Effect of Y-doping on the transport and magnetic properties of La0.5Sr0.5CoO3 and La0.7Sr0.3CoO3

The temperature variation of magnetization, resistivity, and thermoelectric power of undoped and Y-doped La_0.7Sr_0.3CoO₃ and La_0.5Sr_0.5CoO₃ samples have been investigated. Y-doping decreases the magnetization possibly due to the spin-state transition of Co ions. The low temperature conduction in (La_1?y Y _y )_0.7Sr_0.3CoO₃ is consistent with the variable range hopping. With Y-doping, value of the Seebeck coefficient increases as Y-doping decreases bandwidth and increases distortion. Seebeck coefficient value also reflects that the orbital stability increases with Sr concentration.     

La0.5Sr0.5CoO3底电极上的BiNdFeO3薄膜的电性质。

用溶胶．凝胶方法在La0.5Sr0.5CoO3底电极上制备了纯的和10％A位Nd替代的BiFeO3薄膜。对薄膜的结构、形貌和电性质进行了研究。XRD研究表明两薄膜呈随机取向。而SEM测试结果表明，通过Nd替代，构成薄膜的晶粒显著减小，表面出现了针孔。铁电性研究表明，通过Nd替代，薄膜的铁电性得到显著增强，剩余极化强度由1．87μC/cm^2。增加到3．11μC/cm^2。而漏电流特性测试表明，Nd替代有效地限制了BiFeO3薄膜在正向偏压下的漏电流。     

Influence of Ag-doping in La0.5Sr0.5CoO3 on Its Structural and Magnetic Properties

The structural and magnetic properties of the mixed valent cobaltites La_0.5Sr_0.5?x Ag _x CoO₃ (0≤x≤0.15) with perovskite structure have been investigated. X-ray diffraction (XRD) analysis using Rietveld refinement shows that all our samples crystallize in the orthorhombic structure with $R\overline{3}c$ space group. Although the unit cell volume remains almost constant, the Co–O bond length and the Co–O–Co bond angle are sensitive to the Ag addition. All our studied samples undergo a paramagnetic-ferromagnetic transition with decreasing temperature. The zero field cooled (ZFC) and field cooled (FC) magnetization curves at 50 mT of La_0.5Sr_0.5CoO₃ (x=0) sample exhibits thermomagnetic irreversibility indicating frustration and competition of both antiferromagnetic and ferromagnetic interactions. It is suggested that Co³⁺ ions are in the intermediate spin state but Co⁴⁺ ions stay in a mixture of intermediate and high spin states. Ag addition leads to dramatic changes in magnetic properties. An interesting result has been obtained for La_0.5Sr_0.4Ag_0.1CoO₃, where the ZFC and FC curves coincide. For La_0.5Sr_0.35Ag_0.15CoO₃ (x=0.15) sample, the observed thermomagnetic irreversibility is much higher than that observed in x=0 sample. Hysteresis loops were recorded for x=0, 0.05, 0.1, and 0.15. The coercitive field is found to increase with Ag content reaching 0.26 T for x=0.15 whereas the remanent magnetization decreases.     

Magnetocaloric Effect of Perovskite Eu0.5Sr0.5CoO3

Magnetocaloric properties of the Eu_0.5Sr_0.5CoO₃ system near a phase transition from a ferromagnetic to a paramagnetic state are investigated. It is shown that the magnetic entropy change (ΔS _M ) peak spanning over a broad range of temperature leads to a remarkably wide working temperature region, yielding a significant performance in terms of refrigerant efficiency. Moreover, ΔS _M distribution is very uniform, which is desirable for Ericsson-cycle magnetic refrigerator. Eu_0.5Sr_0.5CoO₃ can be used as a working material of an apparatus based on the active magnetic regenerator cycle that cools hydrogen gas.     

离子束溅射La0.5Sr0.5CoO3-δ薄膜的XRD和XPS研究

采用离子束溅射法在氧化钇稳定的氧化锆（YSZ）和铝酸镧（LaAlO3）上溅射RE0.5Sr0．5CoO3-δ薄膜，测试了薄膜的XRD、XPS谱、分析了表面微结构及化学状态。实验表明。随着热处理温度的升高。La0．5Sr0．5CoO3-δ薄膜在YSZ和LaAlO3上生长时有取向增强的趋势，并且晶粒度增大，晶格氧减少。氧空位增加。氧输运性能提高。当热处理温度为750℃时薄膜结晶度最好，晶粒度最大，氧输运性能最好。     

La0.5-xCexBa0.5CoO3化合物的磁性和输运特性

在La0.5Ba0.5CoO3中,系统研究了Ce对La的替代效应.Ce的掺入首先产生了电荷转移效应.材料高温磁化率测量表明,每个Ce原子向Co的3d壳层转移2.86个电子,结果随Ce掺入量增加,材料磁矩成线性下降.另外,随Ce含量增加,材料居里温度单调下降,这是由于稀土离子的尺寸效应.在所研究的温度范围内,所有材料的导电机理都属于极化子的变程跳跃导电.由于电荷转移效应,使材料电阻率随Ce掺入量增加而迅速加大.当La全部被Ce替代后,室温下材料的电阻率提高了五个数量级以上.     

