Synthesis of Nd0.7Ba0.3MnO3 via Microwave Processing

Nd_0.7Ba_0.3MnO₃ is synthesized via microwave processing of neodymium, barium, and manganese nitrates in solution. It is shown that microwave processing allows the temperature and duration of Nd_0.7Ba_0.3MnO₃ synthesis to be reduced as compared to the conventional ceramic route. The physicochemical properties of the resultant material compare well with those of samples prepared by ceramic processing techniques.     

Low-field magnetoresistance in La0.7Sr0.3MnO3/BaTiO3 composites

A manganite composite series of (1 ? x)La_0.7Sr_0.3MnO₃/xBaTiO₃ (x = 0, 0.06, 0.12, and 0.18) has been fabricated by solid-state reaction combined with a high-energy mechanical milling method. Experimental results revealed that the insulator–metal transition temperature was shifted towards lower temperatures, and resistivity increases with increasing BaTiO₃ content in (1 ? x)La_0.7Sr_0.3MnO₃/xBaTiO₃. Meanwhile, the ferromagnetic–paramagnetic transition temperature was almost unchanged. The increase in magnetoresistance was observed in the all composites at whole measurement temperatures under an applied magnetic field of 3 kOe. Here, temperature dependences of magnetoresistance display a Curie–Weiss law-like behavior. The nature of this phenomenon is explained in detail.     

Synthesis of Different-Sized La0.7Sr0.3MnO3 Nanoparticles via a Polyacrylamide Gel Route and Their Magnetocaloric Properties

In this work, a polyacrylamide gel route was used to synthesize La_0.7Sr_0.3MnO₃ nanoparticles. It is shown that single-phase La_0.7Sr_0.3MnO₃ nanoparticles with a rhombohedral perovskite structure can be prepared at a calcination temperature of 600 °C when separately using citric acid, tartaric acid, acetic acid, oxalic acid, and EDTA as the chelating agent. The particles prepared using the five chelating agents are nearly spherical in shape with a narrow diameter distribution, and have an average size of 24, 28, 36, 38, and 50 nm, respectively. Magnetic properties of the 24 and 50 nm samples (termed as samples S1 and S5, respectively) were investigated. It is revealed that the samples have a similar ferromagnetic Curie temperature T _C , and exhibit a typical second-order magnetic phase transition near T _C . The maximum magnetic entropy change at an applied magnetic field of 15 kOe is obtained to be about 0.71 and 0.74 J?kg^?1?K^?1 for the 24 and 50 nm samples, respectively.     

高能球磨法制备Nd0.7Sr0.3MnO3氧化物

经4h的高能球磨，钙钛矿结构氧化物Nd0.7Sr0.3Mn03完全合成，合成得到的样品颗粒尺寸范围在60～120nm，且大多数呈椭球状。继续球磨8h，出现了非晶无序相。然而，令人奇怪的是，再球磨非晶态样品6h，Nd0.7Sr0.3MnO3钙钛矿结构相又一次被合成。电子衍射结果也反应了初始原料随球磨时间的延长从多晶到非晶再到多晶的系列变化过程。     

Simulation of Magnetocaloric Effect in La0.7Ca0.3MnO3 Ceramics Fabricated by Fast Sintering Process

The enhanced low-field magnetocaloric effect (MCE) is simulated for La_0.7Ca_0.3MnO₃ (LCMO) ceramics that were fabricated by fast sintering process with different Al₂O₃ contents. It is shown that LCMO exhibits magnetic entropy change (ΔS _M ) much more uniform than that of gadolinium. The results show that the peak in the MCE at the ferromagnetic to paramagnetic phase transition is improved as the sintering temperature decreases. Furthermore, the samples open up a new way in which to tune the intrinsic properties of mixed-valence manganites. Through these results, LCMO has some potential applications for magnetic refrigerants in an extended high-temperature range. It is suggested that the fast sintering process with different Al₂O₃ contents for LCMO is an efficient way to obtain a working material of an apparatus based on the active magnetic regenerator cycle that cools hydrogen gas.     

Tuning magnetic domain structure in nanoscale La0.7Sr0.3MnO3 islands

The realization of spin-based devices requires high density, ordered arrays of magnetic materials with a high degree of spin polarization at surfaces. We have synthesized, for the first time, highly spin polarized complex magnetic oxide nanostructures embedded in a paramagnetic matrix by electron beam lithography and ion implantation. Imaging the magnetic domains with X-ray photoemission electron microscopy and magnetic force microscopy reveals a delicate balance between magnetocrystalline, magnetoelastic, and magnetostatic energies that can be tuned by the choice of SrTiO3 substrate orientation, film thickness, island size, and island shape.     

