非正分钙钛矿锰氧化物(La0.8 Sr0.2)1-xMnO3的电磁特性

报道了A位缺位的钙钛矿锰氧化物(La0.8Sr0.2)1-xMnO3(0≤x≤0.30)多晶样品的相结构、磁性和磁电阻效应.实验表明,当x≥0.20时,化合物主要由磁性钙钛矿相和非磁性Mn3O4相所组成.它们的电阻率随温度的变化曲线均具有双峰特征,高温侧的电阻率峰出现在钙钛矿相的居里温度附近,低温侧的电阻率宽峰则是金属导电性的钙钛矿晶粒和高电阻率的半导体或绝缘体导电性的晶界或相界共同作用的结果.样品的零场电阻率ρ0随着A位缺位量x的增大而增大.适当改变x值,可以改善磁电阻比的温度稳定性.当x=0.30时,化合物磁电阻比MR在一个相对宽的温度范围内(175～328K)基本上保持不变,即MR=(9.1±0.5)%.     

Aluminum oxide formation at Al/La(1-x)Sr(x)MnO3 interface: a computational study for resistance random access memory applications

Resistance random access memory (ReRAM) is emerging as a next-generation nonvolatile memory. One of the most promising materials for the ReRAM application is a composite of a reactive metal [such as aluminum (Al)] and a mixed-valance manganite [such as La(1-x)Ca(x)MnO3 (LCMO) and La(1-x)Sr(x)MnO3 (LSMO)]. One of the current hypotheses regarding the origin of the resistive switching of such systems is a voltage-controlled reversible formation of a high-resistance aluminum oxide (AlO(x)) layer at the Al/LC(S)MO interface through oxygen migration from LC(S)MO. To validate this hypothesis, quantum mechanics (density functional theory) calculations were carried out on an atomistic model of the resistive-switching phenomena at the Al/LSMO interface (the composite systems of Al/LSMO and AlO(x)/LSMO) as well as on the component materials such as Al, AlO(x), LaMnO3, LaMnO(3-delta), La(1-x)Sr(x)MnO3, and La(1-x)Sr(x)MnO(3-delta). The changes in the structure, energy, and electronic structure of these systems during the oxygen vacancy formation in LSMO, the oxygen migration through the Al/LSMO interface, and the AlO(x) formation were investigated.     

Effect of Bi doping on magnetoresistance in La(0.7-x) Bi(x)Sr0.3MnO3

It is shown that upon increasing Bi content (x) in La(0.7-x)Bi(x)Sr0.3MnO3, the ground state changes from ferromagnetic metal (x = 0) to charge ordered antiferromagnetic insulator (x > 0.4). The x = 0.3 compound shows unusual magnetic and magnetoresistive properties: it shows hysteresis in magnetization as a function of temperature, field-induced metamagnetic transition in the paramagnetic state, and nearly 100% magnetoresistance. The magnetoresistance as a function of composition at microH = 5 T increases from 38% for x = 0.05 to 99.6% for x = 0.3 and then drops to 60% for x = 0.4. The unusual behavior of x = 0.3 composition is suggested to coexistence of short-range charge-ordered clusters and ferromagnetic domains. The field-induced melting of these charge-ordered clusters leads to large magnetoresistance effect.     

Studies on Low-Field and Room-Temperature Magnetoresistance in La2/3(Ca1−x Sr x )1/3MnO3 Perovskites

