论文标题:第二相引入对庞磁电阻材料磁和电子输运性质的影响
Effects of Introducting the Second Phases on Magnetic and Electronic Transport Properties in Colossal Magnetoresistance Materials
论文作者
论文导师 袁松柳,论文学位 博士,论文专业 材料物理与化学
论文单位 华中科技大学,点击次数 798,论文页数 134页File Size6370K
2006-10-01论文网 http://www.lw23.com/lunwen_128927/
Manganites Perovskites;; Low Field Magnetoresistance;; grain composites;; magnetoelectric coupling effect
近年来,在形如Ln_(1-x)T_xMnO_3(Ln是稀土元素,T是二价阳离子)的锰基钙钛矿多晶材料中的颗粒边界效应引起人们极大的研究兴趣。早期人们在这类材料中发现庞磁电阻效应(CMR),但CMR仅仅出现在居里温度(Tc)附近狭窄的温度范围,而且需要几个特斯拉的高磁场才可以实现,因而限制了它的应用。最近,人们发现在锰基钙钛矿多晶材料中颗粒边界效应剧烈地影响其物理性能,最吸引人的特征就是在很低的磁场下就可以获得很高的磁电阻,而且低场磁电阻出现在整个绝缘体金属转变温度(Tp)以下宽的温度范围。一般认为,这种低场磁电阻是由于载流子通过颗粒边界的能量势垒发生自旋极化隧穿而引起的。颗粒边界的无序结构是载流子能量势垒的主要原因,因此对颗粒边界进行优化处理可以改善这类磁电阻,最有效的方法就是引入第二相形成锰基钙钛矿颗粒复合材料,然而在这类复合材料中增强的磁电阻往往发生在更低的温区,这是目前需解决的主要问题。本文以锰基钙钛矿氧化物La_(0.67)T_(0.33)MnO_3(T=Ca, Sr)的颗粒复合系统为研究对象,通过采取不同的工艺制备方法和选取不同的第二相材料,讨论了第二相的引入对复合样品的结构、输运性质的影响。利用交替沉积的方法,我们成功制备了La_(2/3)Sr_(1/3)MnO_3(LSMO)/CuO和LSMO/Al_2O_3多层颗粒膜。研究表明多层颗粒膜层数和第二相的含量对材料的电输运和磁电阻有很大的影响。对单层的纯LSMO薄膜,金属绝缘体转变温度Tp=236K,最大的磁电阻MR=23%;对40层的LSMO/CuO多层颗粒膜,Tp降到174K,但最大MR增加到31%。和CuO相比Al_2O_3的引入使得增加的磁电阻发生在更高的温度。根据实验结果和理论计算,我们提出通过选择禁带宽度较高的绝缘体氧化物作为CMR氧化物复合体系中的第二相,可以提高增强的磁电阻发生的温度范围。 我们还用水浴包覆的方法设计制备了La_(0.67)Ca_(0.33)3Mn_3(LCMO)/MgO颗粒复合体系,发现和纯LCMO相比,MgO的引入使得复合样品的低场磁电阻得以显著提高,在0.3T的外磁场下,纯LCMO的最大零场磁电阻值为8%,而MgO含量x=0.07的复合样品的零场磁电阻达到28%。另外,我们还发现LCMO原粉的颗粒尺寸对复合体系电输运和磁电阻也有较大的影响。结合对样品微观形貌的分析,我们认为复合样品中的这些输运性质和LCMO颗粒之间的联接性有关,在复合样品中观察到的两级磁转变现象进一步证明了这一点。然而,和其它LCMO复合系统一样,增强的磁电阻仅仅发生在Tp以下更低的温区。 铁磁和铁电氧化物复合系统中强烈的磁电耦合效应为设计锰氧化物颗粒复合系统提供了一个新的思路。我们在低温900℃烧结的LCMO/BaTiO_3复合体系样品中观察到不同于一般的LCMO复合体系的实验现象,随BaTiO_3含量x的增加,样品的电阻率降低,Tp升高,从纯LCMO的180K增加到x=0.3复合样品的225K,这使得在复合样品中低场磁电阻和高场磁电阻在整个温区都有所增强。用铁磁和铁电之间的磁电耦合效应可以对这些实验现象加以解释。然而对高温1200℃烧结的样品由于BaTiO_3进入LCMO颗粒表面的晶格,其电输运和磁电阻特性和普通的LCMO复合系统相似。对中温1100℃和1000℃烧结的样品,R-T曲线和MR-T曲线都出现两个峰,这是由于较高的烧结温度或较高的BaTiO_3含量都使得部分的BaTiO_3进入到颗粒表面的晶格,在这种情况下,颗粒复合系统中两类颗粒边界共存。 另外,我们还研究了其它绝缘体氧化物作为第二相的锰氧化物颗粒复合系统。在LSMO/CeO_2复合体系中,我们观察到样品的电阻率随温度降低的变化关系为: ,其中α是一个常数,对所有不同CeO_2含量的LSMO/CeO_2复合样品保持不变。基于表面磁化机理,我们对这一现象进行了初步的解释。对LCMO/SrO复合体系,和LCMO/MgO体系比较,T_p下降的较少,R-T曲线的峰也较为平缓,这使得增强的磁电阻发生在较高的温区,基于结构分析和磁性质的测量,我们认为这些不同是由于复合样品中两相之间的界面反应产生不同的界面相引起的。 由此可见,不同的第二相材料和工艺制备方法对锰氧化物颗粒复合系统的颗粒边界效应有显著的影响,通过选择合适的第二相材料可以提高磁电阻增强效应所发生的温区。
Recently growing attention has been paid on the grain boundary effect in polycrystalline perovskite manganites of the type Ln_(1-x)TxMnO_3 (Ln is rare earth, T is divalent cation). Last years, colossal magnetoresistance (CMR) effect was observed in this kind of materials. However, the intrinsic CMR effect in the perovskite manganites is only triggered at high magnetic fields of several tesla, which restrains its use for practical applications. Recently growing attention is being paid to polycrystalline manganites in which the grain boundary effects dramatically modify their physical properties. An attractive feature for the polycrystalline manganites is a large magnetoresistance(MR) at very low magnetic field over wide temperature range below Tp. The low field magnetoresistance in polycrystalline manganites is usually thought to be a result of the spin polarized tunnelling through energy barriers at the grain boundaries. The structural disordered interfaces play a role of the energy barrier for carriers. The magnetoresistance can be enhanced through controlling the grain boundary effect by forming composites of the CMR oxides with secondary phases. However, in these composite systems, the enhanced LFMR was usually observed at lower temperature, which restrains its use for practical applications. In this paper, CMR granular composite systems were investigated. Using different mothed, We discussed the effects of the second phase on the structure and transport properties of the CMR composites. La_(2/3)Sr_(1/3)MnO_3 (LSMO)/CuO and LSMO/Al_2O_3 multilayers granular thin films were