(La2/3Ca1/3)(Mn(3-x)/3)Fex/3)O3体系磁电阻行为的研究

通过系统地测量（La_2/3Ca_1/3）（Mn_{（3-x）／3}Fe_x／3）O₃（x＝0、0.1、0．2、0．3的体系样品的电阻率-温度关系以及一定温度下磁电阻率与磁场的关系,发现随x的变化其磁电阻率峰和电阻率峰均发生位移,磁电阻率峰值增大,并伴生磁电阻率峰展宽效应．作者认为由于Fe的替代,引起体系中Mn³⁺／Mn⁴⁺比率及磁矩的变化,加之外场对磁有序结构的调制作用,从而影响了Mn³⁺－O^－Mn⁴⁺的双交换作用,最终导致磁电阻行为发生变化．     

(La2/3Ca1/3)(Mn(3-x)/3)Fex/3)O3体系磁电阻行为的研究

通过系统地测量（La2/3Ca1/3）（Mn（3-x）／3Fex／3）O3（x＝0、0.1、0．2、0．3的体系样品的电阻率-温度关系以及一定温度下磁电阻率与磁场的关系,发现随x的变化其磁电阻率峰和电阻率峰均发生位移,磁电阻率峰值增大,并伴生磁电阻率峰展宽效应．作者认为由于Fe的替代,引起体系中Mn3 ／Mn4 比率及磁矩的变化,加之外场对磁有序结构的调制作用,从而影响了Mn3 －O－Mn4 的双交换作用,最终导致磁电阻行为发生变化．     

磁电阻效应之巨磁电阻效应研究

体积越来越小,容量越来越大——在如今这个信息时代,存储信息的硬盘自然而然被人们寄予了这样的期待。得益于"巨磁电阻"效应运一重大发现,最近20多年来,我们开始能够在笔记本电脑、音乐播放器等所安装的越来越小的硬盘中存储海量信息。     

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。     

La-Gd-Ca-Mn-O的不可逆磁电阻效应

随掺Gd量的增加，La-Gd-Ca-Mn-O化合物的金属-绝缘体相变温度逐渐下降，对应的峰值电阻率大幅度增加，磁电阻比明显提高，掺入11%的Gd可以使材料的磁电阻比提高一个数量级。Gd的掺入还引起材料磁电阻出现明显的不可逆效应。随掺Gd量增加，不可逆效应明显增大。     

Co_xAg_(100-x)颗粒膜的巨磁电阻效应及其物理机制研究

用超高真空电子束蒸发沉积方法制备了ＣｏｘＡｇ１００－ｘ颗粒并研究了其中的巨磁电阻（ＧＭＲ）效应。实验结果表明：１５ｋＯｅ外磁场下，玻璃衬底上制备态Ｃｏ３９Ａｇ６１颗粒膜室温ＧＭＲ效应值高达２０２％，并随氮气中退火温度上升而单调减小；颗粒膜的电阻与其磁化强度直接相关，可用四次多项式偶函数来描述。     

Fe/Al2O3/Fe隧道结的巨磁电阻效应

研究了Fe(200℃退火)/Al2O3/Fe多层膜隧道结的巨磁电阻效应,在室温下获得了5.89％的巨磁电阻效应,并且测量了样品的伏安曲线,证明了隧道效应的存在。     

微观缺陷和3d电子对La2/3Ca1/3Mn1-xFexO3陶瓷磁电阻性能的影响

用符合正电子湮没辐射Doppler展宽技术研究了La2/3Ca1/3Mn1-xFexO3陶瓷的缺陷和3d电子行为。结果表明:对x≤0.33的样品,随着样品中Fe含量的增加,3d电子数量增加,样品的商谱谱峰升高;对x>0.50的样品,随着样品中Fe含量的增加,样品中有新相生成,体系缺陷增加,样品的商谱谱峰降低。测试了La2/3Ca1/3Mn1-x-FexO3多晶陶瓷的磁电阻性能,讨论了微观缺陷和3d电子对La2/3Ca1/3Mn1-xFexO3多晶陶瓷磁电阻性能的影响。     

