钙钛矿La0.8Ca0.2MnO3多晶样品的制备及低温磁学特性

我们利用固相反应法制备了粉末状La0.8Ca0.2MnO3样品,其空间群为pnma,样品中存在超顺磁性粒子,其截止态和超顺磁态的转变温度为185K,从dσ/dT的负峰得到铁磁-顺磁相变居里温度为210K,在顺磁领域线性拟合而得到顺磁居里温度为216.4K,分子式有效磁矩为6.25μB,低温领域磁化曲线服从布洛赫T32定律,其自旋波劲度系数为49.2meV2.     

Raman光谱研究厚度对La0.9Sr0.1MnO3/Si薄膜结构的影响

通过溶胶-凝胶旋涂方法结合后退火工艺在Si（100）上制备了不同厚度的La_0.9Sr_0.1MnO₃（LSMO）薄膜,利用X射线衍射（XRD）和共焦显微拉曼散射（Raman）研究了LSMO/Si（100）薄膜的微结构．研究结果表明90 nm厚的LSMO薄膜具有正交相结构,当厚度大于150 nm时,薄膜具有菱方相结构. 150 nm厚的薄膜的Raman图谱中,490 cm^-1和602 cm^-1正交结构 关键词： 薄膜 1-xSr_xMnO₃')" href="#">La_1-xSr_xMnO₃ 共焦显微拉曼 微结构     

沉积于MgO基片上的La0.7Ca0.3MnO3薄膜的应变弛豫和磁电阻特性

利用90°离轴射频磁控溅射方法将La_0.7Ca_0.3MnO₃(LCMO)沉积于(001)取向的MgO单晶基片上,薄膜厚度变化范围为5nm到200nm. 通过掠入射X射线衍射技术测量了LCMO/MgO薄膜的面内晶格常数, 结合常规X射线衍射研究了LCMO薄膜的晶格应变及其弛豫情况, 用四探针法测量了薄膜的磁电阻特性.结果表明, LCMO/MgO薄膜均为(001)取向生长, 在厚度小于5nm时已经发生应变弛豫, 当薄膜厚度为100nm以上时, 薄膜的微应变接近于完全弛豫, 并表现出与块体材料类似的磁电阻特性, 具有较大的磁电阻和较高的磁电阻峰值温度.     

La0.8-xCa0.2MnO3纳米颗粒的居里温度与磁热效应

采用溶胶凝胶法制备了系列La_0.8-xCa_0.2MnO₃多晶样品,用X射线衍射分析确定了样品的钙钛矿结构,用透射电子显微镜观察了样品的形貌及粒径分布情况,用PAR155型振动样品磁强计测量了样品的磁性随外场和温度的变化,确定样品的居里温度并计算了各样品的磁熵变.磁测量及计算结果表明制备的各样品的居里温度在180—260K的范围内且随焙烧温度和La³⁺离子空位浓度的不同而变化,不同温度焙烧的样品均有较大的磁熵变值,其中1100℃焙烧的La_0.77Ca_0.2MnO₃,多晶样品在240.5K,H=1.0T的外场下的磁熵变达3.76J/kg·K,对实验结果做了定性的分析.该材料具有较高的居里温度和较大的磁熵变,所需外场强度适中,电阻率高,性能稳定,适合做高温磁制冷材料. 关键词： 钙钛矿 居里温度 磁热效应     

La1-xTexMnO3晶格结构的X射线粉末衍射分析

通过X射线粉末衍射数据，用Rietveld精修方法分析了Te部分替换LaMnO₃中La后，其晶格参数及其结构对称性所发生的变化.结果表明：Te掺杂LaMnO₃系列样品具有R3C的晶格结构对称性，其MnO₆八面体晶格还产生了伸张畸变，畸变程度随Te掺杂量的增加而增大.此外根据Mn—O—Mn键角、e_g电子能带的带宽、A位离子平均半径及A位离子尺寸失配度等的变化特点，推测Te掺杂LaMnO₃样品除居里温度等相变物理量将随x增加而非线性变化外，还可能产生自旋玻璃态、相分离等宏观现象. 关键词： 庞磁电阻效应 La-Te-Mn-O X射线衍射 Rietveld精修     

晶粒尺寸对Bi0.2Ca0.8MnO3电荷有序的影响

利用溶胶-凝胶法制备了平均晶粒尺寸在38—1500 nm的系列Bi_0.2Ca_0.8MnO₃多晶样品,研究了它们的结构、电输运性质和磁性.研究结果表明,随着晶粒尺寸的减小,电荷有序逐渐消失、最后被完全压制,导致低温下出现自旋玻璃态.利用马氏体相变图像和表面效应解释了相关现象. 关键词： 钙钛矿锰氧化物 电荷有序 马氏体相变 自旋玻璃     

