Mo低掺杂La0.67Sr0.33MnO3的磁电特性及室温磁电阻增强

采用固相反应法成功制备了Mo低掺杂La0.67Sr0.33Mn1-xMoxOc(x=0,0.02)单相多晶样品,系统研究了样品的磁性、电输运特性和磁电阻效应.发现Mo低掺杂对La0.67Sr0.33MnO3 的居里温度影响很小,但在La0.67Sr0.33MnO3铁磁基态背景下诱导出团簇自旋玻璃态行为.在低温区(0.1相似文献   

钙钛矿结构La0.7Ca0.3MnO3-δ外延薄膜中氧的扩散

利用脉冲激光淀积方法，我们制备了外延La0．7Ca0．3MnO3-δ薄膜．实验发现，在30Pa氧分压下制备的薄膜晶格中仅有少量氧空位．可以通过高真空条件下的热处理(热处理温度为780℃)减少样品晶格中的氧含量．室温x射线衍射分析表明，在780℃通过高真空热处理的La0．7Ca0．3MnO3-δ外延薄膜结构不发生改变．通过延长高真空热处理时间可以逐渐减少外延薄膜样品晶格中的氧含量，这反映在样品晶格常数有系统变化及样品绝缘体-金属(I-M)转变温度降低．对样品电阻率．温度曲线的拟合表明，样品晶格中的氧含量是均匀的，这与X射线衍射分析的结果是一致的．通过长时间高真空条件下的热处理可以使样品的I-M转变温度低于10K，而短时间氧气氛中的充氧热处理可以使转变温度回复．这说明样品晶格中的氧扩散出和充入是有区别的．在同一外延薄膜样品可以反复通过高真空条件下的高温脱氧热处理和充氧热处理来改变样品中氧含量，这将方便于将薄膜性能与氧含量的关系进行比较、分析和讨论．     

混价锰氧化物超巨磁电阻材料研究进展

综述了钙钛矿结构锰氧化物超巨磁电阻材料的研究进展，主要包括Ｊａｈｎ－Ｔｅｌｅｒ畸变、磁性、电阻率、热电势、霍尔效应、比热、热膨胀、电荷有序、中子衍射和非弹性散射、电子能谱，以及同位素效应等方面     

巨磁电阻材料La0.9Sr0.1 MnO3与半导体Si组成的二极管的整流特性

掺杂锰氧化物La0 .9Sr0 .1 MnO3薄膜被直接沉积在n型硅基片上 ,构成p -n结 .这种p -n结在很宽的温度范围内都有很好的整流特性 .研究结果表明 ,这种p- n结的结电阻对低磁场敏感 ,在 3× 10 - 2 T的磁场下 ,磁电阻可达 70 % .磁电阻的正负依赖于温度 .磁电阻的大小可通过加在p- n结上的电压调节 .     

La1-xCaxMnO3化合物正电子湮没研究

本文对不同Ca含量的La1-xCaxMnO3一系列样品（其中0.05≤x≤0.8）在室温下进行了正电子寿命谱和多普勒展宽谱的测量。结果表明：对应多出现巨磁阻效应的Ca成分范围（0.3≤x≤0.5），正电子的平均寿命τm和多普勒谱S参数随Ca含量的增加而降低，说明在这一成分范围电子局域化程度增强，具有CMR效应的样品La0.7Ca0.3MnO3正电子参数在Tc附近出现与晶格结构不稳定有关的反常变化，反映出电子结构与CMR效应间的关联。     

La0.7-xGdxSr0.3MnO3体系中的相分离

采用固相反应法制备了样品La0.7-xGdxSr0.3MnO3(x=0.00,0.20,0.30,0.40,0.50).通过测量样品的M～T曲线,ESR曲线,研究了La0.7-xGdxSrO0.3MnO3,体系中的相分离现象及相分离与CMR之间关系.结果表明:当x=0.30,0.40时,在PM-FM相变区产生相分离,并且磁电阻MR得到增强.研究结果表明:La0.7-xGdx Sr0.3MnO3中的相分离与Gd在A位的占位情况紧密相关,磁电阻MR的增强是在外磁场作用下,铁磁区扩大顺磁区减小引起的.     

Mo掺杂La0.7Ca0.2Ba0.1Mn1-xMoxO3体系的磁电性质

本文采用固相反应法制备了高价态离子Mo掺杂的La0.7Ca0.2Ba0.1Mn1-xMoxO3(x=0,0.01,…,0.06)多晶样品,研究了Mo掺杂对样品的结构、磁性和磁电阻的影响.X射线衍射谱证实所有样品均为具有正交对称性的钙钛矿结构.零场冷却(ZFC)和加场冷却(FCH=0.01T)下其磁化～温度(M～T)曲线的测量表明样品随温度降低发生了从顺磁(PM)到铁磁(FM)的相变,T相似文献   

