复相钙钛矿锰氧化物电磁输运特性研究
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摘要
具有庞磁电阻效应(CMR)的稀土掺杂钙钛矿锰氧化物中属于强关联电子体系，表现出许多复杂的物理效应，在电荷—自旋—轨道—晶格之间存在各种相互作用，从而诱发绝缘体—金属(I—M)相变，有序化和相分离等一系列奇异效应，是近年来凝聚态物理和材料物理领域的热点课题。LFMR(低场磁电阻)是CMR研究领域中的一个新的研究方向，LFMR是非本征的磁电阻，一般发生在晶界效应明显的多晶样品中。LFMR只需要很小的驱动磁场，而且对温度不是非常敏感，有利于CMR材料进入实用化阶段。在体系中引入第二相物质形成复合相是低场增强磁电阻的一个有效方法。本文的工作是以低场增强磁电阻为出发点，以复合相钙钛矿锰氧化物为研究对象。按照传统的固相烧结法以及新型的粘接方法制备了含有不同第二相物质的复合相钙钛矿锰氧化物，研究不同的复合体系的电磁性质。
全文主要包括以下方面内容：
1．按照传统烧结陶瓷方法制备了烧结型的LaNiO_3/La_(0.7)Sr_(0.3)MnO_3、Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3、La_(0.7)Ca_(0.3)MnO_3/La_(0.7)Sr_(0.3)MnO_3复合相样品。研究顺磁导电氧化物、绝缘性氧化物、铁磁导电氧化物作为第二相物质时，复合体系性质的差异。同时考察母体粉颗粒大小对复合体系性质的影响。
在溶胶-凝胶前驱粉的LaNiO_3/La_(0.7)Sr_(0.3)MnO_3复合相样品中，当LaNiO_3含量x不超过3%时，饱和磁化强度随x的增加而增加；但x超过3%以后，饱和磁化强度随x的增加而降低。LaNiO_3含量对顺磁—铁磁相变温度(T_c)的影响很小，但对绝缘体—金属相变温度(T_(IM))影响大。由于复合体系的电阻率强烈依赖于晶粒表面自旋极化电子散射作用，复合样品电阻率并没有随LaNi03含量增加而降低。不同含量的复合相样品表现为明显的低场磁电阻特征。低温段，LaNiO_3含量1%的样品具有最大的磁电阻。LaNiO_3含量5%的样品磁电阻值(MR)在室温附近基本上不随温度改变，在接近350K时，MR有增加的趋势。
对于Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3复合相样品，用溶胶-凝胶法和固相反应法两种前驱母粉制备了不同的复合相样品。利用溶胶-凝胶前驱母粉制备的复合相样品，晶粒较小，第二相物质易与母相发生反应。Ta_2O_5含量增加，不仅一些Mn~(2+)被Ta~(5+)离子替代，而且Mn~(3+)、Mn~(4+)离子也会被替代，替代作用仅发生在晶粒表面，使得晶胞体积降低。Ta_2O_5含量增加使得样品饱和磁化强度降低。样品T_c随Ta_2O_5含量增加有幅度很小的增加，这是由于晶胞体积降低、双交换作用增加引起的。T_(IM)随着Ta_2O_5含量增加而增加。不同含量的复合相样品表现为明显的低场磁电阻特征。Ta_2O_5含量为4%的样品具有最大的磁电阻值。利用溶胶-凝胶前驱母粉制备的Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3复合相样品中，低温短时间烧结的样品的磁化率微分曲线中可以观察到两个磁相变峰，相应的T_(c1)是晶界的贡献，T_(c2)是晶粒的贡献，第二相主要影响晶界的性质；高温长时间烧结的样品晶界相的磁性作用变得不明显，只观察到一个铁磁相变，随着烧结温度的增加，第二相的掺杂效应越来越明显，使得T_c显著的降低。对于颗粒相对很大的固相前驱母粉的Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3复合相样品，晶粒为微米级。Ta_2O_5含量引起的晶胞体积变化很小。样品的电阻率随Ta_2O_5含量增加而增加。Ta_2O_5含量改变T_(IM)只有较小的变化，远小于溶胶-凝胶前驱粉复合相样品的变化幅度。不同含量的复合相样品表现为明显的低场磁电阻特征。Ta_2O_5含量为4%的样品在整个温区范围内，都具有最大的MR。固相前驱母粉的复合相样品中，Ta_2O_5含量引起电阻率的变化比溶胶-凝胶前驱母粉的复合相样品中电阻率的变化大很多。这种差异是跟晶粒尺寸相关的。不同前驱粉的复合相样品中，磁电阻随Ta_2O_5含量变化具有类似的特性，即低含量的Ta_2O_5会使得体系的低温和高温磁电阻得到大幅度的增强，随着Ta_2O_5含量进一步增加，增强效果减弱。对于低含量Ta_2O_5的样品，固相前驱粉复合体系比溶胶前驱粉体系的低场增强效果要强。
