【作者】 李派；

【导师】 袁松柳；

【作者基本信息】 华中科技大学 ， 材料物理与化学， 2009， 博士

【摘要】 锰基钙钛矿氧化物由于庞磁电阻效应的发现成为近年来凝聚态与材料物理领域中热点的研究。另外一方面，锰基钙钛矿氧化物具有～100％的自旋极化度，利用这一特性，将其作为铁磁金属以形成三明治式铁磁多层膜或铁磁金属颗粒系统，可以实现与边界有关的非本征磁电阻效应，这方面的研究正受到越来越多的关注。本论文在锰基钙钛矿氧化物颗粒系统的基础上，通过样品设计和制备工艺的探索，试图将不同磁行为的第二相引入到锰基钙钛矿氧化物颗粒系统中，以得到“锰基钙钛矿氧化物／不同磁行为的第二相”的颗粒复合系统，对这些复合系统的磁行为、电子输运、磁电阻等性质进行了系统的实验研究，主要研究内容和结果如下：1)通过溶胶凝胶法制备出La_2／3Ca_1／3MnO₃（LCMO）／CuMn₂O₄复合体系，从实验上研究了低居里温度（～72 K）亚铁磁性绝缘体CuMn₂O₄对LCMO磁性及电子输运性质的影响。研究结果表明，在低场0.3 T下，x=0.1复合样品的非本征磁电阻几乎比纯LCMO非本征磁电阻值大2倍。在高场3 T下，相对于纯LCMO，复合样品在相应的绝缘体—金属转变温度T_IM附近的本征磁电阻显著增强。此外，复合样品的磁化小于相应理论计算值。这些结果表明，CuMn₂O₄与LCMO之间存在一定的磁耦合作用。复合系统低温下的电阻率与温度关系遵从经验关系ρ（T）=ρ₀+ρ₁Tⁿ，但随着CuMn₂O₄复合量的增加，n数值变大，意味着亚铁磁性CuMn₂O₄的引入增加了LCMO颗粒边界的磁无序，使得电子—磁振子散射的增强。2)通过溶胶凝胶法成功将高居里温度亚铁磁性绝缘体CuFe₂O₄引入到LCMO颗粒边界，研究了CuFe₂O₄对LCMO磁性及电子输运性质的影响。研究结果表明，在外场3 T下，复合体系样品出现了两个磁电阻平台，其中一个磁电阻平台的起始温度在相应复合样品的金属—绝缘体温度附近，另一个磁电阻平台的起始温度在纯LCMO样品的金属—绝缘体温度附近。我们认为此奇异现象源于亚铁磁性CuFe₂O₄与LCMO颗粒边界磁矩存在的反铁磁耦合作用。实验结果表明，x=0.1复合样品在金属—绝缘体转变温度T_IM附近温区出现了明显的热滞现象，且其能够被外加磁场抑制。此外，还观测到向水平轴负方向移动的交换偏置现象，在矫顽力仅为100 Oe的情况下，交换偏置场高达10 Oe。复合样品中的热滞现象与交换偏置现象进一步说明LCMO与CuFe₂O₄之间存在反铁磁耦合作用。3)通过溶胶凝胶法成功将反铁磁性绝缘体LaMnO₃引入到LCMO的颗粒边界，研究了LCMO／LaMnO₃复合体系磁性及电子输运性质。在高场低场下，复合样品的磁电阻都显著增加。我们认为，第二相反铁磁性绝缘体LaMnO₃的引入增加了相邻LCMO颗粒间电子隧穿过程的磁势垒。加场后，磁势垒迅速减小，电子隧穿几率大幅度增加，从而导致磁电阻效应大大增强。同样采用经验关系ρ（T）=ρ₀+ρ₁Tⁿ对低温下电阻率随温度变化关系进行拟合，随着LaMnO₃复合量的增加，拟合参数n迅速增加，且高达数值8。这表明复合样品中电子—磁振子散射为重要的散射机制，同时还有很多未知电输运散射机制有待进一步研究。4)通过溶胶凝胶方法以及控制烧结温度，成功制备了不同颗粒尺寸的LCMO样品，研究了其电输运行为中的低温极小值现象。实验结果表明，LCMO颗粒尺寸越小，低温极小值现象越明显。随着外磁场的增加，所有颗粒尺寸样品的极小值现象减弱。通过考虑库仑阻塞效应，对Yuan隧穿模型进行扩展，扩展后的模型在整个温区以及零场、加场下都能够很好地拟合不同颗粒尺寸样品的阻温关系。这不仅说明Yuan隧穿模型的可适性，也说明对于锰基钙钛矿氧化物颗粒系统，库仑阻塞效应是低温极小值现象的主要起因。5)通过常规固相反应法制备出La_1-xBa_xMnO₃（x＞0.33）结构相分离体系，研究了其电子输运性质和渗流行为。实验结果表明，当x＞0.33，La_1-xBa_xMnO₃结构相分离为金属相La_0.67Ba_0.33MnO₃与绝缘体相BaMnO₃。不论是在150 K还是300 K，La_1-xBa_xMnO₃（x＞0.33）结构相分离体系都表现出渗流行为。经典渗流公式能够很好地拟合实验数据，在150 K下，临界参数t=1.6，s=0.6；在300 K下，临界参数t=1.7，s=0.7，且渗流阈值同为0.18。我们认为，这是源于样品的蜂窝状结构与颗粒界面效应的共同作用。此外，界面效应导致La_1-xBa_xMnO₃（x＞0.33）体系在200 K附近出现第二个电阻峰，且随着BaMnO₃含量的增加，界面效应引起的电阻峰变大。这进一步说明了界面效应对La_1-xBa_xMnO₃（x＞0.33）体系的电输运性质和渗流行为存在一定影响。更多还原

