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．     

水热合成单晶La 1-x Ca x MnO3系列纳米线

采用水热法，成功地合成了一系列单晶La1-xCaxMnO3（x=0．3、0．5、0．75）纳米线．X射线衍射（XRD）的结果表明所制备的样品是纯的La1-xCaxMnO3正交相（x=0．3、0．5、0．75）纳米线．透射电子显微镜（TEM）的照片清晰地显示所制备的样品是由大量直径均匀（约90nm）、长度从几微米至几十微米的La1-xCaxMnO3的单晶纳米线构成．La0．5Ca0．5MnO3纳米线清晰的高分辨电子显微镜（HRTEM）的图片表明纳米线的表面是光滑的。没有任何缺陷且La0．5Ca0．5MnO3纳米线沿〈100〉方向生长．La0．5Ca0．5MnO3纳米线磁测量的结果表明相对于块材来说纳米线的疋有明显地提高，这是由于晶胞收缩和形状各向异性所致．     

颗粒表面覆Cu的La2/3Ca1/3MnO3复合颗粒系统中的低场高磁电阻效应

用化学方法将Cu有关的材料引入到La2／3Ca1／3MnO3颗粒表面以形成复合颗粒系统．低磁场(0．3T)下磁电阻与温度关系测量表明，伴随着Cu有关材料的引入，磁电阻图像发生了明显变化．对纯La2／3Cal／3MnO3颗粒系统，磁电阻随温度降低而单调增加；而对复合颗粒系统，磁电阻最明显的特征是绝缘体-金属转变温度附近表现出尖的磁电阻峰，且有较高的磁电阻值．结合低场磁化实验对实验结果进行了讨论．     

La0.60Sr0.40-xNaxMnO3钙钛矿材料在室温附近磁电阻的温度稳定性

采用溶胶-凝胶法(sol-gel)制备了名义组分为La0.60Sr0.40-xNaxMnO3(x=0.00,0.10,0.15,0.20,0.30,0.33,0.35)的类钙钛矿型稀土锰氧化物多晶样品。发现用Na+替代部分Sr+2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的磁场作用下,样品La0.60Sr0.07Na0.33MnO3在292K时磁电阻为24.4%,比不含Na的La0.60Sr0.40MnO3增大了2.8倍;样品La0.60Sr0.25Na0.15MnO3在285K～345K温区内磁电阻保持在3.9%(±0.2%)左右,受温度影响不大,因此显著提高了样品的室温磁电阻和其温度稳定性。迄今为止还未见这类材料在室温附近具有如此宽范围和高温度上限的MR温度稳定性报道。这对于该类磁电阻材料的应用有很大意义。     

La0.6Sr0.15Na0.1□0.15MnO3+xAgNO3复合体系的结构、磁性及磁电阻的温度稳定性研究

采用固相反应法制备了La0.6Sr0.15Na0.1□0.15MnO3+xAgNO3(x=0.08、0.16、1.00,x为物质的量比,□代表空位)系列复合材料样品,对复合样品的相结构、磁性质及其磁电阻效应进行了研究。通过相结构的研究发现:复合样品近似呈现La0.6Sr0.15Na0.1□0.15MnO3/Ag/Mn3O4的特殊复合结构。通过对磁电阻的研究发现:复合样品不同程度地提高了母体样品的MR峰值,降低了峰值温度,使其更接近室温。在204～280K的温度范围内和1.8T外磁场作用下,当x=0.08时,MR保持在(4.70±0.25)%;x=0.16时,MR保持在(7.20±0.28)%。所以,复合样品较之母体样品,磁电阻的温度稳定性有了很大提高。     

(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.     

La0.67 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。     

La2/3Sr1/3MnO3掺杂RE(RE=Pr、Eu、Y、Tb)的结构、红外及磁学性质

采用空气中固相反应烧结法制备了一系列钙钛矿结构的(La1-xREx)2/3Sr1/3MnO3(RE=Pr、Eu、Y、Tb;x=0、0.3或0.4)掺杂稀土锰氧化物多晶样品.X射线衍射(XRD)分析表明随着RE离子半径的减小,样品XRD的衍射峰位置普遍向高角度偏移,2θ增大0.02～0.62°.扫描电镜(SEM)观测的结果表明掺杂RE离子的半径越小,形成多晶样品的晶粒越小,未掺杂RE的La2/3Sr1/3MnO3在所有的样品中晶粒最大.红外吸收光谱测量发现样品在599～629cm-1范围出现了吸收峰并且峰的位置随掺杂RE离子半径的减小而向低频方向偏移.样品的磁性质测量表明掺杂稀土离子的半径及磁矩对材料的磁电阻有明显影响.     

(1-x)La2/3Ca 1/3 MnO 3/xSb2O 5复合体系的电子输运和磁电阻行为

用两步法制备了（1-x）La2／3Ca1／3MnO3／xSb2O5复合样品．零场下电阻温度关系测量表明，对x〈3％范围。随Sb2O5含量增加，复合样品电阻率增大，绝缘体一金属转变温度降低；而对x〉3％范围，随Sb2O5含量增加。复合样品电阻率减小，绝缘体一金属转变温度提高．磁电阻测量表明，少量Sb2O5同La,2／3C＆1／3MnO3复合，可明显增强磁电阻效应．     