在固相前驱粉的La_(0.7)Ca_(0.3)MnO_3/La_(0.7)Sr_(0.3)MnO_3复合相样品中，低含量的第二相样品发生绝缘体—金属(I-M)相变。高含量的复合相样品没有观察的I-M相变。样品的电阻率随着La_(0.7)Ca_(0.3)MnO_3含量的增加而减小。复合相样品的MR在整个温度范围内比单相LSMO样品大。高含量La_(0.7)Ca_(0.3)MnO_3的样品在温度超过325 K后出现本征磁电阻贡献。La_(0.7)Ca_(0.3)MnO_3在增强复合体系低场磁电阻的同时，能同时增强本征磁电阻。
比较不同第二相物质的复合体系，我们看到，当高含量的导电性氧化物作为第二相物质时，在高温时会诱发本征磁电阻的出现，低温时也能得到增强的低场磁电阻，也就是本征磁电阻和非本征(低场)磁电阻同时得到增强。
2．研究了烧结型陶瓷的ZnO/La_xSr_(1-x)MnO_3和ZnO/La_xCa_(1-x)MnO_3复合相体系的电磁输运特性。
在La_(0.7)Sr_(0.3)MnO_3与ZnO复合体系中，当LSMO含量为1%-5%时，样品表现出非线性的电压—电流特性。LSMO含量为2%的样品的非线性系数最大。复合相的样品具有铁磁性。磁场可以改变样品的电学性质，表现为正的磁电阻特性。磁场使得晶界处的势垒高度增加，从而样品电阻增加。
对于(La_xCa_(1-x)MnO_3)_(0.2)/(ZnO)_(0.8)体系，当x=0.7时，两相之间的反应小，对磁性的影响较小；而x=0.9和0.6的样品两相之间反应大，对磁性的影响大。大量的非磁性相存在使得样品的绝缘体—金属相变温度远低于其铁磁—顺磁相变温度。当x=0.7时，样品磁电阻与温度的曲线出现峰值，表现出本征磁电阻性质，磁电阻峰具有温度展宽效应。
在La_(0.7)Sr_(0.3)MnO_3含量大于15%的La_(0.7)Sr_(0.3)MnO_3/ZnO复合体系中，观察到明显的磁电阻效应。烧结温度的增加，使得I-M相变温度几乎线性的增加。烧结温度增加，电阻率降低，这是晶界作用弱化的结果。LSMO的含量为33%、烧结温度为1100℃的样品的磁电阻效应最强。
在特定样品(ZnO)_(0.8)/(La_(0.4)Ca_(0.6)MnO_3)_(0.2)中出现了亚稳态GCMR(giantcolossal magnetoresistance)现象。磁电阻值在270 K时约为-1.5×10~5%，但是仅仅发生在样品经历外加磁场由小到大，并且温度降低到50K的特定测试过程中。亚稳态GCMR现象来源于样品中出现了电荷有序相变。
3．传统的烧结陶瓷复合相体系，由于存在烧结过程，母体相与第二相之间存在扩散反应，并不是真正意思的复合相。为此，首次提出粘接型钙钛矿锰氧化物的概念，制备了高分子(环氧树脂)粘接、金属(锡)粘接、复合(环氧树脂+锡)粘接的三种样品。利用粘接方法可以得到真正意义的复合相钙钛矿锰氧化物样品。
对于环氧树脂粘接粘接的样品，粘接样品的相结构在粘接过程中没有任何改变。除了有机物的磁性稀释作用，样品的本征磁性没有任何改变。所有的样品都表现为纯半导体行为，没有观察到I-M相变。环氧树脂含量1%的样品。在温度为250K-350 K范围内，磁电阻几乎不随温度改变。
在锡粘接的样品中，含量低的样品发生I-M相变。含量超过10%的样品表现为纯金属行为。锡含量为5%和8%的样品，表现为典型的低场磁电阻特征。含量10%的样品既没有出现本征磁电阻峰，低场磁电阻特征也不明显。而含量为20%的样品，几乎不表现磁电阻效应。
对于固定锡含量(10%)而改变环氧树脂含量(1％-4％)的复合粘接样品，在环氧树脂含量1％样品中出现结晶态的有机物。环氧树脂含量为2％的样品出现I-M相变，T_(IM)高于单独锡粘接的样品。环氧树脂含量为1％和4％的样品不出现I-M相变。环氧树脂含量为1％的样品，电阻较大。含量为2％的样品电阻率比单独Sn粘接样品的电阻小。环氧树脂含量1％的样品表现为明显的低场磁电阻特征，环氧树脂含量2％的样品在整个测量温区范围内表现为小的负磁电阻值，本征磁电阻和低场磁电阻都没有表现出来。当环氧树脂含量4％时，在从50K到300K的广泛温区内，磁电阻都保持较大的数值(-20％)，且温度变化时磁电阻值变化很小。当温度超过300K后，出现本征磁电阻行为。
粘接样品没有经过高温烧结过程，是一种真正意义的复合相材料。如果在粘接体系中引入其它的物质并改变制备工艺，有可能获得非温度敏感性、具有较高磁电阻值、可以实用化的材料。