prepared by using tandem deposition method via a magnetron sputtering system. The results show that the electrical transport and magnetoresistance(MR) strongly depends on the layers of the thin films and the concent of the second phase. For pure LSMO films, the insulator-metal transition temperature Tp is 236K and the max MR is 23%. For LSMO/CuO composite film with 40 layers, The Tp value decreases to 174K while the max MR increases to 31%. As compared to CuO, the MR increases at higher temperature as a result of the introduction of Al2O_3. In terms of the experiment and theory calculating results, We suggest that the high-T grain boundary magnetoresistance is possible to be obtained by selecting high activation energy insulator as the second phase in manganite-based composite films. La_(2/3)Ca_(1/3)MnO_3 (LCMO)/MgO granular composite systems were prepared by coating method. As compared to pure LCMO, MR is enhanced in the LCMO/MgO composites. For pure LCMO, the max low field MR is 8% measured at 0.3T. For LCMO/xMgO composite with x=0.07, the max low field MR increases to 28%. Furthermore, the electrical transport properties and MR of the LCMO/MgO composites are related to the grain size of the parent LCMO powers. In terms of the SEM images of the samples, we suggest that the transport properties of the composites are related to the coupling properties between the neighboring grains, which is approved by the two magnetic transitions observed in the composites. However, similar to the other LCMO composites, MR is enhanced only at low temperature range below Tp. The strong magnetoelectric coupling effect is observed in the composites of a ferromagnetic and a ferroelectric compound (called bi- or multiferroic), which suggests a new idea for designing the CMR composites. The experiment results observed in the LCMO/BaTiO_3 composites sintered at 900℃are different from other LCMO composites. The insulator-metal transition temperature (Tp) is shifted to higher temperature and resistivity decreases with increasing the BaTiO_3 content. The Tp value increases from 185 K for pure LCMO to 230 K for x = 0.3 sample. Magnetoresistance (MR) of the composites is enhanced over the whole temperature range as a result of the introduction of BaTiO_3. The experimental results can be explained in terms of the magnetoelectric coupling effect. However, for the samples sintered at 1200℃, BaTiO_3 enters the LCMO lattice at the grain surface. The electrical transport properties and MR are similar to the general LCMO composites. For the composites sintered at 1000℃and 1100℃, the R-T and MR-T curves appear two peaks, which could be due to that partial BaTiO_3 enter the LCMO lattice at the grain surface at the medium sinter temperature. In this case, two kinds of grain boundary coexist in the composites. Moreover, the CMR composites with other insulator oxides were investigated in this paper. In the LSMO/CeO_2 composites, the resistivity varies asρ=ρTp exp( ?α(T ?Tp)2). The constantαdoes not vary for all samples with different CeO_2 content x. The experimental results can be interpreted in terms of a surface magnetization mechanism. Different electrical and magnetic transport properties were observed between LCMO/SrO and LCMO/MgO composites. As compared to MgO, the SrO content has little effect on Tp and the low field MR is enhanced at higher temperature range. The different properties could be attributed to the formation of the different interfacial phase at the grain boundary in two kinds of composites. In summary, the grain boundary effect in the CMR granular composites strongly depends on the second phase propertied and material preparation method. MR can be enhanced at higher temperature range by appropriate choice of the second phase.
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