Fabrication and Magnetic Properties of Electrospun La0.7Sr0.3MnO3 Nanostructures

La_0.7Sr_0.3MnO₃ (abbreviated as LSMO) nanostructures were fabricated by a simple electrospinning using a solution that contained poly(vinylpyrrolidone) (PVP), lanthanum, strontium and manganese nitrates. The LSMO nanostructures were successfully obtained from calcination of the as-spun LSMO/PVP composite nanofibers at 500–900 °C in air for 7 h. The as-spun and calcined LSMO/PVP composite nanofibers were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Analysis of phase composition by XRD revealed that all the calcined samples have a single rhombohedral LSMO phase. The SEM results showed that the crystal structure and morphology of the LSMO nanofibers were affected by the calcination temperature. Crystallite size of the nanoparticles contained in nanofibers increased with an increase in calcination temperature. The specific saturation magnetization (M _s ) values were obtained to be 1.23, 28.61, and 40.52 emu/g at 10 kOe for the LSMO samples calcined respectively at 500, 700, and 900 °C. It is found that the increase of the tendency of M _s is consistent with the enhancement of crystallinity, and the values of M _s for the calcined LSMO samples were observed to increase with increasing crystallite size. This increase in M _s for the calcined LSMO samples with increasing crystallite size may be explained by considering a magnetic domain of the samples.     

Exchange coupling and exchange bias in La0.7Sr0.3MnO3-SrRuO3 superlattices

La(0.7)Sr(0.3)MnO(3)-SrRuO(3) superlattices with and without nanometrically thin SrTiO(3), BaTiO(3) and Ba(0.7)Sr(0.3)TiO(3) interlayers were grown by pulsed laser deposition. Transmission electron microscopy studies showed coherent growth of La(0.7)Sr(0.3)MnO(3), SrRuO(3) and SrTiO(3) layers with atomically sharp interfaces, even if individual layers were as thin as one or two unit cells. In contrast, misfit dislocations and unit cell high interfacial steps were observed at the interfaces between BaTiO(3) and one of the ferromagnetic layers. The presence of the interlayers as well as these extended defects had a significant influence on the magnetic properties of the superlattices, especially on the antiferromagnetic interlayer exchange coupling between the La(0.7)Sr(0.3)MnO(3) and SrRuO(3) layers and the exchange biasing. Surprisingly, exchange biasing was found to increase with decreasing strength of the antiferromagnetic interlayer exchange coupling. This was explained by different magnetization reversal mechanisms acting in the regimes of strong and weak interlayer exchange coupling.     

Microwave synthesis of magnetoresistive La0.7Ba0.3MnO3 using inorganic precursors

We report here the use of inorganic precursors as good microwave absorbers, by virtue of its polarity and high dielectric constant, to synthesize high temperature stable rare earth manganite, La_0.7Ba_0.3MnO₃. Compared to other wet chemical methods, the oxides prepared by microwave assisted route give fine particle oxides (<30 nm) with effective BET surface area of 25 m²/g. Two factors contribute to the rapid synthesis of these high temperature phases. Firstly, the dielectric constant of the precursors employed increases the microwave power loss in the material and this leads to a local heating effect. Secondly decomposition of these precursors, lead to formation of finely divided oxides often accompanied by an exothermic reaction which provides the needed energy to effect the formation of the product. This method offers a new approach to employ inorganic precursors as starting materials to realize fast and effective reaction in microwave assisted material synthesis.     

La空位掺杂对La-Ca-Sr-MnO_3磁热性质的影响

用溶胶-凝胶法制备了不同空位掺杂的系列样品La_(0.7-x)Ca_(0.28)Sr_(0.02)MnO_3,研究了La~(3+)空位浓度对样品居里温度和磁热效应的影响。结果表明,在La位掺入少量空位(x0.06),可以实现将样品的居里温度有效调整至室温,同时也促进了样品磁熵变的提高。当La空位掺杂x=0.06时,与未空位掺杂样品相比,居里温度由227 K提高到264 K,近室温,其磁熵变值为3.01 J·kg-1·K-1(外加磁场1 T)。该系列样品在室温附近,较低磁场下,有较强的磁制冷能力。     

纳米多晶La0. 7Sr0.3MnO3磁性相变临界行为研究

用溶胶一凝胶方法制备了纳米多晶La0.7Sr0.3MnO3样品.测量了不同温度下烧结的样品的零场冷却交流磁化率与温度和直流磁场的依赖关系.通过对铁磁-顺磁转变点附近临界峰的分析,得到973K烧结的多晶样品居里温度为312.1K±0.2K,临界指数为:δ=3.040,γ=1.007,β=0.493;1173K烧结的多晶样品居里温度为331.7K±0.1K,临界指数分别为:δ=2.950,γ=0.993,β=0.508.两组数据均与平均场理论预言结果一致,表明纳米多晶La0.7Sr0.3MnO3样品在磁性相变点附近存在长程相互作用.     

