用内耗技术研究La0.7Pb0.3MnO3巨磁电阻材料

用低频扭摆法在多功能内耗仪上测量了巨磁电阻材料La_0.7Pb_O.3MnO₃(LPMO)的温度内耗谱和弹性模量.结果表明,内耗峰位与测量频率无关,并且峰高与频率成反比,弹性模量在对应的内耗峰处有明显的转折,内耗峰表现为相变峰的特征.结合电阻和磁化率的测试,解释了内耗和电阻-温度曲线的双峰现象,高温内耗峰和高温电阻峰与居里温度有很好的对应,来源于顺磁半导体向铁磁金属的转变,低温内耗峰和磁化率的单调下降来源于铁磁相分离过程,而较大的低温电阻峰部分来源于相分离过程.     

La2/3(Ca,Ba)1/3MnO3系统的顺磁共振研究

报道了对La_2/3Ca_1/3MnO₃和La_2/3Ba_1/3MnO₃系统不同温度下的电子顺磁共振(EPR)实验研究,当温度高于某一特征温度Tmin时,共振线宽随温度降低而线性变窄;当温度低于Tmin时,共振线宽表现出反常展宽.基于“瓶颈(bottleneck)”自旋-晶格弛豫模型我们对这种行为进行了讨论,并且指出A位离子平均半径强烈影响着自旋-晶格交换作用和自旋-自旋相互作用的强弱.     

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.     

La0.5Ca0.5MnO3中Ca位上Ba掺杂效应研究

用固态反应法制备了具有单相结构的La_0.5Ca_0.5-xBa_xMnO₃系列样品,其结构随Ba含量增加由立方晶系转变为典型的正交晶系．零场下输运性质实验表明,在较低掺杂量时,系统在整个温区显示半导体型导电特征,而当掺杂量超过0.14时,系统发生由半导体到金属的转变,且转变的温度随掺杂量增加而移向高温瑞．基于对实验结果的分析,就Ca位上Ba掺杂所起的作用进行了初步探讨．     

用内耗技术研究La0.7Pb0.3MnO3巨磁电阻材料

用低频扭摆法在多功能内耗仪上测量了巨磁电阻材料La0.7Pb0.3MnO3(LPMO)的温度内耗谱和弹性模量。结果表明，内耗峰位与测量频率无关，并且峰高与频率成反比，弹性模量在对应的内耗峰处有明显的转折，内耗峰表现为相变峰的特征，结合电阻和磁化率的测试，解释了内耗和电阻-温度曲线的双峰现象，高温内耗峰和高温电阻峰与居里温度有很好的对应，来源于顺磁半导体向铁磁金属的转变，低温内耗峰和磁化率的单调下降来源于铁磁相分离过程。而较大的低温电阻峰部分来源于相分离过程。     

La2/3Sr1/3MnO3/ZnO混合物薄膜的磁电阻和伏安特性研究

利用脉冲激光沉积的方法在Si（100）氧化成SiO₂的基片上制备了(La_2/3Sr_1/3MnO₃)_x/(ZnO)_1-x混合物薄膜,研究了薄膜的磁电阻和伏安特性. X射线衍射分析表明,除了衬底SiO₂的衍射峰以外,分别出现了La_2/3Sr_1/3MnO₃（101）的衍射峰和ZnO（002）的衍射峰,且它们形成了两相共存体系. 实验表明：x=0.3的混合物薄膜表现为半导体导电特性,而x=0.4的样品则出现了典型的金属绝缘相变. 所制备的样品表现出了低场磁电阻效应和非线性伏安特性. 在0.7T磁场的作用下,x=0.3的样品在温度为60K时取得的最大磁电阻值为28.8%. 通过对伏安关系拟合表明,在La_2/3Sr_1/3MnO₃和ZnO颗粒之间存在一定的耗尽层,且产生了界面缺陷态.     

