The Tb Doping Effect on the Room Temperature Magnetoresistance in (La1-xTbx)0.67Sr0.33MnO3 (x≤ 0.4)

by Tb in (La1-xTbx)0.67Sr0.33MnO3, the room temperature magnetoresistance △R/R0 drops at first, then undergoes an increase near x≈0.1, and finally drops again. The value of room temperature magnetoresistance at a field H=12 kOe for (La1-xTbx)0.67Sr0.33MnO3 is -3.56%. The enhancement of the room temperature magnetoresistance induced by an appropriate Tb substitution in (La1-xTbx)0.67Sr0.33MnO3 is correlated with the shifts of the Curie temperature and metal-insulator temperature to near room temperature. The drop of the room temperature magnetoresistance at large Tb doping-contents may be due to its lower TC and TMI far from the room temperature.     

Research on a New Process of Preparation for Nano-SiO2 with High Activity and Mesopores

Nano-SiO_2 with high activity and mesopores was prepared through sol-gel synthesis followed by low-temperatureheat treatment and ball milling firstly in our experiments. TEM was performed to measure particle sizes. Nitrogenadsorption experiments were carried out to estimate specific surface area, porous distribution and porous ratio by BETand BJH methods. The content of Si-OH in SiO_2 surface was calculated by analysis of the results of hydrogen-oxygencontent mensuration (HOCM). As a result, appropriate heat treatment system and ball milling time are important topreparation for nano-SiO_2 with high activity and mesopores, which are 5～50 nm particles, 5～6 nm average aperture,85%～93% porous ratio, and 51%～55% Si-OH content in surface. Nano-SiO_2 with that structure has high surfaceenergy and activity. This process, which has simple facilities and operation rules, is a new way of preparation fornano-SiO_2 with high activity and mesopores.     

(La0.52Gd0.15)Sr0.33MnO3多晶颗粒的磁热效应

利用非晶态分子合金作前驱体,在相对低的热分解温度800℃、10h成功合成了钙钛矿结构(La0.52Gd0.15)Sr0.33MnO3多晶颗粒.TEM观察表明,颗粒的尺寸范围为100～150nm.研究了多晶颗粒(La0.52Gd0.15)Sr0.33MnO3的居里温度和磁熵变化(MCE).在多晶颗粒(La0.52Gd0.15)Sr0.33MnO3中,居里温度(343K)附近观察到较大的磁熵变和较宽的峰值温度范围,较大的磁熵变来源于磁场条件下的铁磁转变贡献.这些结果表明,该材料是室温附近磁制冷合适的工作物质.     

La0.67Sr0.33MnO3中Ag-Ti的掺杂效应

将La0.67Sr0.33MnO3（LSMO）、Ag2O及TiO2粉混合经高温烧结后制备了钙钛矿相/xAg两相复合体系（x是Ag与钙钛矿材料的物质的量比）,系统地研究了Ag-Ti的共掺杂对LSMO电性和磁电阻效应的影响.0.07摩尔比Ti4＋离子的B位掺杂使LSMO的居里温度降至室温.Ag的掺入对Tc影响不大,Tp逐渐升高.由于钙钛矿颗粒属性的改善和金属导电通道的出现,材料的电阻率明显下降.Ag掺杂使室温磁电阻得到显著增强,室温下从x=0.30样品中得到最大的磁电阻,约为32%,是La0.67Sr0.33MnO3样品的8倍,La0.67Sr0.33Mn0.93Ti0.07O3样品的1.6倍.     

La0.67Sr0.33Co1-xFexO3体系的输运行为

研究了Co位Fe掺杂对La0．67Sr0．33Col—xFexo3体系的电输运性质和超大磁阻效应的影响．实验结果表明：在低掺杂(x≤0．1)时电阻率表现出金属性输运行为，在高掺杂(x＝0．2和0．3)时则为半导体行为．Fe掺杂削弱了Te处的MR峰值，但增加了低温下(T《Te)的MR值．La0．67Sr0．33Col—xFe2o3体系的磁电阻的起源可由外加磁场导致的自旋态转变来解释。     

La0.67Sr0.33-xCuxMnO3的室温磁电阻效应

采用溶胶一凝胶(sol-gel)法制备了名义组分为La0.67Sr0.33-XCuXMnO3(x=0～0.33)的多晶样品,发现用Cu替代少部分Sr后样品的室温磁电阻比替代前的明显增大.在1.8T磁场作用下,当x＝0.15时,磁电阻峰值为27.7%,峰值温度为306K,当温度低于306K时磁电阻值随温度的升高而增大,当温度高于306K时磁电阻值随温度的升高而减小;当x＝0.1时,在295K-310K温度之间磁电阻值达19%左右,受温度影响很小;因此在提高了室温磁电阻值的同时,又提高了磁电阻的温度稳定性.这对于该类磁电阻材料的应用具有很大意义.     

