Nd0.7Sr0.3MnO3顺磁-铁磁转变的电子自旋共振谱

用电子自旋共振(ESR)在100～300K温区对Nd0.7Sr0.3MnO3的顺磁-铁磁相变进行了细致的观测.所得ESR信号显示,在居里温度附近温区,顺磁相中存在着铁磁团簇,铁磁相中也存在顺磁成分.巨磁电阻峰值出现在ESR谱形状变化急剧的温区.ESR线宽在顺磁态随温度升高而线性增大,在居里温度附近呈现了极小值.     

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La0.7-xGdxSr0.3MnO3(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MR-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.     

La0.7-xDyxSr0.3MnO3(0.00≤x≤0.30)体系低温磁行为和电阻率的反常

通过测量样品的磁化强度-温度曲线、电阻率-温度曲线及磁电阻-温度曲线.研究了 Dy 掺杂(0.00≤x≤0.30)对 La_(0.7-x)Dy_xSr_(0.3)MnO_3 体系磁电性质的影响.实验发现,随 Dy 掺杂量的增加,体系磁结构从长程铁磁有序向自旋团簇玻璃态、反铁磁状态转变；x=0.20、0.30时的低温磁行为发生异常,电行为存在低温电阻率极小值现象.这些奇特现象不仅来源于掺杂引起的晶格效应,也来源于掺杂引起的额外磁性耦合.     

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     

磁电阻效应的研究进展

介绍了磁电阻效应的研究状况和进展,总结了铁磁金属的磁电阻效应、磁性多层膜和颗粒膜的巨磁阻效应、磁隧道电阻效应及氧化物铁磁体的超大磁阻效应的理论模型,并简要分析了磁电阻效应的物理机制。     

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     

晶界对庞磁电阻颗粒薄膜的磁学和输运性能的影响

采用脉冲电子束沉积技术，在Si(100)单晶衬底上沉积庞磁电阻La_0.67Ca_0.33MnO₃颗粒薄膜，并对它的磁学性能和电学输运性能进行了表征.研究晶界对庞磁电阻薄膜的物理性能的影响，结果表明，晶界的存在使得晶粒之间的耦合变弱，在变温磁化过程中表现出团簇玻璃态行为，金属—绝缘体转变温度(T_p)远远低于铁磁—顺磁转变温度(T_c).低温下电子输运具有弱局域化行为.在低磁场下，晶界的存在掩盖了La_0.67Ca_0.33MnO₃的本征磁电阻行为. 关键词： 脉冲电子束沉积 晶界 磁学和电学输运性能 庞磁电阻     

磁电子学中的若干问题

本文综述了自旋极化输运过程中巡游电子的自旋极化、自旋相关的散射及自旋弛豫等三方面的内容；全面总结了铁磁金属的磁电阻效应（ＡＭＲ）、磁性金属多层膜和颗粒膜的巨磁电阻效应（ＧＭＲ）、氧化物铁磁体的特大磁电阻效应（ＣＭＲ）以及磁隧道结的巨大隧道电阻效应（ＴＭＲ）研究中具有代表性的实验结果及理论模型；简单介绍了新生的磁电子器件—磁电阻型随机存取存储器（ＭＲＡＭ）和全金属自旋晶体管的工作原理和工作过程。     

磁电子学中的若干问题

本文综述了自旋极化输运过程中巡游电子的自旋极化、自旋相关的散射及自旋弛豫等三方面的内容;全面总结了铁磁金属的磁电阻效应（ＡＭＲ）、磁性金属多层膜和颗粒膜的巨磁电阻效应（ＧＭＲ）、氧化物铁磁体的特大磁电阻效应（ＣＭＲ）以及磁隧道结的巨大隧道电阻效应（ＴＭＲ）研究中具有代表性的实验结果及理论模型;简单介绍了新生的磁电子器件—磁电阻型随机存取存储器（ＭＲＡＭ）和全金属自旋晶体管的工作原理和工作过程。     

互不固溶磁性颗粒合金巨磁电阻的一个唯象理论

利用随机的自相关函数来描述互不固溶的磁性-非磁性颗粒薄膜体系中传导电子的散射势,从而求得了该体系的电阻率与颗粒尺度的一个函数关系。进而通过引入一个唯象因子以刻划在饱和外磁场下关联长度的增加,得到了巨磁电阻系数的解析表达式,计算结果显示理论能定性地解释磁电阻随铁磁颗粒大小变化的实验结果。     

Effect of grain boundary on electrical properties of polycrystalline lanthanum nickel oxide thin films

In the present work, lanthanum nickel oxide (LNO) thin films were prepared by the sol-gel method. Microstructures of the films were tailored by changing sol concentration so as to investigate the effect of grain boundary on the transport properties of electrons in the polycrystalline LNO films. Based on the temperature dependence of the resistivity and the magnetic field dependence of the magnetoresistance (MR) at various temperatures, the factors that dominate the transport behavior in the polycrystalline LNO films were explored in terms of weak localization and strong localization. The results show that the grain boundary has a significant influence on the transport behavior of the electrons in LNO films at a low-temperature region, which can be captured by a variable-range hopping (VRH) model. The increase of metal–insulator (M–I) transition temperature is ascribed to Anderson localization in grain boundary. At a high-temperature region, electron–electron scattering and electron–phonon scattering predominates in the films. In this case, the existence of more grain boundary shows a minor effect on the transport behavior of the electrons but elevates the residual resistivity of the films.     

