Dy的高掺杂对La0.7Sr0.3MnO3体系磁电性质的影响

通过实验研究了La07-xDyxSr03MnO3(x=000，030，040，050，060，070)体系的M_T曲线，M_H曲线，ρ_T曲线和MR_T曲线. 实验结果表明：随着Dy掺杂的增加，体系从长程铁磁有序向自旋团簇玻璃态、反铁磁及 亚铁磁状态转变. x=030样品的M_T曲线上在TN附近出现磁化强度尖峰， x=040时尖峰消失. 对x=050样品，ZFC的M_T曲线随温度增加出现一个谷，随 后在TN处出现峰；而FC的M_T曲线在低温下呈现负磁矩. 对x=060和070 样品，不论FC还是ZFC的M_T曲线都类似于x=050的ZFC的M_T曲线. 高掺杂时的输 运性质在其磁背景下发生异常. 这些奇异现象用Nel的双格子模型并联合M_H回滞曲线 给予很好地解释. 关键词： 磁结构 输运行为 庞磁电阻效应     

A位的Gd掺杂对La0.7Sr0.3MnO3体系磁电性质的影响

通过实验研究了La_0.7-xGd_xSr_0.3MnO₃(x= 0.00，0.10，0.15，0.20，0.30 ，0.40，0.50，0.60，0.70)体系的M-T曲线、M-H曲线、红外光谱、拉曼光谱、ρ-T曲线和M R-T曲线.实验结果表明：随着Gd掺杂的增加，体系从长程铁磁有序向自旋团簇玻璃态和反铁 磁状态转变，高掺杂时的输运性质在其磁背景下发生异常.Gd掺杂引起的磁结构变化和额外 的磁性耦合将导致庞磁电阻效应. 关键词： 磁结构 输运行为 庞磁电阻效应     

La0.67Ca0.33MnO3的磁特性研究

采用高温固态反应法制备了具有钙钛矿结构的锰氧化物La0.67Ca0.33MnO3,利用X射线衍射、超导量子干涉仪等实验技术研究其结构和磁性质。应用标度律对起依赖于磁场的始磁化强度在t→0时的行为用临界指数β随磁场变化进行描述。临界指数β随着磁场的增大而减小,说明在临界点附近,磁场的增大导致自旋有序度的增加。     

(La0.55Gd0.15)Sr0.3MnO3中的磁热效应

本对A位掺杂的超大磁阻材料（La0.55Gd0.15)Sr0.3MnO3的磁热效应进行了研究，通过不同温度下的等温磁化（M-H）曲线的测量和计算，发现伴随铁磁-顺磁相变出现大的磁热效应，额外的磁性交换作用将导致额外的磁熵变化。     

La0.7-xDyxSr0.3MnO3中的相分离和输运行为

通过测量样品的M-T曲线、 M-H曲线、 ESR曲线、ρ-T曲线和MR-T曲线, 研究了Dy掺杂(0.0≤x≤0.30)对La0.7Sr0.3MnO3磁电性质的影响. 实验结果表明 在TC处所有样品都经历了顺磁到铁磁的转变; 当T＜TC时, 掺杂样品进入自旋团簇玻璃态, 低温时显示出反铁磁性; x=0.20时, 样品在TC以上温区发生相分离; Dy掺杂引起的磁结构变化将导致CMR效应.     

Transport properties and anomalous fatigue effect of Ag/Bi0.9La0.lFeO3/La0.7Sr0.3MnO3 heterostructures

The transport properties and fatigue effect of Ag/Bi0.9La0.lFeO3/La0.7Sr0.3MnO3 heterostructures are described. By examining the I-V curves, an anomalous fatigue effect was found and its influences on resistive states were studied. I-V curves combined with C-f spectra were used to directly analyze the transport properties and fatigue effect. Compared to the first I-V cycle state, this structure shows more than one order increase of resistance after 100 cycles of ＂I-V curve training＂. The redistribution of oxygen vacancies in the depletion layer of Ag/Bi0.9La0.lFeO3 is believed to be responsible for the different resistance mechanisms and tenfold magnitude drop in resistance. The resistive switching is understood to be caused by electric field-induced carrier trapping and detrapping, which changes the depletion layer thickness at the Ag/Bi0.9La0.lFeO3 interface.     