As a strongly correlated electron system, the rare earth doped perovskite manganese which has colossal magnetoresistance (CMR) effect is the hot topic in the research of condensed physics and materials physics, due to exhibiting intriguing physical properties such as insulator-metal transition, charge ordering, orbital ordering and phase separation. In recent years, low field magnetoresistance (LFMR) has become a new research aspect in the research area of CMR. Very different from CMR, LFMR is an extrinsic characteristic which commonly happens in polycrystalline material. The advantage of LFMR-low drive magnetic field and non temperature sensitive-favors the practical application of CMR materials. Ordinarily, enhanced LFMR is obtained by making a composite of the perovskite manganese oxides with a secondary phase. This is called the manganese-based two-phase composition. In this work, the main research content is enhanced low field magnetoresistance in two-phase perovskite manganese composition. The perovskite manganese composition containing different second phase is prepared using conventional sintering method and newly bonded method, and the corresponding electrical and magnetic properties are studied.
The main contents are as follows:
1, The perovskite manganese composition containing different second phase, that is LaNiO_3/La_(0.7)Sr_(0.3)MnO_3, Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3 andLa_(0.7)Ca_(0.3)MnO_3/La_(0.7)Sr_(0.3)MnO_3, is p repared using conventional ceramic sintering method.
In the LaNiO_3/La_(0.7)Sr_(0.3)MnO_3 composite with sol-gel precursor powders, if LaNiO_3 contention is low, the value of saturate magnetization increases with the concentration of LaNiO_3. The PM-FM transition temperature varies slightly with LaNiO_3 content. The insulator-metal transition temperature is smaller and the variation is much larger. The LFMR enhancement is observed over a wide range of temperature up the room temperature. The composite of (LSMO)_(0.95)/(LNO)_(0.05) had remarkable MR value and it is not sensitive to temperature near room temperature.