晶粒尺寸对La0.83Na0.17Mn0.90Fe0.10O3磁性和输运性质的影响

本文研究了纳米相和体相多晶样品La5/6Na1/6Mn0.90Fe0.10O3的结构、磁性和输运性质.XRD谱图表明两样品都是单相的钙钛矿结构.随着晶粒尺寸的减小晶粒表面处的自旋无序增多,使居里温度降低,同时使自旋相关电子在晶界处的散射增强,提高了材料电阻率.纳米粒子的尺寸效应提高了材料的低场磁电阻;晶粒表面自旋无序的增多使电子在晶粒表面的二阶隧道效应增强,提高了高场磁电阻效应.零场电阻率的拟合结果也表明晶粒尺寸的减小使自旋无序增加.     

Study of magnetoresistance and conductance of bicrystal grain boundary in La0.67Ba0.33MnO3 thin film

La_0.67Ba_0.33MnO₃ (LBMO) thin film is deposited on a 36.7°C SrTiO₃ bicrystal substrate using laser ablation technique. A microbridge is created across bicrystal grain boundary and its characteristics are compared with a microbridge on the LBMO film having no grain boundary. Presence of grain boundary exhibits substantial magnetoresistance ratio (MRR) in the low field and low temperature region. Bicrystal grain boundary contribution in MRR disappears at temperature T>175 K. At low temperature, I-V characteristic of the microbridge across bicrystal grain boundary is nonlinear. Analysis of temperature dependence of dynamic conductance-voltage characteristics of the bicrystal grain boundary indicates that at low temperatures (T<175 K) carrier transport across the grain boundary in LBMO film is dominated by inelastic tunneling via pairs of manganese atoms and tunneling through disordered oxides. At higher temperatures (T>175 K), magnetic scattering process is dominating. Decrease of bicrystal grain boundary contribution in magnetoresistance with the increase in temperature is due to enhanced spin-flip scattering process.     

Electrical properties and enhanced room temperature magnetoresistance in La0.7Ca0.2Sr0.1MnO3/Pd composites prepared by chemical plating

A series of bulk polycrystalline La_0.7Ca_0.2Sr_0.1MnO₃ (LCSMO)/Pd composites were prepared by chemical plating and structural, electrical, magnetic, and magnetoresistance (MR) properties were investigated. It is found that Pd additions are uniformly distributed on the grain boundaries of the LCSMO grains, which decrease the resistivity and the saturation magnetic moment of the matrix. An interesting phenomenon is observed that at a given field, when the plating time increases, the MR increases at low addition level (0>t (plating time)<40 min) and decreases at high addition level (t>40 min), indicating an optimal plating time of 40 min, at which the MR value is maximum. Our analysis suggests that the improvement of grain boundaries originating from Pd addition plays an important role in enhancing the MR.     

Absence of traditional magnetoresistivity mechanisms in Sr(2)FeMoO(6) thin films grown on SrTiO(3), MgO and NdGaO(3) substrates

Magnetoresistive double perovskite Sr(2)FeMoO(6) thin films were grown with two different deposition pressures on SrTiO(3), MgO and NdGaO(3) substrates by pulsed laser deposition and thorough structural, magnetic and magneto-transport characterization was made. According to x-ray diffraction, all the films were phase pure and fully textured. Indication of substrate dependent strain and low angle grain boundaries was found, especially in films on MgO. Both the deposition pressure and the choice of the substrate have a strong influence on the saturation magnetization, M(s), and Curie temperature, T(C). The structural and magnetic data indicate the presence of anti-site disorder (ASD) in the films. The temperature dependence of resistivity showed semiconductive behaviour at temperatures below 100 K and metallic behaviour at higher temperatures. The semiconductive behaviour was found to increase with increasing ASD. In good quality films, up to 12% negative magnetoresistance (MR) was observed and films grown on MgO and NGO substrates also showed low field MR. However, the most significant observation of this study was that the magnetoresistivity of these Sr(2)FeMoO(6) thin films could not be explained with any traditional MR mechanism, but carried the clear signature of superposition of different mechanisms, in particular low angle grain boundary tunnelling and suppression of antiferromagnetically ordered domains under a magnetic field.     