Emulsion processing of SOFC materials Ca0.3La0.7CrO3, Sr0.16La0.84CrO3, and Sr0.2La0.8MnO3

A novel preparation route to the perovskite materials Ca_0.3La_0.7CrO₃, Sr_0.16La_0.84CrO₃, and Sr_0.2La_0.8MnO₃ is described. The method produces the phase pure perovskite phases after calcination at 700°C for 2 hours. The powders produced are unagglomerated, and consist of hollow spherical particles 0.15 m in diameter. EDX has shown that the careful control of reaction conditions is vital to control the phase composition, and that small changes in stoichiometry result in the production of unsinterable powder.     

Low-Temperature Synthesis of Ultrafine La0.7Sr0.3MnO3 Powder via Chelate Homogenization

Ultrafine La_0.7Sr_0.3MnO₃ powders were prepared via homogenization in chelate solutions, followed by the calcination of solid precursors at 700°C in air or oxygen, and their phase composition and average particle size were determined. The solid precursors were obtained from a solution of polynuclear La, Sr, and Mn chelates (diethylenetriaminepentaacetates) by three procedures: (1) gelation of the solution, followed by air drying of the resultant gel; (2) gelation followed by microwave dehydration; and (3) microwave dehydration of the solution. The results demonstrate that the way in which the chelate solution is converted into solid foams has little effect on the phase composition and particle size of the powders. At the same time, the phase composition and particle size of the reaction products depend on the calcination atmosphere (air or oxygen). Calcination in oxygen (700°C, 10 h) ensures the preparation of phase-pure La_0.7Sr_0.3MnO₃ powders, with an average particle size of 30 nm, from the three precursors.     

Electrical Resistivity of La0.7Sr0.3MnO3 Ceramics Prepared via Chelate Homogenization

La_0.7Sr_0.3MnO₃ ceramics are prepared from powders produced via gelation and/or microwave processing of solutions of polynuclear chelates (La, Sr, and Mn diethylenetriaminepentaacetates), and their electrical resistivity is measured as a function of temperature. As the sintering temperature is raised from 800 to 1100°C, the average grain size of the ceramics, evaluated by the Debye–Scherrer method, increases by about a factor of 2.5 and their resistivity drops by about two orders of magnitude. The effect of the sintering temperature on the average grain size depends very little on the preparation procedure. For some of the samples, the room-temperature weak-field magnetoresistance is determined.     

渠道火花烧蚀法制备La0.7Sr0.3MnO3薄膜

采用新型渠道火花烧蚀技术在LaAlO3(001)基片上生长了La0.7S0.3MnO3(LSMO)薄膜.X射线衍射对样品结构的分析表明,制备的LSMO薄膜具有c轴取向生长的特点,薄膜与基片间因晶格不匹配而受面内应力挤压,发生弛豫而出现两相.在室温下采用振动样品磁强计测试样品的面内方向磁滞回线,表明制备的LSMO样品具有软磁性,矫顽力Hc=13 Oe.通过标准四探针法测量了LSMO薄膜的室温薄膜电阻与外加磁场的关系,得知零场电阻率ρ(0)=19.4 mΩ·cm,室温下4800 Oe外场作用下的磁电阻变化率为2.25%,对此用双交换作用机制定性地加以了解释.     

Hall-effect studies of Pr0.7Sr0.3MnO3

Hall-effect measurements of the perovskite manganate Pr_0.7Sr_0.3MnO₃ have been obtained at a magnetic field of 0.6 T in the temperature range 77–400 K. The results show peculiarities in the Hall-coefficient behavior. This is likely to be due to the coexistence of charge ordering and ferromagnetism. Obviously, the transformation between different magnetic phases with increasing temperature is significant for the physical properties, and influences the current transport in the compound.     

Effect of strain and grain boundaries on dielectric properties in La0.7Sr0.3MnO3 thin films

High dielectric constant and its dependence on structural strain and grain boundaries (GB) in La_0.7Sr_0.3MnO₃ (LSMO) thin films are reported. X-ray diffraction, magnetization, and magneto-transport measurements of the LSMO films, made by pulsed laser deposition on two different substrates—MgO and SrTiO₃ (STO), were compared to co-relate magnetic properties with dielectric properties. At room temperature, in the ferromagnetic phase of LSMO, a high dielectric constant (6 × 10⁴) was observed up to 100 kHz frequency for the films on MgO, with polycrystalline properties and more high-angle GB related defects, while for the films on STO, with single-crystalline properties but strained unit cells, high dielectric constant (≈10⁴) was observed until 1 MHz frequency. Also, a large dielectric relaxation time with significant broadening from the Debye single-dielectric relaxation model has been observed in samples with higher GB defects. Impedance spectroscopy further shows that large dielectric constant of the single-crystalline, strained LSMO film is intrinsic in nature while that in the polycrystalline films are mainly extrinsic due to higher amount of GBs. The presence of high dielectric constant value until high frequency range rules out the possibility of “apparent giant dielectric constant” arising from the sample-electrode interface. Coexistence of ferromagnetism and high dielectric constant can be very useful for different microelectronic applications.