Polarized Raman scattering in single crystals of Nd0.7Sr0.3MnO3

We report polarized Raman scattering in single crystals of Nd_0.7Sr_0.3MnO₃. The temperature dependence of the MnO₆ octahedral bending and stretching modes observed in the XX spectra points to the existence of local lattice distortions, possibly polarons. The XY spectra have been analyzed using a collision-dominated model, which allows the extraction of the carrier scattering rate.     

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.     

La0.67Pb0.33MnO3的磁性及输运特性

利用固相反应烧结法制备了La067Pb033MnO3单相多晶样品.研究了其结构、磁性及输运特性.结果表明，样品呈菱面相晶体结构，空间群为R3C，居里温度TC(=353K)非常接近TMI(=360K).在居里温度附近，发生铁磁-顺磁转变，导电特性由金属特征向半导体特征过渡.磁电阻在居里点达到极大值.当H=16T时，磁电阻的极大值为145%；当H=08T时，磁电阻的极大值为9%.输运性质表明，TTMI时 关键词： 磁电阻 输运特性 磁极化子 钙钛石     

用化学镀法制备的铜包覆La0.7Ca0.3MnO3复合体系的巨磁电阻效应

用化学镀法制备了铜包覆的La_(0.7)Ca_(0.3)MnO_3(LCMO)颗粒.这些粉末压片之后在氩气中煅烧,形成铁磁-金属型结构,得到被铜均匀包覆的LCMO复合材料.利用扫描电子显微镜(SEM)观察了LCMO被铜包覆的形貌.研究了不同烧结温度下样品的电输运性质和在0.3T外加磁场下的磁电阻行为.结果表明LCMO在通过包覆铜进行表面修饰后,样品的绝缘体-金属的转变温度(Tp)往高温移动接近室温,室温附近的磁电阻值也得到显著提高.     

Transport properties in La<Subscript>0.7−<Emphasis Type="Italic">x</Emphasis></Subscript>Gd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> system

The influence of Gd doping at La-site on the electrical transport properties and the colossal magnetoresistance of La_0.7−x Gd _x Sr_0.3MnO₃ (x=0.00, 0.10, 0.15, 0.20, 0.30, 0.40, 0.50, 0.60, and 0.70) is studied. The experimental results indicate that the transport properties exhibit abnormal behavior under high doping condition. Forx=0.50, we find that a transition from metal to insulator occurs after the occurrence of insulator-metal transition nearT _c, which seldom occurs in ABO₃ structure. For samplesx=0.60 and 0.70, it exhibits insulator behavior far aboveT _c. These abnormal behaviors are attributed to different magnetic background, i.e. the system undergoes a transition from long range ferromagnetic order to the cluster-spin glass state and further to antiferromagnetic order.     

La0.7-xCexCa0.3MnO3低场磁电阻的晶格效应

以稀土元素铈部分替换钙钛矿结构亚锰酸盐中的镧,使A位离子半径减小.由此在晶格中产生的内压将影响到B位d轨道与O位p轨道之间交叠形成的电子活性能带.本文以固相反应法制备了多晶La0.7-xCexCa0.3MnO3样品(x=0,0.05,0.10,0.20),测量并研究了电阻、低场磁电阻与温度(77K～300K)之关系.实验表明,随Ce替代量的增加,磁电阻(MR)峰值无显著变化,而MR峰值出现的温度Tp则递减.关于"减小可能导致Mn-O-Mn键角减小而最终使Tc降低"的见解可解释该实验结果.     

对靶溅射技术制备的La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜中Tc的降低

本文用对靶溅射技术制备了La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜.与YBCO单层薄膜相比,由于超导/铁磁系统中的磁性邻近效应,三层薄膜表现出较低的超导转变温度.薄膜的R～T测量曲线显示出超磁阻(CMR)效应和超导转变,预示着超导和铁磁特性共存于LCMO/YBCO/LCMO三文治结构.     