La2／3Ca1／3—xSrxMnO3系列锰氧化物材料的Rietveld结构分析

用固相反应法制备了La2/3Ca1/3-xSrxMnO3系列样品，利用Rietveld方法精修晶体结构，结果表明，当x≥0.15时，样品由正交结构（Pnma)转变为六方结构（R－3c)。本文详细介绍了Rietveld精修方法并分析了Sr^2 ,Ca^2 对材料结构的影响。     

La0.5(Ca,Sr)0.5MnO3中各种有序相电子结构和磁性的研究

在低温条件下,随着搀杂浓度的增加,La1-X(Ca,Sr)xMnO3材料在x＝0.5时会发生由铁磁态向反铁磁态绝缘体的相变过程.X-射线和中子衍射实验的研究进一步表明,在x＝0.5时由于自旋、电荷和轨道序的相互作用,形成了La1-x(Ca,Sr)xMnO3材料的Charge-Exchange(CE)型反铁磁结构,因此出现了一些反常的磁学性质.本文对La0.5(Ca,Sr)0.5MnO3材料各种有序相进行了深入的研究.以多带Hubbard模型为基础,我们运用平均场近似和实空间Recursion方法,计算了La0.5(Ca,Sr)0.5MnO3材料中各种轨道序的铁磁态,G-型反铁磁态和CE型反铁磁态的态密度和能量.通过对不同状态之间能量的比较,得出在La0.5(Ca,Sr)0.5MnO3材料中CE-型反铁磁态将是系统最稳定状态的结论.同时对CE-型反铁磁态密度的分析,可以看出由于赝能隙的产生,系统将逐渐向绝缘体转变这一趋势.所有这些结论都和中子衍射和X-射线衍射的实验结果相符合.     

La0．5Er0．2Ca0．3Mno3低温磁电阻效应的实验研究

本文简要介绍对La0．5Er0．2Ca0．3Mno3进行低温电磁测量的实验过程,研究掺杂后复合体系的磁电阻效应。     

Magneto-transport properties of La0.7Ca0.3MnO3/SrTiO3/La0.7Ce0.3MnO3 tunnel junction

Hole-doped rare-earth manganite La_0.7Ca_0.3MnO₃ and the electron-doped manganite La_0.7Ce_0.3MnO₃ both show a metal-insulator transition around 250 K associated with a ferromagnetic transition and colossal magnetoresistance. In an earlier publication we have reported the rectifying characteristic of La_0.7Ca_0.3MnO₃/SrTiO₃/La_0.7Ce_0.3MnO₃ tunnel junction at room temperature, showing that it is possible to fabricate a diode out of the polaronic insulator regime of doped manganites. Here we report the magneto-transport properties of such a tunnel junction above and below the metal-insulator transition. We show, from the large positive magnetoresistance of the tunnel junction at low temperature, that La_0.7Ce_0.3MnO₃ could be a minority spin carrier ferromagnet. The implication of this observation is discussed.     

Activation energy in La0.7Ca0.3MnO3/YBa2Cu3O7-δ / La0.7Ca0.3MnO3 superconducting trilayers

Resistivity vs. temperature measurements on La_0.7Ca_0.3MnO₃/YBa₂Cu₃O_7-δ /La_0.7Ca_0.3MnO₃ (LCMO/YBCO/LCMO) trilayers with different YBCO thickness, were performed in external magnetic field H up to 8 T. By evaluating the activation energy U from the slope of the resistivity Arrhenius plot, a strong depression of U has been observed when decreasing the YBCO layer thickness and the absolute U values appear to be reduced with respect to the values reported in literature in the case of YBCO thin films and YBCO/insulating multilayers. Moreover, a logarithmic U vs. H dependence is shown both in the case of thick and thin YBCO layers indicating the formation of a two dimensional vortex lattice. The experimental data are discussed considering the strong influence of the ferromagnetic LCMO on the superconducting YBCO properties which reduces the effective YBCO thickness more than predicted by the conventional theories.     

Enhanced magnetocaloric effect in Eu-doped La0.7Ca0.3MnO3 compounds

Orthorhombic La_0.7-xEu_xCa_0.3MnO₃ samples (x = 0.04–0.12) with apparent density of ρ = 3.9–4.1 g/cm³ prepared by solid-state reactions have been studied. The analysis of temperature-dependent magnetization for an applied field H = 500 Oe indicated a decrease of the Curie temperature (T_C) from about 225 K for x = 0.04 through 189 K for x = 0.08–146 K for x = 0.12. The magnetocaloric (MC) study upon analyzing M(H, T) data has revealed that the magnetic entropy change around T_C reaches the maximum (|ΔS_max|), which is dependent on both x and H. For an applied field interval of ΔH = 60 kOe, |ΔS_max| values are about 5.88, 4.93, and 4.71 J/kg⋅K for x = 0.04, 0.08, and 0.12, respectively. Though |ΔS_max| decreases with increasing x, relative cooling power (RCP) increases remarkably from 383 J/kg for x = 0.04 to about 428 J/kg for x = 0.08 and 0.12. This is related to the widening of the ferromagnetic-paramagnetic transition region when x increases. Particularly, if combining two compounds with x = 0.04 and 0.08 (or 0.12) as refrigerant blocks for MC applications, a cooling device can work in a large temperature range of 145–270 K, corresponding to RCP ≈ 640 J/kg for H = 60 kOe. M(H) analyses around T_C have proved x = 0.04 exhibiting the mixture of first- and second-order phase transitions while x = 0.08 and 0.12 exhibit a second-order nature. The obtained results show potential applications of Eu-doped La_0.7Ca_0.3MnO₃ materials for magnetic refrigeration below room temperature.     

