Possible origins for paramagnetic-ferromagnetic and insulator-metal transitions as well as colossal magnetoresistance in manganites

Systematical investigations of zero-field resistivity, magnetoresistance and magnetization were performed for a typical manganese compound La_2/3Ca_1/3MnO₃. It is argued that the common origin for insulator-metal and paramagenetic ferromagnetic-transitions as well as colossal magnetoresistance is due to the formation of ferromagnetic clusters in the paramagnetic background. The transition to metallic state is resulted from percolation of ferromagnetic metallic clusters, while the colossal magnetoresistance is due to the application of magnetic field, which accelerates the growth of ferromagnetic metallic clusters and causes the shift of the onset temperature for the metallic percolation to higher temperature. Based on the random resistor network model, the zero-field resistivity versus temperature dependence is simulated by using experimental parameters, and experimental data well agree with those in whole temperature range, giving a strong support to our approach.     

化学共沉淀合成LCMO纳米结构的热化学行为研究

采用化学共沉淀法,并经热处理的方法制备了La1-xCaxMnO3(x=0.4)钙钛矿型氧化物,经XRD确定样品为钙钛矿型结构,TEM分析表明,样品直径约为200 nm,为棒状.结合热分析曲线对共沉淀产物的热化学行为进行了系统分析.采用该法,可以大大降低反应温度,并可控制产品的尺寸形貌均一.     

钙钛矿锰氧化物La_(0.9)Sb_(0.1)MnO_3电子自旋共振谱的研究

制备了电子掺杂钙钛矿锰氧化物庞磁电阻材料La0.9Sb0.1MnO3,并在不同温度测量了La0.9Sb0.1MnO3的电子自旋共振谱.观察到La0.9Sb0.1MnO3体系存在相分离现象,实验证明,这种电子掺杂的锰氧化物具有复杂的磁结构,随温度升高样品逐渐由铁磁态向顺磁态转变,在220～260K的过渡温区,样品的磁状态最复杂,存在铁磁、顺磁多个磁相共存的现象.通过分解电子自旋共振的积分吸收谱,分析了La0.9Sb0.1MnO3样品磁性随温度的演变过程,并讨论了电子掺杂锰氧化物的磁不均匀性和相分离现象.     

La1-xZnxMnO3和La2/3Sr_((1-x)/3)Znx/3MnO3的磁电输运特性

用固相反应方法制备La1-xZnxMnO3和La2/3Sr(1-x)/3Znx/3MnO3化合物。La1-xZnxMnO3在不同掺杂浓度x的研究表明:Zn2+在一定浓度范围内掺杂,具有钙钛矿结构,但掺杂浓度在50%~70%时有ZnO衍射峰,其电阻率在100 K以上随温度而下降,当掺杂浓度为x=0.3时电阻率最低。当Zn2+和Sr2+共掺而     

Transport-magnetism Correlation and Colossal Magnetoresistance in Mixed-valent Manganites

A phenomenological model based on phase separation between ferromagnetic metallic and paramagnetic insulating domains was applied to analyze the electrical transport and colossal magntoresistance for mixed-valent manganites of RE2/3 AE1/3 MnO3 . The results show that the model can yield results in agreement with experimental observatioas in these mangardtes. The present approach provides a simple picture to visualize the reason that the temperature dependence of resistance ( with and without applied magnetic fields ) in these compounds has the peculiar shape, without invoking any complicated concept.     