In the Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3 composite with sol-gel precursor powders, a small amount of Ta ions enter into LSMO grains near the grain surface region, resulting in the deduction of the cell volume, and consequently, an extra deduction of the saturate magnetization. The paramagnetism-to-ferromagnetism transition temperature is increased. The LFMR enhancement is observed over a wide range of temperature from 50 to 350K when the second phase material (Ta_2O_5) is introduced. The magnetic disorder caused by the secondary phase at grain boundaries and the Ta doping effect on the surface of LSMO grains are believed to contribute to the enhanced MR. And in the Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3 composite with sol-gel precursor powders, if the sintering temperature and time is low, two peaks are observed in the dM/dT－T curve. The peak temperature T_(c1) is the grainboudary contribution and T_(c2) is the grain contribution, correspondingly. In the Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3 composite with solid state reaction precursor powders, the variation of cell volume insulator-metal transition temperature is slightly.
As for the La_(0.7)Ca_(0.3)MnO_3/La_(0.7)Sr_(0.3)MnO_3 composite with solid state reaction precursor powders, the insulator-metal transition is only observed with low content of second phase. The resistivity increases with the content of La_(0.7)Ca_(0.3)MnO_3. The intrinsic CMR contribution is observed in the composite with high content ofLa_(0.7)Ca_(0.3)MnO_3.
2, The properties of sintering composite of ZnO/La_xSr_(1-x)MnO_3 and ZnO/ La_xCa_(1-x)MnO_3 are studies.
The La_(0.7)Sr_(0.3)MnO_3/ZnO composites with low content of LSMO about 1%-5% exhibit nonlinear voltage-current properties. Applied magnetic filed could change the electrical property. Resistance increased after magnetic field was applied, which was a positive magnetoresistance (PMR) phenomenon. The existence of PMR is duo to the broadening of barrier at the grain boundaries caused by the magnetic field.
In the composite of (La_xCa_(1-x)MnO_3)_(0.2)/(ZnO)_(0.8), the insulator-metal transition temperature is much smaller than the PM-FM transition temperature due to the existence of large content of ZnO. For the sample of x=0.7, intrinsic CMR is observed with temperature-broadening effect.
The La_(0.7)Sr_(0.3)MnO_3/ZnO composites with high content of LSMO above 15% exhibit obvious magnetoresistant effect. The insulator-metal transition temperature increases with the sintering temperature. The composite with La_(0.7)Sr_(0.3)MnO_3 content of 33% and sintering temperature of 1100℃has the biggest value of MR.
In the composite of (ZnO)_(0.8)/(La_(0.4)Ca_(0.6)MnO_3)_(0.2), a giant CMR effect is observed for the measuring field from low to high up to 3 T and measuring temperature down to 50 K. The giant CMR effect is meta-stable which vanishes for different measuring process. The charge ordering state and capacity effect caused by the insulation ZnO between La_(0.4)Ca_(0.6)MnO_3 grains contribute to the electrical relaxation behavior.
3, The concept of bonded perovskite manganite oxide is firstly putted forward in this thesis. Three different types of bonder samples, i.e. polymer (epoxy resin), metal (tin) and complex (epoxy resin and tin), are prepared.
In the polymer bonded samples, all the samples show purely semiconducting behavior in the whole temperature range. There is a slight peak in the MR-T curve for the sample containing 2% binder and no peak is observed for the samples containing lower concentration of binder (0.5% and 1%). MR value for the sample containing 1% binder keeps at the basically same value in the temperature range of 250K-350K.
In the metal (Sn) bonded samples, the insulator-metal transition is only observed with low content of Sn. If Sn content exceeds 10%, the samples show basically pure metal behavior. The bonded samples with Sn content of 5% and 8% exhibit obviously low field magnetoresistance properties.