La0.67Sr0.33MnO3中Bi的掺杂效应

将Bi2O3掺杂到用溶胶—凝胶法制备的La0.6Sr0．33MnO3(LSMO)微粉中,XRD测量结果证实有过量的Bi析出。随着Bi掺杂量的增加,LSMO／(Bi2O3)x/2材料电阻率发生明显变化,在x=(0—0．10)摩尔比的掺杂范围内,电阻率先上升后突然下降。当X=0．1时,电阻率比未掺杂样品下降了一个数量级。Bi掺杂对低温和室温磁电阻有着完全不同的影响。低温下,随掺杂量增加,磁电阻下降;室温下Bi的微量掺杂可以使磁电阻增大,掺入x=0.03Bi使室温磁电阻由-4.4％提高到-5．6％。     

Annealing Effect on Transport and Magnetic Properties of La_(0.67)Sr_(0.33)MnO_3 Thin Films Grown on Glass Substrates by RF Magnetron Sputtering

The manganite La0.67Sr0.33MnO3(LSMO) thin films were grown on glass substrates in a mixed argon and oxygen atmosphere by using RF magnetron sputtering.The structural characteristics,transport behaviors and magnetic properties of LSMO films were studied by annealing the films in air at 550 and 620 ℃.The out-of-plane lattice parameter a LSMO contracted after annealing and was close to that of bulk LSMO abulk,indicating that the internal strain was fully relaxed.Nanocrystalline grains were observed in the annealed films.Enhanced saturation magnetization and metal-to-insulator transition temperature(TMI=268 K) were also obtained.Curie temperatures(Tc) of the as-grown films was 340 K with the same as that of annealed at 550 ℃,but dropped to 315 K when the annealing temperature increased to 620 ℃,which can be attributed to the oxygen release during annealing in atmosphere.     

Piezoresistivity in films of nanocrystalline manganites

Rare earth manganites having perovskite structure are susceptible to lattice strain. So far most investigations have been done with hydrostatic pressure or biaxial strain. We have observed that hole doped rare-earth manganites, which are known to display colossal magnetoresistance (CMR) also show change in its resistance under the influence of uniaxial strain. We report the direct measurement of piezoresistive response of La0.67Ca0.33MnO3 (LCMO) and La0.67Sr0.33MnO3 (LSMO) of this manganite family. The measurements were carried out on nanostructured polycrystalline films of LCMO and LSMO grown on oxidized Si(100) substrates. The piezoresistance was measured by bending the Si cantilevers (on which the film is grown) in flexural mode both with compressive and tensile strain. At room temperature the gauge factor approximately 10-20 and it increases to a large value near metal-insulator transition temperature (Tp) where the resistivity shows a peak.     

La_(0.60)Sr_(0.40-x)K_xMnO_3系列材料的磁电阻效应

利用溶胶-凝胶法(sol-gel)制备了多晶类钙钛矿型稀土锰氧化物La0.60Sr0.40-xKxMnO3(x=0.00,0.15,0.20,0.30)。发现K+取代部分Sr2+后,可使样品的居里温度降至室温附近,并且使样品的室温磁电阻比替代前明显增大。在1.8T的磁场下,x=0.30的样品磁电阻峰值为21%,相应的峰值温度为304K。而母体La0.60Sr0.40MnO3的磁电阻峰值仅为6.4%,峰值温度为373K。可见K+离子替代使室温附近样品的庞磁电阻效应有了明显的改善。     

Influence of particle size on electrical transport properties of La0.67Sr0.33MnO3 manganite system

A systematic investigation of lanthanum-based manganite, La0.67Sr0.33MnO3, has been undertaken with a view to understand the influence of varying particle sizes on electrical transport properties. With a view to obtain materials with varying particle size, they were prepared by sol-gel route, sintering at four different temperatures. The samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD data has been analyzed by Rietveld refinement technique and it has been confirmed that the materials have rhombohedral crystal structure with R3c space group. Metal-insulator transition temperatures (Tp) were found to decrease continuously with decreasing particle size where as ferro to paramagnetic transition temperatures (Tc) are found to remain constant. The magnetoresistance (MR) values are found to increase with decreasing particle size. With a view to understand the conduction mechanism, the electrical resistivity data have been analyzed both in the ferromagnetic metallic (T < Tp) as well as high temperature paramagnetic insulating (T > Tp) regions.     

