution to the thermopower is tuned by the spin state transition.     

Hydrothermal synthesis,sintering and characterization of nano La-manganite perovskite doped with Ca or Sr

Two lanthanum manganite perovskite-nanostructures, namely; La_xCa_(1-x)MnO₃ and La_xSr_(1-x)MnO₃ (x?=?0.1, 0.3, 0.5 and 1.0), were synthesized by hydrothermal method. To follow up the composition of formed phases, the synthesized powders were calcined at different temperatures. The obtained materials were investigated by X-ray diffraction (XRD) and transmission electron microscope (TEM). Moreover, the calcined powders were pressed at 100?MPa and sintered at variable temperatures; i.e. 1250, 1300, 1350 and 1450?°C. The phase composition and microstructural characteristics of sintered pervoskites were examined by XRD and scanning electron microscope (SEM). Furthermore, physical (bulk density and apparent porosity), electrical resistivity and magnetic properties were also determined. The results revealed that La_xCa_(1-x)MnO₃ and La_xSr_(1-x)MnO₃ nanostructures were successfully prepared by hydrothermal method. The physical properties of sintered pervoskites were strongly depended on dopant type and concentration. The maximum sintering temperature for La_xCa_(1-x)MnO₃ was 1400?°C while for La_xSr_(1-x)MnO₃ was 1450?°C, after which the materials have been fused. Materials doped with Ca or Sr exhibited lower resistivity. On the other side, the magnetic properties have been also improved after addition of Ca or Sr. This has been discussed based on the double exchange mechanism. La_xSr_1-xMnO₃ exhibited better magnetic properties than La_xCa_1-xMnO₃ and LaMnO₃. La_0.5Ca_0.5MnO₃ and La_0.5Sr_0.5MnO₃ exhibited the highest magnetization among the other pervoskites.     

Electrical transport properties of Sm-doped La0.7Ca0.3MnO3 polycrystalline ceramics

Improving the magnetoresistance effect of perovskite ceramic materials under a low applied magnetic field to expand its application range is one of the main research directions of this type of material. In this study, La_0.7Ca_0.3MnO₃ was doped with different levels of Sm by the sol-gel method to yield a series of La_0.7-xSm_xCa_0.3MnO₃ (LSCMO) polycrystalline ceramics. X-ray diffraction (XRD) results revealed that LSCMO ceramics possessed standard perovskite structures. Scanning electron microscopy (SEM) showed grains closely connected without obvious holes. In addition, the grain size gradually decreased with the increase in Sm doping content. The resistivity temperature curves displayed a clear metal-insulator transition behavior of LSCMO accompanied by a steep change from ferromagnetic to paramagnetic behavior (FM-PM). The metal-insulator transition temperature (T_p) values of the as-obtained LSCMO gradually shifted toward lower temperatures with increase in Sm content. Moreover, resistivity temperature coefficient (TCR) and magnetoresistance (MR) values also gradually increased with Sm doping content. The transport properties in polycrystalline ceramics could be adequately explained by the double exchange model, which would be useful for interpreting the CMR effects when used in magnetic devices.     

Synthesis and photoluminescence enhancement of Ca3Sr3(VO4)4:Eu3+ red phosphors by co-doping with La3+