Spin‐dependent‐magnetoresistance control by regulation of heat treatment temperature for magnetite nano‐particle sinter

The control of spin‐dependent‐magnetoresistance by regulation of the heat treatment (HT) temperature for magnetite (Fe₃O₃) nano‐particle sinter (MNPS) has been studied. The average nano‐particle size in the MNPS is 30nm and the HT was carried out from 400°C to 800°C. The HT of the MNPS varies the coupling form between adjacent magnetite nano‐particles and the crystallinity of that. The measurements on electrical resistance (ER), magnetoresistance (MR) and magnetization were performed between 4K and 300K. The behavior of the ER and MR considerably changes at the HT temperature of ～600°C. Below ～600°C the ER indicates the variable‐range‐hopping conduction behavior and the MR shows the large intensity in a wide temperature region. Above ～600°C the ER shows the indication of the Verwey transition near 110K like a bulk single crystal and the MR designates the smaller intensity. We consider that below ～600°C the ER and MR are dominated by the grain‐boundary conduction and above ～600°C those are determined by the inter‐grain conduction. The magnetic field application to the grain‐boundary region is inferred to cause the large enhancement of the MR.     

高自旋极化氧化物材料的颗粒边界磁电阻效应

颗粒边界磁电阻是高自旋极化氧化物颗粒体系中由于颗粒边界的存在而导致显著的磁电阻效应。本文将这种磁电阻效应定义为颗粒边界磁电阻效应。这里所说的颗粒边界,包括各种自然和人工晶界、粉末颗粒表面、复合材料中的颗粒界面等多种情况;所涉及的材料包括高自旋极化氧化物多晶、压缩粉末和各种复合材料等。对颗粒边界磁电阻效应的研究,不仅有助于人们进一步理解高自旋极化氧化物磁输运性质的基本机制,并为寻求具有高磁电阻效应的新型自旋电子学器件提供理论基础。本文综述了高自旋极化氧化物颗粒边界磁电阻研究的主要背景和发展现状,介绍了该领域中主要的实验发现和理论模型,展望了未来的发展。     

Enhancing low-field magnetoresistance of La0.67Ca0.33MnO3 films deposited on anodized aluminium-oxide membranes

In this paper we report a new method to fabricate nanostructured films, La0.67Ca0.33MnO3 （LCMO） nanostructured films have been fabricated by using pulsed electron beam deposition （PED） on anodized aluminium oxide （AAO） membranes, The magnetic and electronic transport properties are investigated by using the Quantum Design physics properties measurement system （PPMS） and magnetic properties measurement system （MPMS）. The resistance peak temperature （Tp） is about 85 K and the Curie temperature （To） is about 250 K for the LCMO film on an AAO membrane with a pore diameter of 20nm. Large magnetoresistance ratio （MR） is observed near Tp. The MR is as high as 85% under 1 T magnetic field. The great enhancement of MR at low magnetic fields could be attributed to the lattice distortion and the grain boundary that are induced by the nanopores on the AAO membrane.     

具有人工晶界的Sr2FeMoO6粉末磁电阻的温度特性

利用固相反应合成了纯相的Sr2FeMoO6多晶块体，并通过机械球磨方法引入了人工晶界，研究了具有此种晶界的Sr2FeMoO6粉末磁电阻的温度特性．X射线衍射分析表明，机械球磨过程没有改变Sr2FeMoO6的晶体结构，但却在晶粒间界处引入了SrMoO4绝缘相，其量随着球磨时间的增加而增加．不同磁场下的磁电阻测量结果表明，由于一定量SrMoO4绝缘相的存在，晶粒间的绝缘隧穿势垒得到加强，更有利于自旋极化电子在晶粒间的隧穿，从而提高了Sr2FeMoO6多晶粉末的低温磁电阻值．然而随着温度的升高，磁电阻值迅速下降，表现出较强的温度依赖关系．这种现象是由于随着温度的升高，电子在晶界局域态间的非弹性跳跃逐渐增强引起的，而晶界局域态是由在晶界附近的大量缺陷构成．分析表明，晶界状态对Sr2FeMoO6多晶粉末磁电阻的温度特性有十分重要的影响．     

Giant room-temperature magnetoresistance in polycrystalline Zn(0.41)Fe(2.59)O4 with alpha-Fe2O3 grain boundaries

A tunneling-type magnetoresistance (MR) as large as 158% is observed at T = 300 K in a polycrystalline Zn0.41Fe2.59O4 sample, in which the Zn0.41Fe2.59O4 grains are separated by insulating alpha-Fe2O3 boundaries. The huge room-temperature MR is attributed to the high spin polarization of Zn(0.41)Fe(2.59)O4 grains and antiferromagnetic correlations between magnetic domains on both sides of the insulating alpha-Fe2O3 boundary. The MR exhibits strong temperature dependence below 100 K and its magnitude is enhanced to reach 1280% at 4.2 K, which may arise from the Coulomb blockade effect.