Magnetoresistance plateau in La2/3Ca1/3Bi x Mn1 − x O3 granular system

The effect of doping additional Bi on the magnetoresistance (MR) of La_2/3Ca_1/3B _x Mn_1−x O₃ was investigated. It is found that traditional colossal magnetoresistance (CMR) peak can only be observed in the x<0.05 samples and the peak value decreases with the increase of x, but the x≥0.05 samples show a magnetoresistance plateau above 200 K because of the presence of additional (La,Ca,Bi)-O layers. Moreover, this MR plateau is enhanced for the segregation of the La, Ca, and Bi elements. Funded by the Foundation for the Author of National Excellent Doctoral Dissertation of China (FANEDD) (No. 200233)     

Enhancement of magnetic properties and thermal stability of nanocrystalline （NdDyTb）12.3（FeZrNbCu）81.7B6.0 alloy with Co substitution

The major drawbacks of Nd-Fe-B magnets are relatively low Curie temperature and poor thermal stability. Ribbons with the near stoichiometric 2:14:1 composition of Nd_(10.8)Dy_(0.75)Tb_(0.75)Fe_(79.7-x)Co_xZr_(0.8)Nb_(0.8)Cu_(0.4)B_(6.0)(x=0, 3, 6, 9, 12, 15) were prepared by rapid quenching and subsequent heat treatment. The effect of Co element on the magnetic properties, thermal stability, and microstructure of the ribbons was systematically studied by vibrating sample magnetometer(VSM), thermal magnetic analysis, atomic force microscopy (AFM), and transmission electron microscopy (TEM).It was found that Co substitution was significantly effective in improving the magnetic properties and the thermal stability of nanocrystalline ribbons. Although the intrinsic coercivity decreased from 1308.7 kA/m for x=0 to 817.4 kA/m for x=15, the remanence polarization and maximum energy product increased from 0.839 T and 116.5 kJ/m~3 for the Co-free samples to 1.041 T and 155.1 kJ/m~3 for the 12at% Co-substituted samples,respectively.About 10 K increase in Curie temperature was observed for the 2:14:1 phase with lat% Co substitution. The absolute values of temperature coefficients of induction and coercivity were significantly decreased with Co substitution, which may be attractive for high operational temperature applications. The microstructure of nanocrystalline ribbons was slightly refined with Co substitution.     

固相烧结法制备高纯LaCoO3粉末

LaCoO3是典型的ABO3型钙钛矿化合物。它具有优异的化学稳定性和高温下的良导电性，在环境催化分解、汽车尾气处理、巨磁阻材料、二次电池和燃料电池的电极材料等方面都有着十分广泛的应用前景。通过XRD谱和XPS研究发现，固相烧结法制备LaCoO3的最佳配比为Co La=105%，最佳烧结时间为50小时。     

Mn基钙钛矿样品庞磁电阻效应的Monte Carlo模拟

基于顺磁性绝缘体和铁磁金属两相随机分布的假设，对居里温度Tc分别为330K和250K的La2／3Ba1／3MnO3和Laa2/3Ca1／3MnO3两个典型样品的电阻随温度和磁场的变化进行了Monte Carlo模拟，模拟曲线在行为上相同于文献上普遍报道的实验曲线。研究表明，Mn基钙钛矿样品中Tc附近观察到的庞磁电阻是由于磁场引起Tc附近铁磁金属相体积分数的实质性增加所致。     

Gd5(Ge2-xSi2-xM2x)化合物的晶体结构和磁性研究

研究了用稳定元素Mn、Mo、Al和V代替非磁性的Si和Ge对Gd5Ge2Si2化合物的居里温度的影响；并且研究了Gd5(Ge2-xSi2-xM2x)(M=Mn、Mo、Al和V)样品退火处理对居里温度的影响．结果表明少量的合金化元素替代会提高Gd5Ge2Si2化合物的居里温度；退火后居里温度随替代元素的含量变化比退火前更敏感．X射线粉末衍射分析表明少量的合金化元素(Mn、Mo、Al和V)替代不会影响母体Gd5Ge2Si2化合物的晶体结构．对样品的晶态显微组织和表面微结构研究发现退火后组织分布比退火前更均匀。     