In the complex bonded samples with fixed Sn content and alterable epoxy resin content (1%-4%), the insulator-metal transition is observed with polymer content of 2%. Other samples do not show insulator-metal transition. The complex bonded sample with polymer content of 1% has biggest value of resistivity. The complex bonded sample with polymer content of 2% does not show obvious magnetoresistant property. In the complex bonded sample with polymer content of 4%, enhanced LFMR and intrinsic CMR are simultaneously observed.
And if another kind of material, such as insulating oxide and metal, is added to this bonded sample, the real manganite-based two-phase composition could be made, as there is no high-temperature reaction in the preparing process.
全文主要包括以下方面内容：
1．按照传统烧结陶瓷方法制备了烧结型的LaNiO_3/La_(0.7)Sr_(0.3)MnO_3、Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3、La_(0.7)Ca_(0.3)MnO_3/La_(0.7)Sr_(0.3)MnO_3复合相样品。研究顺磁导电氧化物、绝缘性氧化物、铁磁导电氧化物作为第二相物质时，复合体系性质的差异。同时考察母体粉颗粒大小对复合体系性质的影响。
在溶胶-凝胶前驱粉的LaNiO_3/La_(0.7)Sr_(0.3)MnO_3复合相样品中，当LaNiO_3含量x不超过3%时，饱和磁化强度随x的增加而增加；但x超过3%以后，饱和磁化强度随x的增加而降低。LaNiO_3含量对顺磁—铁磁相变温度(T_c)的影响很小，但对绝缘体—金属相变温度(T_(IM))影响大。由于复合体系的电阻率强烈依赖于晶粒表面自旋极化电子散射作用，复合样品电阻率并没有随LaNi03含量增加而降低。不同含量的复合相样品表现为明显的低场磁电阻特征。低温段，LaNiO_3含量1%的样品具有最大的磁电阻。LaNiO_3含量5%的样品磁电阻值(MR)在室温附近基本上不随温度改变，在接近350K时，MR有增加的趋势。
对于Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3复合相样品，用溶胶-凝胶法和固相反应法两种前驱母粉制备了不同的复合相样品。利用溶胶-凝胶前驱母粉制备的复合相样品，晶粒较小，第二相物质易与母相发生反应。Ta_2O_5含量增加，不仅一些Mn~(2+)被Ta~(5+)离子替代，而且Mn~(3+)、Mn~(4+)离子也会被替代，替代作用仅发生在晶粒表面，使得晶胞体积降低。Ta_2O_5含量增加使得样品饱和磁化强度降低。样品T_c随Ta_2O_5含量增加有幅度很小的增加，这是由于晶胞体积降低、双交换作用增加引起的。T_(IM)随着Ta_2O_5含量增加而增加。不同含量的复合相样品表现为明显的低场磁电阻特征。Ta_2O_5含量为4%的样品具有最大的磁电阻值。利用溶胶-凝胶前驱母粉制备的Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3复合相样品中，低温短时间烧结的样品的磁化率微分曲线中可以观察到两个磁相变峰，相应的T_(c1)是晶界的贡献，T_(c2)是晶粒的贡献，第二相主要影响晶界的性质；高温长时间烧结的样品晶界相的磁性作用变得不明显，只观察到一个铁磁相变，随着烧结温度的增加，第二相的掺杂效应越来越明显，使得T_c显著的降低。对于颗粒相对很大的固相前驱母粉的Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3复合相样品，晶粒为微米级。Ta_2O_5含量引起的晶胞体积变化很小。样品的电阻率随Ta_2O_5含量增加而增加。Ta_2O_5含量改变T_(IM)只有较小的变化，远小于溶胶-凝胶前驱粉复合相样品的变化幅度。不同含量的复合相样品表现为明显的低场磁电阻特征。Ta_2O_5含量为4%的样品在整个温区范围内，都具有最大的MR。固相前驱母粉的复合相样品中，Ta_2O_5含量引起电阻率的变化比溶胶-凝胶前驱母粉的复合相样品中电阻率的变化大很多。这种差异是跟晶粒尺寸相关的。不同前驱粉的复合相样品中，磁电阻随Ta_2O_5含量变化具有类似的特性，即低含量的Ta_2O_5会使得体系的低温和高温磁电阻得到大幅度的增强，随着Ta_2O_5含量进一步增加，增强效果减弱。对于低含量Ta_2O_5的样品，固相前驱粉复合体系比溶胶前驱粉体系的低场增强效果要强。
在固相前驱粉的La_(0.7)Ca_(0.3)MnO_3/La_(0.7)Sr_(0.3)MnO_3复合相样品中，低含量的第二相样品发生绝缘体—金属(I-M)相变。高含量的复合相样品没有观察的I-M相变。样品的电阻率随着La_(0.7)Ca_(0.3)MnO_3含量的增加而减小。复合相样品的MR在整个温度范围内比单相LSMO样品大。高含量La_(0.7)Ca_(0.3)MnO_3的样品在温度超过325 K后出现本征磁电阻贡献。La_(0.7)Ca_(0.3)MnO_3在增强复合体系低场磁电阻的同时，能同时增强本征磁电阻。