Direct injection of spin-polarized carriers across YBa2Cu3O7-δ-La0.3Ca0.7MnO3 interface at 77 K

We report here injection of spin-polarized carriers from a half-metallic La_0.3Ca_0.7MnO₃ (LCMO) colossal magnetoresistive (CMR) thin film into a high-temperature superconducting YBa₂Cu₃O_7-δ (YBCO) thin film studied using a micro-bridge. The LCMO and YBCO films were grown on 〈100〉 LaAlO₃ (LAO) substrate sequentially using pulsed laser deposition (PLD). I-V measurements carried out at 77 K show that while normal critical current, I _c ⁿ , of the micro-bridge is 80 mA, the critical current, I _c ^p , through the micro-bridge when injected from the CMR layer is 38 mA. This clearly shows that spin-polarized quasiparticles injected from the the CMR layer into the YBCO layer suppress the critical current of the superconductor via the pair-breaking phenomena.     

Hyperfine Fields at the Cd Site in La0.67Cd0.25MnO3 CMR Manganites

Although Cd and Ca ions have the same valence and cation size, their incorporation into vacancy-doped La manganites induce different properties. While the incorporation of Ca leads to high Tc up to 250 K and induces a metallic-like behaviour, the incorporation of Cd severely reduces Tc and promotes insulator-like behaviour. In this work, the Cd hyperfine fields have been measured with the Perturbed Angular Correlations (PAC) technique after implantation and annealing of ^111m Cd in La–Cd–MnO₃ samples. The PAC results are compared with measurements of the resistivity and magnetization performed on the same samples. The mixed La and Mn site Cd occupancy is suggested as a possibility to explain the properties of the La–Cd–MnO₃ system.     

纳米多晶La0.7Sr0.3MnO3-δ的输运性质和磁电阻效应研究

利用溶胶-凝胶法制备了纳米多晶La0.7Sr0.3MnO3-δ(LSM)块体样品.详细研究了在不同烧结温度下的ISM样品电阻率随测量温度的变化关系和磁电阻效应.随着测量温度从室温降低,电阻率ρ都在250K附近存在最大值,低于该温度后,样品表现为金属导电特性,随后在50K左右存在一极小值.即随着温度从50K左右降低到4.2K,ρ反而逐渐升高,表现为绝缘体性的导电特性.研究表明,在低温下(＜50K),ρ随温度降低而升高的现象与隧穿效应的理论模型(lnρT1/1符合得很好,表明这种现象是由于传导电子在通过邻近LSM晶粒间表面/界面层时的隧道效应所致.而在50-250K的温度范围内,其电阻率与T2成正比,表现为LSM本征的金属导电特性.因此这种低温下电阻率的极小值现象来源于隧穿效应和LSM晶粒本征的金属导电特性的相互竞争.本文还详细研究了相应的隧道磁电阻效应.     

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.     

A位的Sm掺杂对La0.67Sr0.33MnO3体系磁电性质的影响

通过实验研究了La067-xSmxSr033MnO3(x=000，010，020，030)体系的M_T曲线、ESR曲线、红外光谱、拉曼光谱、ρ_T曲线和MR_T曲线. 实验结果表明：随着Sm掺杂的增加，体系从长程铁磁有序向自旋团簇玻璃态和反铁磁状态转变，Sm掺杂引起的磁结构变化和额外磁性耦合将导致CMR效应. 关键词： 磁结构 输运行为 庞磁电阻效应     

多晶La0.7Sr0.3MnO3的低温输运性质和磁电阻效应

详细研究了由纳米晶粒组成的块体多晶La0.7Sr0.3MnO3(LSM)的电阻率和磁电阻效应,以及它们的温度依赖性.随着温度从室温降低,电阻率(ρ)在250K附近存在一最大值,低于该温度后,样品表现为金属导电特性,随后在50K附近存在一极小值.也就是说在低于50K的温度范围内,随着温度降低ρ反而升高,表现为绝缘体性的导电特性.经研究发现,这种随温度降低ρ反而增加的现象与隧穿效应的理论模型(lnρ∝T-1/2)符合得很好.这种现象是由于传导电子在通过邻近LSM晶粒间表面/界面层时的隧道效应所导致的.这种低温下电阻率的极小值现象来源于隧穿效应和LSM晶粒本征的金属导电特性的相互竞争.还详细研究了相应的隧道磁电阻效应的温度依赖性.