La0.7Ca0.3MnO3多晶材料制备及其电磁特性

以化学溶液分解法制备La0.7Ca0.3MnO3粉体,常压烧结制备多晶块体样品。对La0.7Ca0.3MnO3样品的磁矩、磁致电阻效应（CMR）和电脉冲诱导电阻转变效应（EPIR）进行了研究。磁矩零场冷和场冷试验结果发现,居里温度以下,零场冷和场冷条件下磁矩具有较大差别。这说明材料中缺少真正的长程磁有序。La0.7Ca0.3MnO3多晶具有磁致电阻效应,在9 T磁场作用下,磁致电阻变化大约为70%。在80 V,20 ns电脉冲作用下,材料具有电脉冲诱导电阻转变效应。从理论上对磁致电阻效应与电脉冲诱导电阻转变效应的机理进行了分析。     

Magnetic properties of Fe3（1-x）Cr3xC alloys

The magnetic properties of Fe33(1-x)Cr3xC alloys with x=0.05, 0.1, 0.15, and 0.2, which crystallize in the cementiteFe3C-type structure with space group Pnma, were investigated by means of magnetization measurements. These alloys show tem-perature-induced second-order magnetic phase transitions. The Curie temperature (Tc) of these alloys decreases with increasing x. The isothermal magnetic-entropy changes of these alloys were derived from the magnetic isotherms measured with increasing tem-x = 0.05 in a magnetic field change from 0 to 1 T and 0 to 5 T, respectively.     

Sr离子含量的减少对于La0.60Sr0.25Na0.15MnO3材料性能的影响

利用溶胶-凝胶法（sol-gel）制备了稀土锰氧化物多晶样品La0.60Sr0.25-xNa0.15MnO3,发现随着Sr离子含量的减少,样品在室温附近庞磁电阻效应有了明显的改善。在1.8 T的磁场下,对于x=0.05的样品,其CMR值在240~320 K的温区范围内均保持在5.3%（±0.2%）,温度稳定性有显著改善。     

Transport Properties of Polycrystalline La_(2/3)Ca_(1/3)Mn_(1-x)Cu_xO_3 Prepared by Sol-Gel

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Electrical Transport and Magnetic Properties of La0.67Ca0.33Mn1-xCrxO3 and La0.67＋xCa0.33-x Mn1-xCrxO3 （0.04≤x≤0.08）

A comparative study for two different series,La0.67Ca0.33Mn1-xCrxO3 and La0.67+xCa0.33-xMn1-x CrxO3,is performed with x changing from 0.04 to 0.08 through measurements of zero-field resistivity (ρ) magnetoresistance(MR) and ac susceptibility (x) as functions of temperature(T). For the second group,a single insulator-metal transition was characterized by the resistivity maximum appears in ρ VS.T curve.For the first group,besides the resistivity peak appearing at higher temperatures,the sample shows evidence for the existence of another peak appearing at lower temperatures.Compared to the first group,the second group shows a significant enhancement in MR effect,while the measurement of x-T dependence does not display obvious differences for the two series.Different behaviors observed in the tWO series are discussed by considering possible exchange interaction between Mn3+ ion and Cr3+ ion.     

亚锰酸盐La0.7-xYxCa0.3MnO3的电磁特性和磁电阻效应

研究了用固相反应法制备亚锰酸盐多晶样品La0.7-xYxCa0.3MnO3,并研究了Y的不同掺杂(x=0.1,0.2)对样品的晶体结构、电磁特性和磁电阻效应的影响.实验结果表明,随着温度的降低,x=0.1样品的R-T曲线没有出现峰值,而x=0.2样品出现了峰值,显示了复杂的输运特性.两种样品的磁电阻Rm都随着温度的降低而升高,但都没有出现峰值.x=0.2的样品在较宽的温区内,磁电阻效应都很显著,而且Y掺杂由x=0.1提高至x=0.2,磁电阻效应增强.     