比较不同第二相物质的复合体系，我们看到，当高含量的导电性氧化物作为第二相物质时，在高温时会诱发本征磁电阻的出现，低温时也能得到增强的低场磁电阻，也就是本征磁电阻和非本征(低场)磁电阻同时得到增强。
2．研究了烧结型陶瓷的ZnO/La_xSr_(1-x)MnO_3和ZnO/La_xCa_(1-x)MnO_3复合相体系的电磁输运特性。
在La_(0.7)Sr_(0.3)MnO_3与ZnO复合体系中，当LSMO含量为1%-5%时，样品表现出非线性的电压—电流特性。LSMO含量为2%的样品的非线性系数最大。复合相的样品具有铁磁性。磁场可以改变样品的电学性质，表现为正的磁电阻特性。磁场使得晶界处的势垒高度增加，从而样品电阻增加。
对于(La_xCa_(1-x)MnO_3)_(0.2)/(ZnO)_(0.8)体系，当x=0.7时，两相之间的反应小，对磁性的影响较小；而x=0.9和0.6的样品两相之间反应大，对磁性的影响大。大量的非磁性相存在使得样品的绝缘体—金属相变温度远低于其铁磁—顺磁相变温度。当x=0.7时，样品磁电阻与温度的曲线出现峰值，表现出本征磁电阻性质，磁电阻峰具有温度展宽效应。
在La_(0.7)Sr_(0.3)MnO_3含量大于15%的La_(0.7)Sr_(0.3)MnO_3/ZnO复合体系中，观察到明显的磁电阻效应。烧结温度的增加，使得I-M相变温度几乎线性的增加。烧结温度增加，电阻率降低，这是晶界作用弱化的结果。LSMO的含量为33%、烧结温度为1100℃的样品的磁电阻效应最强。
在特定样品(ZnO)_(0.8)/(La_(0.4)Ca_(0.6)MnO_3)_(0.2)中出现了亚稳态GCMR(giantcolossal magnetoresistance)现象。磁电阻值在270 K时约为-1.5×10~5%，但是仅仅发生在样品经历外加磁场由小到大，并且温度降低到50K的特定测试过程中。亚稳态GCMR现象来源于样品中出现了电荷有序相变。
3．传统的烧结陶瓷复合相体系，由于存在烧结过程，母体相与第二相之间存在扩散反应，并不是真正意思的复合相。为此，首次提出粘接型钙钛矿锰氧化物的概念，制备了高分子(环氧树脂)粘接、金属(锡)粘接、复合(环氧树脂+锡)粘接的三种样品。利用粘接方法可以得到真正意义的复合相钙钛矿锰氧化物样品。
对于环氧树脂粘接粘接的样品，粘接样品的相结构在粘接过程中没有任何改变。除了有机物的磁性稀释作用，样品的本征磁性没有任何改变。所有的样品都表现为纯半导体行为，没有观察到I-M相变。环氧树脂含量1%的样品。在温度为250K-350 K范围内，磁电阻几乎不随温度改变。
在锡粘接的样品中，含量低的样品发生I-M相变。含量超过10%的样品表现为纯金属行为。锡含量为5%和8%的样品，表现为典型的低场磁电阻特征。含量10%的样品既没有出现本征磁电阻峰，低场磁电阻特征也不明显。而含量为20%的样品，几乎不表现磁电阻效应。
对于固定锡含量(10%)而改变环氧树脂含量(1％-4％)的复合粘接样品，在环氧树脂含量1％样品中出现结晶态的有机物。环氧树脂含量为2％的样品出现I-M相变，T_(IM)高于单独锡粘接的样品。环氧树脂含量为1％和4％的样品不出现I-M相变。环氧树脂含量为1％的样品，电阻较大。含量为2％的样品电阻率比单独Sn粘接样品的电阻小。环氧树脂含量1％的样品表现为明显的低场磁电阻特征，环氧树脂含量2％的样品在整个测量温区范围内表现为小的负磁电阻值，本征磁电阻和低场磁电阻都没有表现出来。当环氧树脂含量4％时，在从50K到300K的广泛温区内，磁电阻都保持较大的数值(-20％)，且温度变化时磁电阻值变化很小。当温度超过300K后，出现本征磁电阻行为。
粘接样品没有经过高温烧结过程，是一种真正意义的复合相材料。如果在粘接体系中引入其它的物质并改变制备工艺，有可能获得非温度敏感性、具有较高磁电阻值、可以实用化的材料。
As a strongly correlated electron system, the rare earth doped perovskite manganese which has colossal magnetoresistance (CMR) effect is the hot topic in the research of condensed physics and materials physics, due to exhibiting intriguing physical properties such as insulator-metal transition, charge ordering, orbital ordering and phase separation. In recent years, low field magnetoresistance (LFMR) has become a new research aspect in the research area of CMR. Very different from CMR, LFMR is an extrinsic characteristic which commonly happens in polycrystalline material. The advantage of LFMR-low drive magnetic field and non temperature sensitive-favors the practical application of CMR materials. Ordinarily, enhanced LFMR is obtained by making a composite of