La0.7-xDyxSr0.3MnO3体系的输运行为研究

研究了La位Dy掺杂对La0.7～xDyxSr0.3MnO3(x=0．00、0．10、0．15、0．20、0．30、0．40、0．50、0．60、0．70)体系输运行为的影响．实验结果表明：随Dy掺杂的增加，体系由长程铁磁有序向自旋团簇玻璃态、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，在x=0．50时，发现了在ABO3结构中很少出现过的在Tc附近发生绝缘体-金属相变之后又发生从金属一绝缘体相变的现象；对x=0．60，0．70，在远离Tc的温区体系表现为绝缘体．     

Neutron powder diffraction and magnetic study of perovskites Pb(Mn1/2Nb1/2)O3 and Pb(Mn1/4Fe1/4Nb1/2)O3

The structural and magnetic properties of the complex metal oxides Pb(Mn_1/2Nb_1/2)O₃ (PMNO) and Pb(Mn_1/4Fe_1/4Nb_1/2)O₃ (PMFNO), which belong to a class of disordered perovskites have been studied. The magnetic susceptibilities of PMNO showed hysteresis between field cooled and zero-field cooled conditions below the transition of 15 K, suggesting that the material has a spin-glass feature. Neutron diffraction patterns of PMNO showed no evidence of a long-range magnetic ordering at 1.5 K, which is consistent with spin-glass behavior. Rietveld refinements of neutron powder diffraction data collected at different temperatures between 1.5 and 700 K have been carried out in order to extract structural information. The crystal structure of this compound is cubic (space group Pmm) within the whole temperature interval. The Mn and Nb ions were found to be disordered over the perovskite B-sites. The main feature of this structure is the positional disorder at the Pb site, the importance of which in connection with the ferroic transitions is briefly discussed. The Pb cations show a positional disorder shifting from their high-symmetry positions along the [1 1 1] direction. The effect of Fe-doping on PMNO has been studied. The substitution of Fe at the Mn site in PMFNO results in a small changes of the magnetic properties without significant differences in the crystal structures. The factors governing the observed structural and magnetic properties of PMNO and PMFNO are discussed and compared with those of other quaternary Mn-containing perovskites. For the PbB³⁺_1/2Nb_1/2O₃ series with the isomorphous substitution B³⁺, graphs of average lattice parameters of the perovskite phase and the temperatures of ferroelectric and magnetic phase transitions as functions of the B³⁺ cation radius were constructed and are discussed. Influence of A-cation sublattice on magnetic properties is also considered.     

Phase developments in the Pb(Zn1/2W1/2)O3-Pb(Zn1/3Ta2/3)O3-Pb(Zn1/3Nb2/3)O3 pseudo-ternary system

The phase stability ranges in the B-site precursor (Zn_1/2W_1/2)O₂-(Zn_1/3Ta_2/3)O₂-(Zn_1/3Nb_2/3)O₂ were determined by X-ray diffraction (XRD), where wolframite, tri-αPbO₂, and columbite phases were identified. Next attempts were carried out (with the addition of PbO) for the system Pb(Zn_1/2W_1/2)O₃-Pb(Zn_1/3Ta_2/3)O₃-Pb(Zn_1/3Nb_2/3)O₃, where the perovskite phase did not develop in the entire compositions investigated. Instead, only the Pb₂WO₅ and pyrochlore phases (along with ZnO) resulted.     

Stress relaxation of La1/2Sr1/2MnO3 and La2/3Ca1/3MnO3 at solid oxide fuel cell interfaces

Interfacial stress is thought to have significant effects on electrical and oxygen transport properties in thin films of importance in solid oxide fuel cell applications. We investigate how in-plane biaxial stress modifies the electronic structure of La_2/3Ca_1/3MnO₃ and La_1/2Sr_1/2MnO₃ thin films prepared by pulsed laser deposition on three different substrates to vary the in-plane stress from tensile to compressive. The electronic structure was probed by X-ray absorption spectroscopy of the Mn L_2,3-edge to characterize the interfacial disruption in this region in an element-specific, site-specific manner. The compressive or tensile interfacial strain modifies the relative concentrations of La and Sr in the interfacial region in order to achieve a better lattice match to the contact material. This atomic migration generates an interfacial region dominated by a compound with a single valency for the transition metal ion, resulting in a severe barrier to oxygen and electron transport through this region.     