In this study, a series of red-emitting Ca₃Sr₃(VO₄)₄:Eu³⁺ phosphors co-doped with La³⁺ was prepared using the combustion method. The microstructures, morphologies, and photoluminescence properties of the phosphors were investigated. All Ca₃Sr₃(VO₄)₄:Eu³⁺, La³⁺ samples synthesized at temperatures greater than 700 ℃ exhibited the same standard rhombohedral structure of Ca₃Sr₃(VO₄)₄. Furthermore, the Ca₃Sr₃(VO₄)₄:Eu³⁺, La³⁺ phosphor was effectively excited by near-ultraviolet light of 393 nm and blue light of 464 nm. The strong excitation peak at 464 nm corresponded to the ⁷F₀→⁵D₂ electron transition of Eu³⁺. The strong emission peak observed at 619 nm corresponded to the ⁵D₀→⁷F₂ electron transition of Eu³⁺. Co-doping with La³⁺ significantly improved the emission intensity of Ca₃Sr₃(VO₄)₄:Eu³⁺ red phosphors. The optimum luminescence of the phosphor was observed at Eu³⁺ and La³⁺ concentrations of 5% and 6%, respectively. Moreover, co-doping with La³⁺ also improved the fluorescence lifetime and thermal stability of the Ca₃Sr₃(VO₄)₄:Eu³⁺ phosphor. The CIE chromaticity coordinate of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.06La³⁺ was closer to the NTSC standard for red phosphors than those of other commercial phosphors; moreover, it had greater color purity than that of all the samples tested. The red emission intensity of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.06La³⁺ at 619 nm was ~1.53 times that of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺ and 2.63 times that of SrS:Eu²⁺. The introduction of charge compensators could further increase the emission intensity of Ca₃Sr₃(VO₄)₄:Eu³⁺, La³⁺ red phosphors. The phosphors synthesized herein are promising red-emitting phosphors for applications in white light-emitting diodes under irradiation by blue chips.     

Magnetic and magnetoresistive properties of La0.65−xEuxSr0.35MnO3

Eu-doped perovskites La_0.65−xEu_xSr_0.35MnO₃ (0.05 ≤ x ≤ 0.30) were synthesized by sol–gel method using citric acid and characterized by X-ray diffraction, magnetization, resistivity and magnetoresistance (MR) experiments. All samples had a single hexagonal perovskite structure. As x increased from 0.05 to 0.30, the Curie temperature T_C for the samples decreased from 352 to 242 K. It was found that two transition points appeared when the resistivity changed with increasing temperature, and upon an application of a magnetic field of 20 kOe the maximum magnetoresistivity of 18% for the La_0.65−xEu_xSr_0.35MnO₃ with x = 0.20 was obtained at room temperature 300 K. The mechanism of the transitions for the samples was explored.     

Critical behavior near the paramagnetic-ferromagnetic phase transition in polycrystalline La0.6Ca0.2Ba0.15□0.05MnO3

In this paper, we present a detailed study on the structural and critical behavior around paramagnetic (PM)-ferromagnetic (FM) phase transition in La_0.6Ca_0.2Ba_0.15□_0.05MnO₃ (LCBMO). Our powder specimen was synthesized using the conventional solid-state reaction method and was analyzed by X-ray diffraction and magnetic measurements. X-ray diffraction analysis, at room temperature, shows that LCBMO adopts an orthorhombic structure with Pbnm space group. Detailed analyses of magnetic-field dependences of magnetization in the vicinity of the paramagnetic-ferromagnetic transition, M(H,T), reveal that LCBMO undergoes a second-order magnetic phase transition. Critical behavior has been studied through various techniques such as modified Arrott plot (MAP), Kouvel-Fisher method (KF) and critical isotherm (CI) analysis. The estimated Critical exponents are close to those expected for the mean-field model (β=0.5, γ=1 and δ=3). The reliability of the critical exponent values was confirmed by the Widom scaling relation, as well as the universal scaling hypothesis.     

Growth and characterization of epitaxial (La2/3Sr1/3)MnO3 films by pulsed laser deposition

High quality epitaxial (La_2/3Sr_1/3)MnO₃ (0 0 1) thin films were grown by pulsed laser deposition on SrTiO₃ (0 0 1) substrate at optimized growth parameters. The films quality was confirmed by both structural and physical properties characterization. Channeling Rutherford Backscattering Spectrometry characterization showed the minimal channeling coefficient as low as 4%. The LSMO thin films growth on SrTiO₃ substrate follows the island growth model. The Curie temperature of LSMO films is around 360 K, which is the one of the highest reported in literature. The resistivity of LSMO films showed the metal-insulate transition temperature coincides with the Curie temperature. This high quality LSMO is suitable for room temperature magnetic devices application.