the perovskite manganese oxides with a secondary phase. This is called the manganese-based two-phase composition. In this work, the main research content is enhanced low field magnetoresistance in two-phase perovskite manganese composition. The perovskite manganese composition containing different second phase is prepared using conventional sintering method and newly bonded method, and the corresponding electrical and magnetic properties are studied.
The main contents are as follows:
1, The perovskite manganese composition containing different second phase, that is LaNiO_3/La_(0.7)Sr_(0.3)MnO_3, Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3 andLa_(0.7)Ca_(0.3)MnO_3/La_(0.7)Sr_(0.3)MnO_3, is p repared using conventional ceramic sintering method.
In the LaNiO_3/La_(0.7)Sr_(0.3)MnO_3 composite with sol-gel precursor powders, if LaNiO_3 contention is low, the value of saturate magnetization increases with the concentration of LaNiO_3. The PM-FM transition temperature varies slightly with LaNiO_3 content. The insulator-metal transition temperature is smaller and the variation is much larger. The LFMR enhancement is observed over a wide range of temperature up the room temperature. The composite of (LSMO)_(0.95)/(LNO)_(0.05) had remarkable MR value and it is not sensitive to temperature near room temperature.
In the Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3 composite with sol-gel precursor powders, a small amount of Ta ions enter into LSMO grains near the grain surface region, resulting in the deduction of the cell volume, and consequently, an extra deduction of the saturate magnetization. The paramagnetism-to-ferromagnetism transition temperature is increased. The LFMR enhancement is observed over a wide range of temperature from 50 to 350K when the second phase material (Ta_2O_5) is introduced. The magnetic disorder caused by the secondary phase at grain boundaries and the Ta doping effect on the surface of LSMO grains are believed to contribute to the enhanced MR. And in the Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3 composite with sol-gel precursor powders, if the sintering temperature and time is low, two peaks are observed in the dM/dT－T curve. The peak temperature T_(c1) is the grainboudary contribution and T_(c2) is the grain contribution, correspondingly. In the Ta_2O_5/La_(0.7)Sr_(0.3)MnO_3 composite with solid state reaction precursor powders, the variation of cell volume insulator-metal transition temperature is slightly.