The polycrystalline perovskites La_2/3(Ca_1?x Sr _x )_1/3MnO₃ (x=0,0.2,0.4,0.6,0.8,1) were successfully prepared by a modified method of solid-state reactions, in which the ingredient mixture with ethanol as a liquid milling medium to form suspension was milled by high-energy ball milling for 10 h and sintered in air at 1400 °C for 10 h. The microstructure, electrical transport, and low-field magnetoresistance (LFMR) of the perovskites were investigated to study the room-temperature magnetoresistance (RTMR) behavior. The results reveal that the metal-to-insulator transition temperature (T _MI) increased with increasing doping level x, and the peak values of the magnetoresistance (MR) near T _MI dropped with the more Ca²⁺ substituted. A single-phase La_2/3Ca_1/3MnO₃ showed T _MI at 263 K and the peak MR of 23 % in the applied field of 3 kOe near T _MI. The LFMR effect at room temperature could be obtained by controlling the doping Sr²⁺-substituted Ca²⁺ level. When x=0.29, transition temperature (T _MI) was 305.30 K, and the MR effect was recorded up to 12.9 % at 298.55 K and 3 kOe. Finally, the possible mechanism is discussed.     

Synthesis of Enhanced-Conductivity La0.74Sr0.26MnO3

Strontium-doped lanthanum manganite of composition La_0.74Sr_0.26MnO₃ is synthesized via carbonate coprecipitation from aqueous nitrate solutions. Its 1170-K electrical conductivity is 197 S/cm, which far exceeds the conductivity of the material of the same composition and close in porosity but prepared by the conventional ceramic route. The proposed synthesis procedure makes it possible to prepare La_0.74Sr_0.26MnO₃ with a relative density of 66% (needed for fuel-cell cathodes) and 1170-K conductivity as high as 125 S/cm. La_0.74Sr_0.26MnO₃ has a rhombohedral structure (sp. gr. R c) with lattice parametersa = 0.55000 nm and c = 1.33491 nm.     

La(0.7)Sr(0.3)MnO3 nanoparticles based ultra-high sensitive ammonia chemical sensor

A facile, reliable, reproducible and ultra-high sensitive aqueous ammonia chemical sensor has been fabricated based on the utilization of La(0.7)Sr(0.3)MnO3 nanoparticles (LSMO NPs), as efficient electron mediators, and reported in this paper. The LSMO NPs were prepared by hydrothermal protocol followed by the annealing process and characterized in detail in terms of their mophological, structural and compositional properties. The I-V technique based aqueous ammonia sensor exhibits an ultra-high sensitivity of 494.68 +/- 0.01 microA cm(-2)mM(-1) and very low-detection limit of 0.2 microM with a response time less than 10 s. To the best of our knowledge, this is the first report in which LSMO is used as an efficient electron mediator for the fabrication of aqueous ammonia chemical sensor. Moreover, by comparing the literature, it is confirmed that the fabricated sensor exhibits highest sensitivity towards the detection of aqueous ammonia. This LSMO nanomaterial based research broadens the range of efficient electron mediators utilized for the fabrication of ultra-high sensitive chemical sensors.     

Equilibration kinetics of (La,Sr)MnO3

The present work reports isothermal changes of oxygen non-stoichiometry for the perovskite-type electrode material (La_0.8Sr_0.2)MnO₃ in the temperature range 945–1255 K. A thermogravimetric method was used to monitor the rate of the gas/solid equilibration. For equilibration degrees larger than 0.5, the equilibration kinetic data can be described by a diffusion equation. The determined chemical diffusion coefficient depends essentially on the oxygen partial pressure. Its temperature dependence can be expressed by the following expressions at low and high p(O₂), respectively:

(La0.52Gd0.15)Sr0.33MnO3多晶颗粒的磁热效应

利用非晶态分子合金作前驱体,在相对低的热分解温度800℃、10h成功合成了钙钛矿结构(La0.52Gd0.15)Sr0.33MnO3多晶颗粒.TEM观察表明,颗粒的尺寸范围为100～150nm.研究了多晶颗粒(La0.52Gd0.15)Sr0.33MnO3的居里温度和磁熵变化(MCE).在多晶颗粒(La0.52Gd0.15)Sr0.33MnO3中,居里温度(343K)附近观察到较大的磁熵变和较宽的峰值温度范围,较大的磁熵变来源于磁场条件下的铁磁转变贡献.这些结果表明,该材料是室温附近磁制冷合适的工作物质.     