Mn的价态对La0.8Ba0.2MnO3电磁性能的影响

在2—390K温度之间研究了La_0.8Ba_0.2MnO₃的 磁矩、磁电阻与温度的关系.发 现以不同价态的Mn元素引入得到的La_0.8Ba_0．2MnO₃ ，性能虽然都存在金属 —绝缘体转变，以及在磁场作用下居里温度附近电阻率变化非常显著的特点，但是价态对磁 性转变温度T_C，金属—绝缘体转变温度T_mi，以及磁电阻极大 值温度T_MR的影 响都非常显著.三种价态相比较，使用二价Mn的电阻率最低以及磁性转变温度更接近室温.认 为影响材料性能的主要因素是材料制备时引进的Mn元素的价态，由于原料价态的不同而形成 的氧空位浓度变化，进而影响了Mn⁴⁺/Mn³⁺的比. 关键词： 价态 磁电阻 居里温度 金属—绝缘体转变     

厚度与应变效应对La0.67Ca0.33MnO3薄膜电输运与居里温度的影响

在厚度为25—400nm范围内，系统地研究了 (001)SrTiO_3(STO), (001)LaAlO_3(LAO)衬底上La_0.67Ca_0.33Mn_O.3 (LCMO)薄膜的电输运与居里温度T_C随薄膜厚度及衬底的变化. 结果表明，随薄膜变薄，电阻率ρ增加，T_C降低. 对于同一薄膜厚度，LCMO/STO薄膜的ρ大于LCMO/LAO基上的薄膜的ρ. T_C衬底的依赖关系则与ρ相反. 分析表明，LCMO薄膜的低温区电阻温度(ρ-T) 符合关系式ρ=ρ_0+Bω_s/sin h^2(ω_s/2/k_BT)+CT^n， 其中ρ_0为剩余电阻；等号右端第二项反映软光学模声子对电子散射的贡献；第三项包括其余可能散射机 理在电输运过程中所起的作用；B，ωs（软光学模声子的平均频率）与C都为拟合系数. 高温区的电输运则由小极化子跃迁模型ρ=DT×exp（E_a/k_BT）描述（E_a为极化子激 发能）. 根据ρ_0，ωs，E_a以及T_Ｃ变化，初步讨论了薄膜中的厚度与应变效应. 进一步 研究发现ωs，E_a的变化与T_C相关，从而说明极化子效应为影响T_C变化的主要因素. 关键词： 锰氧化物薄膜 电输运 居里温度 极化子     

Co掺杂对铁磁金属La0.8Sr0.2MnO3磁电阻影响机理

通过对La_0.8Sr_0.2Mn_1-yCo_yO₃(y≤02)饱和磁矩和输运的测量,研究了Co对La_0.8Sr_0.2MnO₃的磁电阻影响机制.结果表明,在La_0.8Sr_0.2Mn_1-yCo_yO₃（y≤02）中Co³⁺离子是低自旋态.由于Mn³⁺—O—Co³⁺—O—Mn³⁺类型的磁交换与Mn³⁺-Mn⁴⁺离子间双交换作用相比较弱,Curie温度T_C附近的磁电阻随着Co掺杂量的增加而降低.与此相反,由于Co²⁺离子与e_g巡游电子的反铁磁交换耦合作用,低温区间的磁电阻随着Co掺杂量的增加而升高.     

La0.1Bi0.9-xEuxFeO3化合物的结构与性能研究

利用固相反应烧结技术制备La_0.1Bi_0.9-xEu_xFeO₃系列化合物. 利用X射线粉末衍射进行物相鉴定和结构分析,确定了材料的相关系：x≤0.05,材料为R3c结构相;0.08≤x≤0.12,材料为赝R3c结构相;x≥0.15是Pbnm相,其中0.15≤x≤0.20区域Pbnm相存在畸变. 磁测量结果表明,材料具有弱铁磁性,对于x≤0.20材料,磁矩在x=0.12成分存在极值. 利用阻抗分析仪测量了室温介电常数随成分的变化关系.讨论了材料的结构与弱铁磁性和室温介电常数间的关系. 关键词： 0.1Bi_0.9-xEu_xFeO₃')" href="#">La_0.1Bi_0.9-xEu_xFeO₃ X射线衍射 磁性 介电常数     

Transport and magnetic properties of La0.8Ce0.2MnO3 thin films grown by pulsed laser deposition

We report on the growth of La_0.8Ce_0.2MnO₃ thin films on (0 0 1) LaAlO₃ substrates by pulsed laser deposition. CeO₂ as an impurity is present in both the bulk and film samples. The electrical transport and magnetic properties of the films are similar to that of the divalent cation-doped or La deficient LaMnO₃, which show colossal magnetoresistance. Thermopower measurements indicate that the carriers are holes. The results are explained in terms of a La site deficiency due to the existence of CeO₂.     

The structural and magnetic properties of one-step mechanochemical route synthesized La0.8Pb0.2MnO3 manganites

Mechanochemical route has been used to produce La_0.8Pb_0.2MnO₃ (LPMO) nanocrystalline samples from oxide precursors. The samples were characterized using X-ray diffraction, scanning electron microscope and AC susceptibility measurements. The results showed that it is possible to produce LPMO perovskite powders after 10 h of ball milling. The crystallite size and microstrain were estimated using Williamson-Hall equation. The results showed that the crystallite size and microstrain increase initially and then decrease by the increase of milling time. By decreasing particle size the dislocation density (strain) increases and reaches to a saturation point at a particular particle size, further particle size reduction takes place through gliding motion along grain boundaries, which leads to a reduction of the strain. The dynamic properties of 15 h ball-milled sample were investigated by AC susceptibility using the Neel-Brown and Vogel-Fulcher law for superparamagnetism. The frequency dependence of blocking temperature is well described by the Vogel-Fulcher law, and fitting the experimental data with Neel-Brown law gives unphysical value for relaxation time.     