高压下纳米La_(0.8)Sr(0.2)MnO_3块状材料的成型及性能研究

在高压条件下制备出了晶粒尺寸为30nm左右的La0.8Sr0.2MnO3块状材料.采用X射线衍射仪(XRD)和场发射扫描电子显微镜(FESEM)对高压样品的相组成、晶粒尺寸及微观形貌进行了表征;利用显微拉曼光谱仪测量了不同压力下制备出的块状样品在激光功率为400mW时位于100～3000cm-1之内的拉曼光谱.结果表明,高压烧结时样品的晶粒演化受温度和压力的共同影响:300℃以下时,1～3GPa烧结的样品晶粒生长速度随压力的升高而增大,4～5GPa烧结的样品其晶粒生长速度随压力的升高而减小;300℃以上时,其晶粒生长速度随压力和温度的升高而不断增大.测量了高压烧结后纳米La0.8Sr0.2MnO3块状材料的各种物理性能,结果表明,高压烧结后该材料的显微硬度显著提高;当烧结温度为300℃时,不同压力下制得的样品的电阻率随压力的升高呈先减小后增大的趋势;5GPa,300℃时制出的样品在室温下呈典型的铁磁性能.     

Giant sharp and persistent converse magnetoelectric effects in multiferroic epitaxial heterostructures

Magnetoelectric coupling between magnetic and electrical properties presents valuable degrees of freedom for applications. The two most promising scenarios are magnetic-field sensors that could replace low-temperature superconducting quantum interference devices, and electric-write magnetic-read memory devices that combine the best of ferroelectric and magnetic random-access memory. The former scenario requires magnetically induced continuous and reversible changes in electrical polarization. These are commonly observed, but the coupling constants thus obtained are invalid for data-storage applications, where the more difficult to achieve and rarely studied magnetic response to an electric field is required. Here, we demonstrate electrically induced giant, sharp and persistent magnetic changes (up to 2.3 x 10(-7) s m(-1)) at a single epitaxial interface in ferromagnetic 40 nm La(0.67)Sr(0.33)MnO(3) films on 0.5 mm ferroelectric BaTiO(3) substrates. X-ray diffraction confirms strain coupling via ferroelastic non-180( composite function) BaTiO(3) domains. Our findings are valid over a wide range of temperatures including room temperature, and should inspire further study with single epitaxial interfaces.     

Irreversible Colossal Magnetoresistance in La0.58Dy0.09Ca0.33MnO3

The representative sample La0.58Dy0.09Ca0.33MnO3 of Dy doped La0.67Ca0.33MnO3 rare-earth manganites was investigated.The most important effect of Dy doping is to introducethe magnetoimpurity and form the spin clusters which induce dramatically large CMR in Lao.58Dyo.09Cao.33MnO3. The fitting results of field-induced resistivity decrease to the Brillouin function indicate that the CMR is caused by the spin dependent hopping between spin clusters. It is the magnetic field that reduces the size of spin clusters and induces a field-induced irreversible CMR behaviour.     

La0.67-xCa0.33MnO3系列样品的磁热效应

用溶胶-凝胶法制备了不同空位浓度掺杂的La0.67-xCa0.33MnO3系列样品(x=0.00,0.02,0.06,0.10),用扫描电子显微镜(SEM)观察了样品的形态和颗粒大小,X射线衍射实验结果显示样品为单相钙钛矿结构,用振动样品磁强计(VSM)测量了样品的磁化强度随温度变化的M-T曲线和起始磁化M-H曲线.研究了空位浓度对样品的居里温度Tc和磁熵变的影响.结果表明,空位浓度的掺杂将样品的居里温度Tc提高至室温附近,磁熵变得到增强.对于经历了一级相变的样品La0.67-xCa0 33MnO3(x=0.02),在居里温度Tc(277 K)附近和1T外磁场下,最大磁熵变达到了2.78 J/(kg·K).     