High-Temperature Thermoelectric Power of The Metal Oxides: La2− x Sr x CuO4 and Bi1− x Sr x MnO3

We have investigated the high-temperature thermoelectric power (TEP) of La_{2? x} Sr _x CuO₄ (0.05?≤?x?≤?0.35) and Bi_{1? x} Sr _x MnO₃ (0.5?≤?x?≤?0.8) up to 700 K. Based on the TEP results we have discussed the phase transitions on each case. In the case of high-T _C cuprates, La_{2? x} Sr _x CuO₄ (0.05?≤?x?≤?0.35), the TEP shows different temperature dependences in three temperature regions. At low temperature, the positive TEP rises showing a broad peak at temperature T _P, which shifts to lower temperature upon Sr doping. Right above T _P, the TEP decreases linearly as temperature increases. At high temperature, TEP deviates from the linear-T dependence at a certain temperature, T _H, showing a saturation behavior. The systematic change of the TEP behavior is discussed in terms of the two-fluids model, which is an intrinsically inhomogeneous state, consisted of bound pairs and independent carriers in the normal state of the high-T _C superconductors. For Bi_{1? x} Sr _x MnO₃ (0.5?≤?x?≤?0.8), the negative TEP is almost temperature-independent in the high temperature regime (T _CO?<?T?<?700 K). Near the charge ordering temperature (T _CO), however, TEP suddenly decreases with decrease of temperature, indicating the suppression of carrier mobility with charge ordering transition. As Bi concentration decreases, T _CO shifts to lower temperature from T _CO?～?520 K for x?=?0.5 to T _CO?～?435 K for x?=?0.8, which suggests that charge ordering is related to the local lattice distortion due to highly polarizable 6s² character of Bi³⁺ ion. In comparison with the resistivity data, the TEP results have been discussed in terms of the carrier localization accompanied by local lattice distortion.     

X-ray Photoelectron Spectra of La0.8 – x Ce x Sr0.2MnO3

La_{0.8 – x} Ce _x Sr_0.2MnO₃ (x= 0–0.3) mixed-conducting manganites are studied by x-ray photoelectron spectroscopy. The effect of A-site Ce substitution on the surface composition of the samples, charge states of the constituent cations, and bonding configuration is analyzed. The binding energies of the constituent elements are determined. The spectral shapes of the O 1s, La 4d, La 3d, Sr 3d, Mn 2p, Ce 4d, and Ce 3d peaks are examined as a function of Ce content.     

A single electric relaxation time in Ba(1-x)Sr(x)TiO(3) nanoparticles at low temperatures

It is shown that the dielectric response of Ba(0.77)Sr(0.23)TiO(3) nanoparticles at temperatures below 200?K has a frequency and temperature dependence in agreement with the Debye theory with a single relaxation time, which exhibits the Arrhenius law. By contrast, at temperatures above 210?K the dielectric response exhibits a broad range of relaxation times characteristic of relaxor-ferroelectrics. We suggest that the single relaxation time at low temperature originates from a frustration effect, in analogy with frustrated antiferromagnetism.     

Comparison of the magnetotransport properties between (La0.7Pb0.3MnO3)1−xAgx and (La0.7Pb0.3MnO3)1−x(Fe2O3)x composites

Ceramic composites of (La_0.7Pb_0.3MnO₃)_1−xAg_x and (La_0.7Pb_0.3MnO₃)_1−x(Fe₂O₃)_x are prepared by using chemical and solid-sate sintering route. The structural and magnetotransport properties are studied for these composites. Ferromagnetism is gradually suppressed with the increase of Ag or Fe₂O₃ mole percentage for both systems. The resistivity decreases significantly for (La_0.7Pb_0.3MnO₃)_1−xAg_x while resistivity increases dramatically for (La_0.7Pb_0.3MnO₃)_1−x(Fe₂O₃)_x. It is suggested that the introduction of Ag into the niche of grain boundaries forms artificial conducting network and improve the carriers to transport. Contrarily, the Fe₂O₃ in (La_0.7Pb_0.3MnO₃)_1−x(Fe₂O₃)_x composite acts as a separation layer between two grains and forms the artificial tunneling junction. However, the enhancement of magnetoresistance has been observed for both systems.     