Investigation of Anomalous Magnetic Transition in Pr0.5Sr0.5CoO3

Polycrystalline perovskite cobalt oxides Pr_0.5Sr_0.5CoO₃ were prepared by the sol-gel method. We mainly study the anomalous magnetic transition of Pr_0.5Sr_0.5CoO₃. We report the investigations of polycrystalline samples of the metallic ferromagnetic material Pr_0.5Sr_0.5CoO₃ through measurements of X-ray diffraction, the magnetization, and the resistivity. We found an unusual anomaly around T _A=120 K, much below the ferromagnetic transition (T _C=228 K). Further using the variable temperature X-ray diffraction and electron spin resonance (ESR) measurements, we found that as the temperature goes down, the crystal structure changes obviously at T _A=120 K. We show that this actually results in anomalous magnetic transition.     

La0.5Ba0.5CoO3中的Pr掺杂效应

用固相反应法制备了La0.5-xPrxBa0.5CoO3系列化合物，系统研究了Pr不同含量时材料的磁性和电输运特性，结果表明：Pr掺杂没有改变Co的3d电子的巡游特性。随Pr掺杂量增加，材料的分子磁矩单调下降，其原因是当Pr逐渐取代La时，Co离子的3d电子逐渐由中间自旋态向低自放态转变。随Pr含量增加，材料的居里温度逐渐下降，这是由于稀土离子的尺寸效应，电阻测量表明：在所研究的温度范围内，居里温度以下，所有掺Pr材料的导电机制为热扩散型。居里点以上，导电机制为变程跳跃导电。     

Chemical diffusion of (La,Sr)CoO3 and (La,Sr)FeO3

The present work reports equilibration kinetics for (La_0.8,Sr_0.2)CoO₃(LSC) and (La_0.72,Sr_0.18)FeO₃(LSF) in the temperature range 876–1114 K using a gravimetric method. Chemical diffusion determined in this way that depends on oxygen partial pressure, can be expressed by the following temperature dependence at low and high O₂), respectively, for LSC:

Comparison of Pb(Zr,Ti)O3 capacitors sandwiched with conductive La0.5Sr0.5CoO3 and non-conductive Bi3.25La0.75Ti3O12 layers

Ferroelectric capacitors of Bi_3.25La_0.75Ti₃O₁₂/PbZr_0.4Ti_0.6O₃/Bi_3.25La_0.75Ti₃O₁₂ (BLT/PZT/BLT) and La_0.5Sr_0.5CoO₃/PbZr_0.4Ti_0.6O₃/La_0.5Sr_0.5CoO₃ (LSCO/PZT/LSCO) are fabricated, respectively, on Si substrates in order to study the properties of PZT film sandwiched by conductive oxide and non-conductive oxide layers. It is found that the crystallinity of PZT film grown on LSCO electrode is much better than that on BLT film although the growth temperature of BLT/PZT/BLT is 100 °C higher than that of LSCO/PZT/LSCO. Remanent polarization of LSCO/PZT/LSCO capacitors measured at 10 V is 31.1 μC/cm², which is much higher than the value of 19.8 μC/cm² for BLT/PZT/BLT. Both kinds of the capacitors are fatigue-free up to 10¹⁰ switching cycles. The leakage current density measured at 5 V is about 4.7 × 10^− 6 A/cm² and 1.9 × 10^− 5 A/cm² for BLT/PZT/BLT and LSCO/PZT/LSCO, respectively. The differences for the two kinds of capacitors are discussed in the article.     

Microstructure and the influence of spontaneous strain in LaCoO3, La0.8Sr0.2CoO3 and La0.8Ca0.2CoO3

Transmission Electron Microscopy has been conducted on rhombohedral perovskite oxides with composition LaCoO₃, La_0.8Sr_0.2CoO₃, and La_0.8Ca_0.2CoO₃. Thin foils prepared within the first weeks of sintering showed macroscopic strain and a very high defect density, which included fault-related superlattice structures. Samples prepared several months after sintering, showed no significant macroscopic strain and the main defect was found to be domains due to reflection twinning in the pseudo-cube plane. The behaviour is likely to be related to accomodation of spontaneous strain arising upon cooling of the material from the sintering temperature. The presence of twin domains illustrates the lowering of lattice symmetry as compared to the ideal cubic perovskite structure, and is relevant for explaining recent observations of ferroelastic behaviour of these materials.     

La0.5Sr0.5TiO3 nanopowders prepared by the hydrothermal method

La_0.5Sr_0.5TiO₃ nanopowders were prepared by the hydrothermal method. The influence of processing parameters, including KOH concentration, reaction temperature and reaction time on the obtained products were studied. The structure and morphology of the obtained products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The XRD results show that pure phase La_0.5Sr_0.5TiO₃ nanopowders can be successfully synthesized with 2 M KOH concentration at a low temperature of 220 °C for 24 h. In addition, the product has a plate-like shape with particle sizes in the range of 25–100 nm as estimated by TEM.     