Magnetic coupling in La0.67Ca0.33MnO3/La0.67Sr0.33CoO3/La0.67Ca0.33MnO3 sandwiches

La_0.67Ca_0.33MnO₃ (LCMO)/La_0.67Sr_0.33CoO₃ (LSCO)/LCMO trilayer films are fabricated on single-crystal substrates NdGaO₃ (110) and the interlayer coupling are investigated. Compared with LCMO single layer, sandwiches showed the enhanced metal-insulator transition temperature of LCMO layers. The magnetoresistance is dependent on spacer thickness and the peak value dramatically decreases when LSCO layer is thick enough because of shorting by the LSCO layer. The magnetic coercivity H_C shows a nonmonotonic behavior with changing spacer layer thickness and the waist-like hysteresis indicates that there is an indirect exchange coupling between the top and bottom LCMO layers across the spacer layer.     

Electrical properties tailoring in Ni-particle-dispersed (Ba0.95Ca0.05)(Ti0.96Zr0.04)O3 composites

Ni-particle-dispersed (Ba_0.95Ca_0.05)(Ti_0.96Zr_0.04)O₃ (BCTZ/Ni) piezoceramic composites were prepared via sintering at 1300 °C in industrial N₂ gas. Structural characterizations showed that the metallic Ni was not oxidized and the BCTZ preserved the perovskite structure. The Ni particles were uniformly distributed in the BCTZ ceramic matrix. The relative dielectric constant ?_r of the BCTZ/Ni composites increased from 1362 to 3910 with increasing Ni content from 0 to 20 vol.%, which is explained by the Maxwell equation as well as the micro-capacitor model. The percolation theory of insulator–metal transitions is also applied to correlate the rapid increase of dielectric constant with Ni content. The piezoelectric constant d₃₃ gradually decreased from 230 to 50 pC N⁻¹, giving a gradient profile of piezoelectric property. We demonstrate that the electrical properties can be effectively tailored by dispersing metal particles into piezoceramics.     

Structural and dielectric behavior of Pb(Mn1/4Zn1/4W1/2)O3 ceramics

Polycrystalline complex perovskite compound of Pb(Mn_1/4Zn_1/4W_1/2)O₃ (PMZW) was prepared from a solid-state reaction technique. X-ray powder diffraction (XRD) study suggests the formation of single-phase compound in orthorhombic structure at room temperature. Dielectric studies as a function of temperature (305–573 K) yielded a small dielectric anomaly at 477 K. Variation of dc resistivity as a function of temperature suggests that the compound behaves as the negative temperature coefficient resistors (NTCR). Proper hysteresis loop could not be observed to confirm the ferroelectric behavior.     

Synthesis and characterization of the colossal magnetoresistance manganite La1.2(Sr1.4Ca0.4)Mn2O7 by citrate gel

Single-phase La_1.2(Sr_1.4Ca_0.4)Mn₂O₇ has been synthesized from the aqueous solution of metal nitrates and citric acid by the sol-gel technique. Small particle size with high homogeneity of the powder was obtained. The valence of Mn is determined to be 3.45±0.05 by chemical titration. The MR ratio [ρ−ρ(H)]/ρ(H), is 115% (102 K, 1.5 T) for the composition prepared by the citrate route, and is nearly three times larger than that of the sample prepared by solid state reaction.     

Inhomogeneous electronic properties of epitaxial La2/3Ca1/3MnO3 thin films

In this paper, we have reported on epitaxial fully strained La_2/3Ca_1/3MnO₃ films grown on single-crystalline SrTiO₃ substrates, with thicknesses ranging from 6 to 108 nm. ⁵⁵Mn NMR experiments, combined with magnetization data, show that the films have non-homogeneous magnetoelectronic properties and that non-ferromagnetic insulating regions are present close to the interface with the SrTiO₃. This observation may account for the repeatedly described disruption of the performance of manganite-based tunneling devices with increasing temperature and may help to better identify the possible causes of the modification of the La_2/3Ca_1/3MnO₃/SrTiO₃ interface properties.