La0.7Ca0.3Mn1-xCrxO3的激光超声研究

运用激光超声的方法测定了五种不同掺杂的巨磁锰氧化物La0.7Ca0.3Mn1-xCrxO3(0.01≤x≤0.60)室温(300 K)下的超声纵波声速.结合热导率的数据,对声速随掺杂浓度的变化原因进行了深入的分析.得到在顺磁绝缘态Cr3 离子的掺入造成了晶格局部涨落的变化,低掺杂使得晶格局部涨落增强,声子U散射弛豫时间减小.     

ESR study of magnetic structures in La0.7Ca0.3MnO3

Comprehensive measurements of electron spin resonance (ESR) and magnetization of La_0.7Ca_0.3MnO₃ in the ferromagnetic as well as paramagnetic phases were carried out. Quantitative evidences for the inhomogeneous magnetic structure, consisting of ferromagnetic microregions embedded in the antiferromagnetic surrounding near T_c, were found. It is suggested that the microscopic local magnetic structures above and below T_c are qualitatively similar except that the phase below T_c carries long range order between the ferromagnetic microregions whose sizes grow with decreasing temperature.     

A位Eu掺杂对La0.7Ca0.3MnO3电输运的影响

采用传统的固相反应法制备出了高质量的La_(0.7)Ca_(0.3)MnO_3(其中x=0.00,0.05,0.10,0.15,0.20)系列样品,并对其进行了XRD,电阻-温度测量.实验结果表明,随着Eu掺杂量的增加,样品的金属-绝缘体转变温度朝低温附近移动,峰值电阻增加.通过对A位平均离子半径和尺寸无序度的计算,我们发现,由Eu掺杂导致A位平均离子半径〈rA〉减小,尺寸无序度σ2增大,使晶格畸变加剧,从而削弱了eg巡游电子在Mn3 和Mn4 之间的跃迁,同时促进了自旋极化子的形成.我们还对样品电阻-温度曲线的高温部分进行了拟合,对于x=0.00的样品,曲线可以用非绝热近似下的小极化子模型拟合;当掺杂量为x=0.05,0.10,样品的导电机理符合变程跳跃模型;进一步加大掺杂量,当x=0.15,0.20时,样品的电输运行为可以用晶格极化子与自旋极化子共存来解释.     

La0.7Ca0.3MnO3中的双交换作用与小极化子效应

本文研究了La0.7Ca0.3MnO3多晶样品的输运性质、电子顺磁共振(EPR)谱随温度的变化规律.实验发现,材料电阻率、热电势在居里温度附近各形成一个尖锐的峰,而在居里温度上、下有着完全不同的变化规律,EPR谱结果也与之完全吻合.可以认为在居里温度以下双交换作用使eg电子退局域而减小电阻率;而在居里温度以上由Jahn-Teller晶格畸变形成的小极化子使eg电子局域化而增大电阻率.进而证实是双交换作用和小极化子效应共同导致了CMR效应的出现.     

Magnetocaloric effect in nano- and polycrystalline manganite La0.7Ca0.3MnO3

La_0.7Ca_0.3MnO₃ samples were prepared in nano- and polycrystalline forms by the sol–gel and solid state reaction methods, respectively, and structurally characterized by synchrotron X-ray diffraction. The magnetic properties determined by ac susceptibility and dc magnetization measurements are discussed. The magnetocaloric effect in this nanocrystalline manganite is spread over a broader temperature interval than in the polycrystalline case. The relative cooling power of the poly- and nanocrystalline manganites is used to evaluate a possible application for magnetic cooling below room temperature. PACS 75.30.Sg; 75.47.Lx; 77.80.Bh     

Jahn-Teller phonon anomaly and dynamic phase fluctuations in La0.7Ca0.3MnO3

Inelastic neutron scattering was used to study the temperature (T) dependence of the lattice excitations in La 0.7Ca 0.3MnO (3). An optical Jahn-Teller phonon exhibits continuous but anomalous damping with increasing temperature in the ferromagnetic-metallic phase and collapses above the Curie temperature T(C) (240 K). We attribute this anomaly to the growing dynamic phase segregation as T-->T(C), thus providing evidence of local fluctuations associated with the short-range polaron or charge/orbital ordering in the ferromagnetic-metallic state.