Lao.5Sro.5MnO3薄膜的输运特性和光诱导效应

用磁控溅射方法制备了La0.5Sro 5MnO3薄膜,研究了薄膜的输运特性和光诱导效应.La0.5Sr0 5MnO3薄膜具有温度相变特征:低温为铁磁金属相,高温则为顺磁非导体相,相变温度420 K.为铁磁金属相时,光诱导效应导致薄膜的电阻增大,在360 K电阻变化相对值的最大值约为9.62%.根据双交换作用模型,激光导致顺磁非导体相小极化子的浓度增加,使电阻减小.     

Synthesis of manganite perovskite Ca0.5Sr0.5MnO3 nanoparticles in w/o-microemulsion

In this paper, w/o-microemulsions were employed to produce nanoparticles of the perovskite Ca_0.5Sr_0.5MnO₃, which have a size of approximately 20–50 nm. The procedure was carried out using sodium hydroxide or ammonia as co-precipitation agent. The precursor was transformed to perovskite by calcinations at 580 °C. Nanosized particles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and N₂ adsorption (BET).     

Observation of Spin-Glass Behavior in La0.85Sr0.15CoO3 Single Crystals

The magnetic behavior of La_0.85Sr_0.15CoO₃ single crystals and polycrystals has been subjected to a controversial debate for the last several years; while some groups show evidence for phase separation, others show spin-glass (SG) behavior. Here, we present a comprehensive investigation of the structural, ac susceptibility, and dc magnetization properties of La_0.85Sr_0.15CoO₃ single crystals grown by float zone method. The structural analysis of XRD data by Rietveld refinement reveals the single crystallographic phase. The ac susceptibility results exhibit a frequency dependent peak shift (??2 K) and time-dependent memory effect below the freezing temperature. The characteristic time scale ?? _o calculated from this peak shift is found to be of ??10^?13 s which matches very well with typical values observed for a SG system. Further, the peak shift of the zero-field-cooling curves to lower temperature at higher dc fields is well described by the well-known de Almeida Thouless line, a characteristic of SG behavior. Thus, all our experimental findings confirm the existence of SG behavior in La_0.85Sr_0.15CoO₃ single crystals.     

钙钛矿La0.5Sr0.5Mn1-xCoxO3的制备及磁性研究

采用溶胶-凝胶法制备了La0.5Sr0.5Mn1-xCoxO3(x＝0.1～0.5)的系列样品,通过XRD、SEM和VSM对样品进行了表征.结果表明:随着Co3 离子掺杂量的增加,样品的结构由正交晶系向单斜晶系转变,这种现象的产生是由于Co3 离子半径(r＝0.754nm)与Mn3 离子半径(r＝0.68nm)不匹配造成的.样品形貌为均匀的球形,平均直径50nm左右.样品的居里温度和居里温度附近的磁熵变随Co3 掺杂量的增加先下降后升高,这是由于参加双交换作用的离子数目减少和结构畸变导致的.     

Observation of Higher Suppression of Superconducting Transition Temperature in YBa2Cu3O7??? /La0.5Sr0.5CoO3 as?Compared?to?YBa2Cu3O7??? /La0.7Sr0.3MnO3 Bilayers

We report the experimental investigations of the suppression of superconductivity due to the pair breaking effect induced by ferromagnetic layer in YBa₂Cu₃O_7??? /La_0.5Sr_0.5CoO₃ and YBa₂Cu₃O_7??? /La_0.7Sr_0.3MnO₃ bi-layers fabricated by pulsed laser deposition. The current dependent electrical transport studies in the bilayers exhibit a significant reduction in the superconducting transition temperature in accord to I ^2/3 law as compared to single YBa₂Cu₃O_7??? layer, and moreover the superconducting transition temperature in YBa₂Cu₃O_7??? /La_0.7Sr_0.3MnO₃ bilayer is surprisingly found to be much larger than the YBa₂Cu₃O_7??? /La_0.5Sr_0.5CoO₃. It appears that the current driven from a low spin polarization (?11%) material like La_0.5Sr_0.5CoO₃ can also suppress the superconductivity to a larger extent. This indicates that the degree of spin polarization of the ferromagnetic electrode is not the only criteria to determine the suppression of superconductivity by pair breaking effect in superconductor/ferromagnet hybrid structures; rather the transparency of the interface for the spin polarization, the formation of vortex state due to the stray field of ferromagnetic layer and the ferromagnetic domain patterns might play significant roles to determine such effect.     

Itinerant electron behaviour and long-range ordering in La1−x Sr x CoO3

Electrical resistivity of La_1?x Sr _x CoO₃ is reduced significantly by the substitution of Ni³⁺ ions in place of the cobalt ions. In the La_1?x Sr _x Co_1?y Ni _y O₃ system, for small values ofx(x=0·05) superparamagnetic clusters seem to be formed at lower temperatures (<300 K). The formation of such clusters is not affected by the presence of nickel ions. Whenx is large, the occurrence of long-range ordering seems to depend on the itinerancy of charge carriers.