【Abstract】 In the past two decades, substantial attention has been focused on colossalmagnetoresistance（CMR） and extrinsic magnetoresistance due to their potential practicalapplication. As a matter of fact, extrinsic magnetoresistance gained much more attentionrecently as its driving magnetic field is lower compared with the high magnetic fielddriving CMR. However, the effect of grain boundary plays an important role in theextrinsic mangnetoresistance and electrical and magnetic properties. Therefore, we tend tomodify the gain boundaries of manganites by introduction of the second phase, in order toinvestigate the effect of the modified gain boundaries on the electrical and magnetictransport properties. The main investigations are shown as follows:1) The composites La_2/3Ca_1/3MnO₃（LCMO）/CuMn₂O₄ were fabricated. We applied theexperimental formulaρ（T）=ρ₀+ρ₁Tⁿ to fit the low temperature region electricaltransport: for x=0 and 0.04, n=2; For x=0.1 and 0.2, n=3. The extrinsicmagnetoresistance is enhanced substantially. We consider with the increasing amountof the ferrimagnetic insulator CuMn₂O₄, the magnetic disorder of the grain boundariesincreases and have a great effect on the electric and magnetic properties.2) The ferromagnetic insulator CuFe₂O₄ was introduced into the LCMO matrix tofabricate the LCMO/CuFe₂O₄ composites. For x=0.1, a substantial thermal hysteresisbehavior is observed at the vicinity of the metal-insulator transition temperature T_IM.This thermal hysteresis is able to be suppressed by applied magnetic field. Applyingcooling field 0.1 T on the x=0.1 composite from room temperature to 110K, we foundthe exchange bias behavior: the coactivity H_C=100 Oe, and the bias field H_ex=10 Oe.This exchange bias behavior supplies the proof to the antiferromagnetic couplingbetween LCMO and CuFe₂O₄. At H=3 T, there are two platform magnetoresistancefor all the composites; we also consider this behavior origins from theantiferromagnetic coupling between LCMO and CuFe₂O₄. 3) We fabricate the LCMO/LaMnO₃ composites where LaMnO₃ is antiferromaganeticinsulator. We applied the experimental formulaρ（T）=ρ₀+ρ₁Tⁿ to fit the lowtemperature region electrical transport: for x=0, n=3; for x=0.05, n=4; for x=0.15,n=6; for x=0.25, n=8; the n=6 and n=8 are contradictions with the max valueof n is 4.5（the electron-magnon） as far as we know. At 0.3 T and 3 T, the lowtemperature region magnetoresistance of all the composites enhance substantiallycompared with the pure LCMO.4) Through sol-gel and different sintering temperature, we gained LCMO particles ofdifferent size. The unusual low temperature minimum is observed in the electricaltransport of LCMO particles of small size. As grain size is closely related to Coulombblockade（CB）, corresponding changes are made to a phenomenological model whichfailed to fit the low-temperature behavior before, and this modified model is inexcellent accord with the experiment date upon a field or zero field. This result givesa strong support to consider CB as one of the crucial origins of this unusual minimumbehavior.5) Samples of La_1-xBa_xMnO₃（0.33≤x≤0.95） have been fabricated by standard solid statereaction. Microstructural studies show that manganites La_1-xBa_xMnO₃（0.33≤x≤0.95）begin structural phase separation into La_0.67Ba_0.33MnO₃ and BaMnO₃ for x＞0.33.These composites form a cellular-like structure when the volume faction ofLa_0.67Ba_0.33MnO₃(f_LBMO) is near the percolation threshold（f_C）. The percolationthreshold（f_C） for our composites is 0.18. This result is not Consistent with theprevious results which prefer smaller percolation threshold value. This could beattributed to the contribution of grain boundaries. This gain-boundary contributionalso induces the large low-temperature bump in electrical transport. The criticalexponent t gained from the good fitting for the experimental data is 1.6 at 150 K and1.7 at 300 K, which is in good agreement with the previous universal result: t=1.6-2.0 for the three dimensional space.更多还原