金属间化合物Tb(Fe1-xNix)11.3Nb0.7的结构和磁性能研究

利用X射线衍射、扫描电镜和差示扫描量热等方法对化合物Tb（Fe1-xNix）11.3Nb0.7的结构和磁性能进行了研究.结果表明,金属间化合物Tb（Fe1-xNix）11.3Nb0.7（x=0,0.05,0.10,0.15,0.20,0.25）的结构均为ThMn12型结构,并且随Ni含量x的增大,Tb（Fe1-xNix）11.3Nb0.7金属间化合物的晶格常数a,c和单胞体积V均呈减少趋势,居里温度TC呈单调上升趋势.当x〉0.15时,Tb（Fe1-xNix）11.3Nb0.7金属间化合物中的杂质相明显呈现出来.     

NaLa_(1-x)Tb_x(MoO_4)_2的燃烧法制备及发光性能研究

采用燃烧法合成了系列NaLa(1-x)Tbx(MoO4)2绿色荧光粉。分别采用XRD和荧光分光光度计表征了合成样品的结构和发光性能,系统研究了煅烧温度及Tb3+的掺杂量对所得荧光粉物相组成及发光性能的影响。结果表明:采用燃烧法制备的NaLa(1-x)-Tbx(MoO4)2绿色荧光粉结晶良好、晶相单一、在280nm波长激发下具有良好的发光性能。确定煅烧温度为1 000℃,Tb3+的掺杂量x=0.5时发光效果最强。作为对照,采用高温固相法合成了NaLa0.5Tb0.5(MoO4)2荧光粉,结果表明,采用燃烧法合成的NaLa0.5-Tb0.5(MO4)2荧光粉具有更高的发光强度,并且大大缩短了煅烧时间。     

(Ba,Eu)TiO_3细晶粒陶瓷制备中的影响因素

采用冷压陶瓷技术,分别在1 400℃和1 300℃制备了具有四方结构的(Ba_(1-x)Eu_x)TiO_3和(Ba_(1-x)Eu_x)Ti_(1-x/8)O_3陶瓷.应用X射线衍射(XRD)、扫描电子显微镜(SEM)和原子力显微镜(AFM)研究Eu含量、Ti空位缺陷和陶瓷化烧结温度的变化对制备(Ba,Eu)TiO_3细晶粒陶瓷的影响因素.结果表明:当x=0.05时,1 300℃制备的(Ba_(1-x)Eu_x)Ti_(1-x/8)O_3陶瓷细化到平均晶粒尺寸小于1μm,1 400℃制备的(Ba_(1-x)Eu_x)TiO_3陶瓷却畸变生长到5μm.而x=0.03时,(Ba_(1-x)Eu_x)TiO_3陶瓷仍能细化到1μm.说明较高的陶瓷化烧结温度并不是晶粒生长的主要原因,Ti空位的存在起到抑制晶粒生长的作用.x≥0.07时,(Ba_(1-x)Eu_x)TiO_3陶瓷为大于5μm的粗晶粒陶瓷,说明Eu含量的继续增加不能抑制晶粒生长.     

Transport properties and microstructure of La0.7Sr0.3MnO3 nanocrystalline thin films grown by polymer-assisted chemical solution deposition

Perovskite-based materials can be widely used in the aerospace and transportation field. Perovskite manganese oxides La_0.7Sr_0.3MnO₃ (LSMO) thin films were grown on LaAlO₃ (100) and Si (100) single crystal substrates by the polymer-assisted chemical solution deposition (PACSD) method. Electronic transport behavior, microstructure, and magnetoresistance (MR) of LSMO thin films on different substrates were investigated. The resistance of LSMO films fabricated on LaAlO₃ substrates is smaller than that on the Si substrates. The magnetic field reduces resistance of LSMO films both on Si and LAO in the wide temperature region, when the insulator-metal transition temperature shifts to higher temperature. The low-field magnetoresistance of LSMO films on Si in low temperature range at 1 T is larger than that of LSMO films on LAO. However, the MR of LSMO film on LAO films at room-temperature is about 5.17 %. The thin films are smooth and dense with uniform nanocrystal size grain. These results demonstrate that PACSD is an effective technique for producing high quality LSMO films, which is significant to improve the magnetic properties and the application of automotive sensor.