Effect of grain boundaries on the local electronic transport in nanostructured films of colossal magnetoresistive manganites

We have studied the spatially resolved local electronic properties of nanostructured films of a colossal magnetoresistive (CMR) material by local conductance mapping (LCMAP) using a variable temperature scanning tunneling microscope (STM), as a function of temperature and magnetic field. The experiments were carried out on nanostructured (NS) films of rare-earth hole doped manganites like La0.67Sr0.33MnO3 (LSMO), (prepared by Chemical Solution Deposition (CSD)) having average grain sizes around 100 nm or less. Due to the presence of a large number of natural grain boundaries (GBs), these films exhibit interesting transport properties that are not observed in single crystals and epitaxial films of the same composition, such as low field magnetoresistance (LFMR), and significant MR at higher fields over the entire temperature range. Spatially resolved measurements of electronic properties, like the LCMAP, traces the contribution of the grain boundaries to the electronic transport in these NS films. It is found that the application of a low magnetic field enhances the DOS in the GB region, leading to the observed low field response of the MR.     

La0.67 Ca0.33 MnO3和La0.67 Ba0.33 MnO3中Dy的置换作用

在La0.67Ca0.33MnO3和La0.67Ba0.33MnO3中用Dy对La进行了置换研究。结果发现，随掺Dy量的增加，两类材料的居里温度和金属-绝缘体相变温度单调下降，峰值电阻率单调增加。在Ca系样品中，掺入13％的Dy后，在5T的磁场下，最大磁电阻比达到7900％。在Ba系样品中．掺Dy对磁电阻的影响要小得多。掺Dy对材料性质的影响可以用晶格效应来解释，但晶格效应产生的作用与碱土离子的品种有明显关系。     

Cr替代Mn对La0.4Ca0.6MnO3磁性质的影响

用固相反应法制备了La0.4Ca0.6Mn1-xCrxO3(x=0.00,0.02,0.04,0.06,0.08,0.10,0.15)多晶样品,通过X射线衍射(XRD)图谱、磁化强度-温度(M-T)曲线、电子自旋共振(ESR)谱线,研究了Cr替代Mn对La0.4Ca0.6MnO3磁性质的影响.实验结果表明:La0.4Ca0.6MnO3存在复杂的磁结构,在258 K出现电荷有序相,从175 K到50 K,产生强关联CO-AFM(电荷有序?反铁磁)相,温度降到41 K左右出现自旋玻璃态;当替代量x≥0.06时,电荷有序相被融化;当x≥0.10时,自旋玻璃态被融化.对实验结果进行了初步解释:电荷有序相被融化主要原因是Cr3+与Mn4+具有相同的电子结构,Cr3+替代Mn3+破坏了CE型反铁磁的自旋序,从而引起电荷序的坍塌,实验证明了电荷序CE型反铁磁体系中,电荷序和自旋序之间存在强耦合相互作用;自旋玻璃态的融化,是由于Cr替代Mn破坏了自旋玻璃态的生成条件,即反铁磁背景下有少量铁磁成分.     

La0.67Ca0.33MnO3/YBa2Cu4O8/La0.67Ca0.33MnO3三层膜的时间效应研究

报导了La0.67Ca0.33MnO3/YBa2Cu4O8/La0.67Ca0.33MnO3薄膜中超导特性的时间效应.样品在空气中存放11个月后,超导转变温度都降低,而且每个系列样品的临界厚度增加.对用La0.67Ca0.33MnO3作保护层的YBa2Cu4O8薄膜进行时间效应测量,结果表明当La0.67Ca0.33MnO3层的厚度大于40nm时可以防止水对YBa2Cu4O8薄膜的影响.     

Ag／La0.67Sr0.33MnO3/Pt异质结构的电阻开关特性

采用脉冲激光沉积技术在Pt／Ti／SiO2／Si（100）衬底上制备出多晶La0.67Sr0.33MnO3（LSMO）薄膜,对其电脉冲致非挥发可逆电阻开关特性进行研究．结果表明,Ag/LSMO／Pt结构具有明显的室温电脉冲诱发电阻开关特性,且在宽电压脉冲作用下表现出较低的开关电压和较快的变阻饱和速度．由此可见,总脉冲能量或电荷（电流作用）为该结构的电阻开关效应提供驱动力．对Ag／LsMO／Pt结构进行了耐久性测试,表明该结构具有良好的疲劳特性与保持特性,可应用于新型不挥发存储器、传感器及可变电阻等电子元器件的研制