As for the La_(0.7)Ca_(0.3)MnO_3/La_(0.7)Sr_(0.3)MnO_3 composite with solid state reaction precursor powders, the insulator-metal transition is only observed with low content of second phase. The resistivity increases with the content of La_(0.7)Ca_(0.3)MnO_3. The intrinsic CMR contribution is observed in the composite with high content ofLa_(0.7)Ca_(0.3)MnO_3.
2, The properties of sintering composite of ZnO/La_xSr_(1-x)MnO_3 and ZnO/ La_xCa_(1-x)MnO_3 are studies.
The La_(0.7)Sr_(0.3)MnO_3/ZnO composites with low content of LSMO about 1%-5% exhibit nonlinear voltage-current properties. Applied magnetic filed could change the electrical property. Resistance increased after magnetic field was applied, which was a positive magnetoresistance (PMR) phenomenon. The existence of PMR is duo to the broadening of barrier at the grain boundaries caused by the magnetic field.
In the composite of (La_xCa_(1-x)MnO_3)_(0.2)/(ZnO)_(0.8), the insulator-metal transition temperature is much smaller than the PM-FM transition temperature due to the existence of large content of ZnO. For the sample of x=0.7, intrinsic CMR is observed with temperature-broadening effect.
The La_(0.7)Sr_(0.3)MnO_3/ZnO composites with high content of LSMO above 15% exhibit obvious magnetoresistant effect. The insulator-metal transition temperature increases with the sintering temperature. The composite with La_(0.7)Sr_(0.3)MnO_3 content of 33% and sintering temperature of 1100℃has the biggest value of MR.
In the composite of (ZnO)_(0.8)/(La_(0.4)Ca_(0.6)MnO_3)_(0.2), a giant CMR effect is observed for the measuring field from low to high up to 3 T and measuring temperature down to 50 K. The giant CMR effect is meta-stable which vanishes for different measuring process. The charge ordering state and capacity effect caused by the insulation ZnO between La_(0.4)Ca_(0.6)MnO_3 grains contribute to the electrical relaxation behavior.
3, The concept of bonded perovskite manganite oxide is firstly putted forward in this thesis. Three different types of bonder samples, i.e. polymer (epoxy resin), metal (tin) and complex (epoxy resin and tin), are prepared.
In the polymer bonded samples, all the samples show purely semiconducting behavior in the whole temperature range. There is a slight peak in the MR-T curve for the sample containing 2% binder and no peak is observed for the samples containing lower concentration of binder (0.5% and 1%). MR value for the sample containing 1% binder keeps at the basically same value in the temperature range of 250K-350K.
In the metal (Sn) bonded samples, the insulator-metal transition is only observed with low content of Sn. If Sn content exceeds 10%, the samples show basically pure metal behavior. The bonded samples with Sn content of 5% and 8% exhibit obviously low field magnetoresistance properties.
In the complex bonded samples with fixed Sn content and alterable epoxy resin content (1%-4%), the insulator-metal transition is observed with polymer content of 2%. Other samples do not show insulator-metal transition. The complex bonded sample with polymer content of 1% has biggest value of resistivity. The complex bonded sample with polymer content of 2% does not show obvious magnetoresistant property. In the complex bonded sample with polymer content of 4%, enhanced LFMR and intrinsic CMR are simultaneously observed.
And if another kind of material, such as insulating oxide and metal, is added to this bonded sample, the real manganite-based two-phase composition could be made, as there is no high-temperature reaction in the preparing process.
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