La2/3(Ca,Ba)1/3MnO3系统的顺磁共振研究

报道了对La2/3Ca1/3MnO3和La2/3Ba1/3MnO3 同温度下的电子顺磁共振（EPR）实验研究，当温度高于某一特征温度Tmin时，共振线宽随温度降低而线性变窄；当温度低于Tmin时，共振线宽表现出反常展宽。基于“瓶颈（bottleneck)”自旋－晶格弛豫模型我们对这种行为进行了讨论，并且指出A位离子平均半径强烈影响着自旋－晶格交换作用和自旋－自旋相互作用的强弱。     

La—Gd—Ca—Mn—O的磁性、电性和磁电阻效应

在La0.67C0.33MnO3中进行了掺Gd研究，结果发现，经1400℃烧结的样品，获得了最佳的磁电阻效应，随掺Gd量增加，材料的相变温度逐渐下降，对应的降值的电阻率大幅度增加，居里温度逐渐下降，磁电阻比明提高，掺入11％的Gd后，可以使磁电阻比提高一个数量极，这些变化可以用晶格效应来解释。     

Ba缺位型稀土氧化物La0.65BaxMnO3的磁电特性分析

制备了Ba缺位型稀土氧化物La0.65BaxMnO3材料,发现随着Ba缺位量在La0.65BaxMnO3中的增加,材料由高晶体对称性向低晶体对称性转移.x＞0.30时材料保持立方晶体结构,当x≤0.25时材料呈现菱方晶体结构.在La0.65BaxMnO3中,随Ba成分x从0.35减少到0.33,居里温度有一个小峰,室温电阻率有一小谷;随x进一步减小(x≤0.30),居里温度随之下降,室温电阻率上升.极化子电阻模型分析表明在La0.65BaxMnO3中,电子活动能力(x=0.35)＞电子活动能力(x=0.25)＞电子活动能力(x=0.20).这与室温电阻率的Ba成分依赖关系吻合.我们还基于迪尼模型计算了La0.65BaxMnO3中电阻随温度的变化关系曲线.计算结果与实验数据符合的较好.计算结果还表明:晶格效应、电-声作用、载流子-晶格(缺陷)耦合作用在解释稀土锰基氧化物材料中的电输运现象方面必须予以考虑.     

La0.7Ca0.3-xSrxMnO3薄膜的电输运特性研究

采用溶胶-凝胶法制备La_0.7Ca_0.3-xSr_xMnO₃(LCSMO)薄膜, 探讨掺杂对结构、磁性能与电输运特性的影响机制。从X射线衍射(XRD)结果来看, 所有薄膜均具有典型钙钛矿结构。LCSMO薄膜的居里温度(T_C)和金属绝缘体转变温度(T_MI)均随Sr掺杂浓度增加而单调增加。总体看来, 当x ≤0.05时, LCSMO薄膜磁阻率类似于窄带系LCMO 系材料, 在T_MI周围较宽的温度区间内存在相分离, 而相分离过程中多相共存的无序状态是该类材料庞磁阻效应的主要来源。对特定温度下的磁阻率随磁场的变化进行分析, 当温度低于T_MI时, 磁阻率随磁场变化出现双梯度, 低磁场时晶界隧穿效应起主导, 该部分效应对磁场特别敏感, 高磁场时磁阻率主要来源于磁场对自旋波动的压制; 当温度接近或高于T_MI时, 晶界隧穿效应逐渐消失, 磁阻率随磁场线性变化, 磁场对自旋波动的压制起主导作用。     

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.     

X-ray Photoelectron Spectra of La0.7Ca0.3MnO3 and La0.7Ca0.3Mn0.97Cu0.03O3 Perovskite Oxides

X-ray photoelectron spectroscopy was used to identify the oxidation states of cations in La_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3Mn_0.97Cu_0.03O₃ perovskite oxides, to determine the surface composition of the samples, and to assess the effect of the Cu dopant on the electronic spectrum of the material. The binding energies of the La 4d, Ca 2p, Mn 2p, and O 1s core levels were determined, and the effect of the Cu dopant on the shape of the La 4d and O 1s peaks was analyzed.     

Microstructural Characteristics of La0.7Ca0.3MnO3 Films and Multilayers

1. IntroductionEver since discovery of colossal magnetoresistance(CMR) in perovskite type Rel--.A.MnO3(Re=lajnthanides, A=alkaline earth elemellts), therehas been enormous illterest in these materials both inthe bulk and the filmsll~5]. A prominellt feature ofthese materials is a large maximum in the resistivitynear the ferromagnetic transition temperature (To).The resistance is dramatically decreased when an external magnetic field is applied, which leads to a verylarge (colossal) magneto…     

