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.     

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.     

Magnetocaloric Effect in La0.7Cd0.3MnO3, La0.7Ba0.3MnO3, and Nd0.7Sr0.3MnO3

We have based on isothermal magnetization curves to study the magnetocaloric effect (MCE) in fine-grained perovskite manganites of La_0.7Cd_0.3MnO₃ (LCMO), La_0.7Ba_0.3MnO₃ (LBMO), and Nd_0.7Sr_0.3MnO₃ (NSMO) prepared by conventional solid-state reaction. Magnetic measurements were performed on a vibrating sample magnetometer, with a temperature increment of 1.0?K, and the applied field in the range of 0?C1.8?T. Under an applied field of 1.8?T, the maximum magnetic-entropy change obtained for LCMO, LBMO, and NSMO taking place at their Curie temperature are about 2.3, 2.1, and 5.1?J/(kg?K), respectively. The large entropy change in NSMO makes it suitable for magnetic-cooling applications.     

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.     

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.     

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.     

La0.7Sr0.3MnO3 Nanoparticles Synthesized via the (Pechini) Polymeric Precursor Method

In this paper, we report the preparation of La_0.7Sr_0.3MnO₃ colossal magnetoresistance nanoparticles by means of the polymeric precursor method, at a temperature of 650 °C. Rietveld refinement of the X-ray powder diffraction spectra shows that the chemically-synthesized manganite is single-phase with the space group R3C. By using the peak broadening technique and Scherrer’s formula, a grain size of ～15 nm was estimated. The ferromagnetic to paramagnetic transition is sharp with a Curie temperature of T _C ～360 K. In spite of the low annealing temperature, the insulator-metal-like transition temperature (T _p ) is close to T _C . Transport measurements on the prepared samples show a magnetoresistance change of ～10 % at room temperature in a field of 2 T. This high-field MR is probably due to grain-boundary effects.     

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.     

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.     

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.     

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.     

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.     

Ho对La0.7Sr0.3MnO3庞磁阻材料微观结构的影响

运用X射线衍射和透射电子显微分析等方法，探讨了稀土掺杂元素Ho对La0.7Sr0.3MnO3庞磁阻材料微观结构的影响。对于La0.7-xSr0.3MnO3庞磁阻材料，Ho含量较低时（x=0．2），材料主要由典型的菱面体钙钛矿相（La0.7Sr0.3MnO3）和六角非钙钛矿相（HoMnO3）组成，还伴随有少量钙钛矿型正交点阵结构出现。当Ho掺杂量较高时（x=0．6），菱面体体积分数明显降低，而六角相显著增多。另外，稀土Ho的增多引起了菱面体单胞点阵参数（α和c）的缩短，这主要是由于菱面体单胞结构中La/Sr原子比例变小而导致的。同时，Ho的掺杂也降低了材料的晶粒尺寸。     

La0.7Sr0.3MnO3磁性的Preisach模拟

用溶胶-凝胶法制备了多晶La0.7Sr0.3MnO3,块状样品.利用超导量子磁强计测量了样品在不同状态下的场冷却、零场冷却、等温剩磁、热剩磁曲线以及磁滞回线,分析得到了样品的技术磁化参数.用相同的-套参数,利用基于双势阱的Preisach模型再现了样品所有的磁测量曲线,得到了耗散场的大小和分布.     

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.     

Evolvement of atomic diffusion and corresponding effect in La0.7Sr0.3MnO3/SrTiO3 superlattice

Journal of Materials Science: Materials in Electronics - The cation order/disorder is an effective freedom to control manganite properties, and superlattice configuration could be used to design...