Magnetocaloric Effect in La1−x Cd x MnO3

Calculations of the magnetocaloric effect for La_1?x Cd _x MnO₃ (LSCM) upon 0.05 T magnetic field variation have been carried out. It is found that magnetic entropy change distribution of the LSCM is much more uniform than that of gadolinium. This feature is desirable for an Ericson-cycle magnetic refrigerator. Furthermore, at different concentrations of Cd, the temperature range between 150 K and room temperature can be covered using the La_1?x Cd _x MnO₃ system. Therefore, the LSCM system is beneficial for manipulating magnetocaloric refrigeration that occurs in various temperatures.     

Catalytic properties and surface states of La1−x (Th,Sr) x CoO3

The surface chemical states of the perovskite-type thorium- or strontium-doped lanthanum cobalt oxides (La_1–x Th _x CoO₃, La_1–x Sr _x CoO₃) have been investigated using X-ray photoelectron spectroscopy. Catalytic oxidations of CO have been also investigated by flow methods. The ionicity between cobalt and lattice oxygen was increased by substituting thorium for lanthanum, and had a peak atx = 0.02. The catalytic activity also had a peak atx = 0.02. However, the ionicity decreased for the case of strontium substitution, and the activity also decreased.     

Terahertz dielectric response of ferroelectric Ba(x)Sr(1-x)TiO3 thin films

Terahertz time-domain spectroscopy has been used to investigate the dielectric and optical properties of ferroelectric Ba(x)Sr(1-x)TiO(3) thin films for nominal x-values of 0.4, 0.6, and 0.8 in the frequency range of 0.3 to 2.5 THz. The ferroelectric thin films were deposited at approximately 700 nm thickness on [001] MgO substrate by pulsed laser deposition. The measured complex dielectric and optical constants were compared with the Cole-Cole relaxation model. The results show that the Cole-Cole relaxation model fits well with the data throughout the frequency range and the dielectric relaxation behavior of ferroelectric Ba(x)Sr(1-x)TiO(3) thin films varies with the films compositions. Among the compositions of Ba(x)Sr(1-x)TiO(3) films with different Ba/Sr ratios, Ba(0.6)Sr(0.4)TiO(3) has the highest dielectric constants and the shortest dielectric relaxation time.     

Structural phase transition at the percolation threshold in epitaxial (La0.7Ca0.3MnO3)1-x:(MgO)x nanocomposite films

'Colossal magnetoresistance' in perovskite manganites such as La0.7Ca0.3MnO3 (LCMO), is caused by the interplay of ferro-paramagnetic, metal-insulator and structural phase transitions. Moreover, different electronic phases can coexist on a very fine scale resulting in percolative electron transport. Here we report on (LCMO)1-x:(MgO)x (0 < x < or = 0.8) epitaxial nano-composite films in which the structure and magnetotransport properties of the manganite nanoclusters can be tuned by the tensile stress originating from the MgO second phase. With increasing x, the lattice of LCMO was found to expand, yielding a bulk tensile strain. The largest colossal magnetoresistance of 10(5)% was observed at the percolation threshold in the conductivity at xc 0.3, which is coupled to a structural phase transition from orthorhombic (0 < x < or 0.1) to rhombohedral R3c structure (0.33 < or = x < or = 0.8). An increase of the Curie temperature for the Rc phase was observed. These results may provide a general method for controlling the magnetotransport properties of manganite-based composite films by appropriate choice of the second phase.