Electro-magnetic transport behavior of La0.7Ca0.3MnO3/SnO2 composites

The composites of (1 ? x)La_0.7Ca_0.3MnO₃ (LCMO) + xSnO₂ (x = 0.01, 0.05, 0.10, 0.30, 0.50, 0.60, 0.65 and 0.70) were synthesized by conventional solid-state reaction method. The results of X-ray diffraction (XRD) and scanning electronic microscopy (SEM) indicate that SnO₂ and LCMO coexist in the composites and SnO₂ mainly segregates at the grain boundaries of LCMO, which are in accordance with the results of the magnetic measurements. The detailed electrical characterizations for all the samples showed that a new metal–insulator (M–I) transition temperature (T_P2) appeared at a lower temperature compared with the intrinsic metal–insulator (M–I) transition temperature (T_P1) when x < 0.50 (T_P1 > T_P2). When x > 0.50, T_P1 disappeared, leaving only T_P2. The resistivity percolation threshold of the composites occurred at x = 0.60. Corresponding to the two M–I transition peaks, the curves of magnetoresistance against temperature also showed two peaks for all composites. These phenomena can be explained by the segregation of a new phase related to SnO₂ at the grain boundaries or surfaces of the LCMO grains.     

Structure and magnetoelectric properties of Gd-doped La0.7Ca0.3MnO3 polycrystalline ceramics

Journal of Materials Science: Materials in Electronics - The temperature coefficient of resistance (TCR) and magnetoresistive (MR) properties of materials have a large impact on the performance of...     

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.     

Structural Characterization and Magneto Electrical Behavior of Sm Doped La0.7Ca0.3MnO3 Manganites

The La_0.7?x Sm _x Ca_0.3MnO₃ with x=0.0 and 0.1 compounds were fabricated by compositional solid state reaction method and investigated for their structural and magnetoelectrical properties. The samples are characterized structurally by X-ray diffraction, scanning electron microscope, and energy dispersive X-ray spectrometer. The electrical and magnetotransport properties of bulk samples have been investigated in the temperature range 5–300 K and a magnetic field up to 7 T. The metal–insulator transition temperature, T _MI, decreased with samarium (Sm) doping and also it is increased slightly with the application of magnetic field. The results of Sm doped LCMO compound showed that the maximum magneto resistance (MR) is about 54 % and it appears near the transition temperature.     

庞磁电阻薄膜La0.7Ca0.3MnO3的制备和输运特性

用溶胶-凝胶法在LaAlO3衬底上制备了La0．7Ca0．3MnO3薄膜,研究了La0．7Ca0．3MnO3薄膜的结构和输运特性．结果表明：La0．7Ca0．3MnO3薄膜呈现畸变钙钛矿结构,具有好的外延性;在同一温度下,电阻在磁场作用下变小,在283K和88K分别出现了金属-绝缘体转变,产生了双极值现象,并且在双极值附近电阻变化最大;用连续激光作用薄膜时,同样出现了双极值现象,在同一温度下,电阻反而变大,在升温和降温过程中出现了明显的滞后行为．根据双交换和小极化子理论,这些现象与Mn离子的eg电子受激跃迁有关．     

Magnetic and Electrical Transport Properties of Er2O3-Doped La0.7Ca0.3MnO3 Composites

The (La_0.7Ca_0.3MnO₃)_1?x /(Er₂O₃) _x composites with x=0, 1, 2, 3, 4, 7, 9, 15, 17, and 20% were prepared by a solid state reaction process. The magnetic and electrical transport properties of the composites were investigated. The X-ray diffraction (XRD), scanning electronic microscopy (SEM), and the calculation of lattice parameters of La_0.7Ca_0.3MnO₃ (LCMO) indicated that there is no reaction between La_0.7Ca_0.3MnO₃ and Er₂O₃; especially, compared with pure LCMO, a new metal-insulator transition temperature (T _P2) was observed at a lower temperature in some composites with x=1,2,3,4, and 7%, which may result from the existence of a new phase related to Er₂O₃ dopant at grain boundaries or surfaces of LCMO grains. An enhancement of low field magnetoresistance (LFMR) is obtained at temperature below 130 K for the composites with higher Er₂O₃ content, but the intrinsic CMR at T _P1 decreased monotonously with increasing Er₂O₃ content. The increased LFMR results from the spin-dependent scattering and spin-polarized tunneling at the grain boundaries and the decreased intrinsic CMR due to the Double Exchange (DE) interaction between Mn³⁺ and Mn⁴⁺ ions.