Current induced metastable states and abnormal electroresistance effect in epitaxial thin films of La0.8Ca0.2MnO3

By applying an electric current to La_0.8Ca_0.2MnO₃ films at 20 K, a high-resistive state is excited near the Curie point. Such a current induced metastable state turns out to be extremely sensitive to weak measuring currents. When the measuring current changes from 1 to 20 μA, the peak resistance near the metal-insulator transition temperature is reduced from 366 to , showing a remarkably enhanced electroresistance effect (ER=[R(0)−R(I)]/R(0)=70%). Also, in contrast to the previous reports, the metal-insulator transition shifts to a higher temperature. Our experiments show that the high-resistive state is metastable and cannot be removed by reversing the current direction. These behaviors yield a possibility to modulate the metal-insulator phase transition by using electric currents.     

Giant room-temperature magnetoresistance in La0.8Tb0.2MnO3 under the low magnetic fields

Polycrystalline perovskite La_0.8Tb_0.2MnO₃ (LTMO) with an orthorhombic phase was synthesized by conventional solid-state reaction. The magnetic and electric properties of La_0.8Tb_0.2MnO₃ were examined. The striking finding is that the material exhibits giant magnetoresistance at room temperature as high as −31.8% and −35.7% under the low magnetic fields of 100 and 1000 Oe, respectively. This result suggests that La_0.8Tb_0.2MnO₃ has a promising potential in future device developments.     

Electrical and magnetic behavior of La0.7Ca0.3MnO3/La0.7Sr0.2Ca0.1MnO3 composites

The electrical transport properties and the magnetoresistance of La_0.7Ca_0.3MnO₃/La_0.7Sr_0.2Ca_0.1MnO₃ composites are investigated as a function of sintering temperature. On the basis of an analysis by X-ray powder diffraction and scanning electron microscopy we suggest that raising the sintering temperature enhanced the interfacial reaction and creates interfacial phases at the boundaries of the La_0.7Ca_0.3MnO₃ and La_0.7Sr_0.2Ca_0.1MnO₃. Results also show that in 3 kOe, and at the Curie temperature, the magnetoresistance value of 14% was observed for the composite sintered at 1300 °C. Based on the phenomenological equation for conductivity under a percolation approach, which depends on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the experimental resistivity—temperature data from 50-300 K and find that the activation barrier decreases as temperature is increased.     

Lanthanum deficiency effect in magnetic transition and transport property of La0.8−δCa0.2MnO3

The evolution of magnetic and electrical phases in La_0.8−δCa_0.2MnO₃ was investigated in terms of La deficiency. We found that the increase of the La deficiency tends to raise the Curie temperature (T_C) in La_0.8−δCa_0.2MnO₃. The FM clusters formed in compounds with large La deficiency provide percolation paths above T_C. With increasing the La defect, the transport property changes from insulating to metallic state, which is in association with the crossover from a second order to a first order magnetic phase transition in the vicinity of T_C.     

Thickness dependence of electroresistance in La0.67Ca0.33MnO3 epitaxial films

The electroresistance (ER) of La_0.67Ca_0.33MnO₃ (LCMO) epitaxial thin films with different thicknesses was studied. For the 110 nm thick LCMO film, its ER shows a maximum at T_p, where the resistance shows a peak, and decreases to zero at lower temperatures. While for the 30 nm thick LCMO film, its ER is remarkable in a wide temperature range. Another interesting observation in this work is that the electric current can tune the magnetoresistance of the ultrathin LCMO thin film. The results were discussed by considering the coexistence of ferromagnetic metallic phase with the charge ordered phase, and the variation of the phase separation with film thickness and electric current. This work also demonstrates that electric current can tune the magnetoresistance of the manganites, which is helpful for their applications.     

Microstructures and electrical properties of La0.8Sr0.2MnO3 films synthesized by sol-gel method

La_0.8Sr_0.2MnO₃ (LSMO) thin films were fabricated on alumina substrates by an improved sol-gel dip-coating process. It was found that multiple dip-coating process could not be performed until the pre-firing temperature reached 600 °C. Different amounts of LSMO powders were added to precursor solution with an aim to avoid cracks in LSMO thin films during calcining caused by the shrinkage mismatch between the film and the substrate. The structure and surface morphology of the films prepared from precursors with and without LSMO powders were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the addition of 56.4 wt.% LSMO powders into the sol-gel precursor solution significantly modified the microstructure of films. A single LSMO perovskite phase was obtained on alumina substrate after calcining at 800 °C for 4 h by the improved sol-gel method. The sheet resistance of the films prepared with different processing parameters was measured by four-point dc method. Results indicated that the sheet resistance of films decreased with increasing the number of coating applications and the amount of LSMO powders.