低磁场条件下（Gd1-xREx）5Si4（RE：Dy,Tb；x=0、0.1、0.125、0.15）合金磁热效应研究术

用高频真空悬浮炉在氲气保护下制备了（Gd1-xREx）5Si4（RE：Dy,Tb；x=0、0.1、0.125、0.15）和（Gd1-xREx）5Si4（x=0.1、0.125）合金样品，用直接测量方法测定了在永磁体提供的低磁场（H=1．3T）条件下样品的磁热效应曲线（△Tab-T）；结果表明，用稀土金属Dy、Tb分别部分替代Gd5Si4合金中的Gd原子，不能增强合金的磁热效应，但可在333～300K范围改变材料的居里温度，且磁热效应峰值范围变宽．     

Microwave assisted hydrothermal synthesis and magnetocaloric properties of La0.67Sr0.33MnO3 manganite

We report microwave assisted hydrothermal synthesis and magnetocaloric properties of La0.67Sr0.33MnO3 manganite. The synthesized La0.67Sr0.33MnO3 nanoparticles was characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and magnetization measurements. The XRD results indicated that La0.67Sr0.33MnO3 nanoparticles have polycrystalline nature with monoclinic structure. FE-SEM results suggested that La0.67Sr0.33MnO3 nanoparticles are assembled into rod like morphology. Magnetization measurements show that La0.67Sr0.33MnO3 nanoparticles exhibit transition temperature (Tc) above room temperature. The maximum magnetic entropy change (deltaS(M))max was found to be 0.52 J/kg K near Tc approximately 325 K at applied magnetc field of 20 kOe. This compound may considered as potential material for magnetic refrigeration near room temperature.     

涂料型La1-xSrxMnO3智能热控涂层的研究

以固相反应法制备了高纯度La0.8Sr0.2MnO3粉体,并以其为基料,磷酸二氢铝为粘结剂,采用涂覆工艺在铝基片上制备了涂料型La0.8Sr0.2MnO3热控涂层.采用XRD、EDS对La0.8Sr0.2MnO3粉体的成分进行了表征,用稳态卡计法测量了涂层在100~100℃温度区间内热辐射率随温度的变化,并测量了涂层的太阳吸收比.研究结果表明:粉体合成过程中,经过1200℃三次热处理制备的La0.8Sr0.2MnO3粉体纯度高,合成的粉体具有均匀的微米级粒径尺寸.通过适当调整浆料中La0.8Sr0.2MnO3粉体所占质量百分比,获得辐射率变化范围大于0.3的热控涂层,该性能与采用烧结工艺制备的La0.8Sr0.2MnO3陶瓷片材料在变温条件下的辐射率变化范围接近.该涂层在航天器热控技术中具有潜在的应用前景.     

Stabilization of Ferromagnetic Order in La(0.7)Sr(0.3)MnO(3)-SrRuO(3) Superlattices

The study of spatially confined complex oxides is of wide interest, since correlated electrons at interfaces might form exotic phases. Here La(0.7)Sr(0.3)MnO(3)/SrRuO(3) superlattices with coherently grown interfaces were studied by structural techniques, magnetization, and magnetotransport measurements. Magnetization measurements showed that ferromagnetic order in ultrathin La(0.7)Sr(0.3)MnO(3) layers is stabilized in the superlattices down to layer thicknesses of at least two unit cells. This stabilization is destroyed, if the ferromagnetic layers are separated by two unit cell thick SrTiO(3) layers. The resistivity of the superlattices showed metallic behavior and was dominated by the conducting SrRuO(3) layers, the off-diagonal resistivity showed an anomalous Hall effect from both SrRuO(3) and La(0.7)Sr(0.3)MnO(3) layers. This shows that the La(0.7)Sr(0.3)MnO(3) layers are not only ferromagnetic but also highly conducting; probably a conducting hole gas is induced at the interfaces that stabilizes the ferromagnetic order. This result opens up an alternative route for the fabrication of two-dimensional systems with long-range ferromagnetic order.     

Exchange coupling and exchange bias in La0.7Sr0.3MnO3-SrRuO3 superlattices

La(0.7)Sr(0.3)MnO(3)-SrRuO(3) superlattices with and without nanometrically thin SrTiO(3), BaTiO(3) and Ba(0.7)Sr(0.3)TiO(3) interlayers were grown by pulsed laser deposition. Transmission electron microscopy studies showed coherent growth of La(0.7)Sr(0.3)MnO(3), SrRuO(3) and SrTiO(3) layers with atomically sharp interfaces, even if individual layers were as thin as one or two unit cells. In contrast, misfit dislocations and unit cell high interfacial steps were observed at the interfaces between BaTiO(3) and one of the ferromagnetic layers. The presence of the interlayers as well as these extended defects had a significant influence on the magnetic properties of the superlattices, especially on the antiferromagnetic interlayer exchange coupling between the La(0.7)Sr(0.3)MnO(3) and SrRuO(3) layers and the exchange biasing. Surprisingly, exchange biasing was found to increase with decreasing strength of the antiferromagnetic interlayer exchange coupling. This was explained by different magnetization reversal mechanisms acting in the regimes of strong and weak interlayer exchange coupling.     

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.     

异质结构掺锰氧化物（La0.60Sr0．4-xNaxMnO3）的磁电阻研究术

利用掺杂锰氧化物La0.60Sr0.05Na0.05MnO3和La0.60Sr0.15Na0.25MnO3两种块体材料串联焊接成异质结构样品,其磁电阻在不同温度出现两个峰值。得到启示：选取一系列磁电阻峰值在不同温度的材料,制成异质结构样品多层膜,适当调整各层膜厚度的比例,可以使多层膜的磁电阻在一个较宽的温度区间内保持基本稳定,并且有较好的磁场灵敏度,从而成为较好的磁电阻磁场传感器。