【作者】 王才林；

【导师】 石磊；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2013， 博士

【摘要】 自从在CaMnO3薄膜中发现庞磁阻效应并意识到其可能具有极大的应用前景以来,钙钛矿锰氧化物受到了科研工作者的广泛关注。人们对该体系的磁性能,电输运性能,以及体系中的自旋、轨道、晶格和电子之间的相互作用开展了大量的研究。研究过程中发现CaMnO3在室温时的Seebeck系数可达350μV/K,通过掺杂处理,可使其具有n型热电材料的潜能。在本论文中,我们首先研究了Yb0.1Ca0.9Mn1-xNbxO3材料的高温热电性能随掺杂Nb含量的变化行为,并发现该体系在高温段存在金属-绝缘体转变行为；而后研究了在YxCa1-xMnO3中出现的高温金属-绝缘体转变现象,并提出了可能的原因；接着我们研究了YxCa1-xMnO3中出现的Griffith相行为；最后,我们研究了尺寸对Y0.1Ca0.9MnO3纳米颗粒磁性能的影响。具体内容如下：第一章,宏观介绍了钙钛矿锰氧化物的晶体结构和电子结构,基于前人的研究成果,介绍了钙钛矿锰氧化物中存在的双交换和超交换相互作用、常见的磁结构等常见基本概念。然后重点介绍了电子掺杂CaMnO3材料的各种性质及其研究进展和存在的问题。第二章,本章我们对Yb0.1Ca0.9Mn1-xNbxO3化合物的高温热电性能进行了研究。研究发现,随着Nb掺杂量的增多,体系出现了一个结构相变,从O型正交结构变为O*型正交结构,表明了体系的晶格畸变程度在逐渐的加剧。由于晶格畸变可以增强电-声子相互作用,因此在该体系中随着Nb离子的掺入,有利于小极化子的形成。可以发现,在整个测量温区(300K-773K)内,体系的电导率随温度的变化关系都可以用绝热小极化子跃迁模型来很好的描述,并且激活能随着掺杂量的增多而变大。从体系的Seebeck系数随温度的变化关系(S(T))上可以看出,在整个测量温区内Seebeck系数均为负值,并且该值大小基本上与载流子浓度成相反的关系。S(T)在整个测量温区内都符合线性行为,可以用Culter和Mott模型来很好的拟合,这也说明了体系费米面处的电子浓度强烈的受到Nb离子掺杂的影响。同时还发现,该体系与CaMnO3和CaMnO3所不同的是：当Nb的掺杂浓度大于0.05时,体系的Seebeck系数绝对值和电导率随温度的变化具有相同的趋势。第三章,本章利用电子自旋共振技术(ESR)对非磁性Y3+离子掺杂的YxCa1-xMnO3化合物在140K-470K之间的磁性行为进行了研究。从ESR的峰宽随温度的变化关系上看,当x=0.05时,样品的峰宽随温度的升高而逐渐减小,但是当x≥0.08时,样品的峰宽随温度的升高先增大后减小,并且在峰宽增大的温区内,符合瓶颈效应,可以用绝热小极化子跃迁模型来很好的拟合。这也说明,体系中存在自旋-晶格相互作用和自旋-自旋相互作用,并且在峰宽随温度的升高而增大的温区中自旋-晶格相互作用起主导作用,而后自旋-自旋相互作用处于主导地位。从ESR的共振强度上看,对于该化合物,所有样品的ESR强度除了在300K<T<330K之间随着温度升高而变大外,均随温度的升高而减小。从g因子上来看,也存在于ESR强度随温度变化相类似的趋势。我们还研究了体系的电导率随温度的变化关系,发现该体系在室温附近存在金属-绝缘体转变行为,并且在转变温度以下的电导率随温度的变化关系可以用绝热小极化子跃迁模型很好的拟合。结合体系的电导率行为和ESR参数随温度的变化行为,我们认为造成这些行为的原因可能是因为在高温段体系中的小极化子受到温度的扰动后崩塌导致的。这也解释了该类电子掺杂锰氧化物中出现的高温金属-绝缘体转变的原因。第四章,本章我们研究了YxCa1-xMnO3中出现的Griffith相。从YxCa1-xMnO3的1/χ-T关系图上可以很明显的看到：在顺磁区域,当温度TC<T<TG时,1/χ—T曲线很明显的向下偏离顺磁区的居里-外斯拟合线。我们认为这种偏离是因为在这个温区内存在了短程的铁磁团簇而造成的,也即存在Griffith相,并且这种偏离程度随着掺杂量的增大而逐渐减小。从Y0.02Ca0.98MnO3样品的1／X-T曲线也可以看出上述的偏离程度随着外加磁场的增加而逐渐减小。在TC<T<TG区间,我们可以利用描述Griffith相的理论公式很好的拟合,拟合参数符合理论要求的值。Griffith相的存在,说明在此样品中2%的Y掺杂即可引起反铁磁CaMnO3的磁性能有较大变化,进一步说明了在此体系中载流子的作用是至关重要的。第五章,通过溶胶凝胶法成功的合成了具有不同尺寸的Y0.1Ca0.9MnO3纳米颗粒,并研究了尺寸对该化合物的磁性能影响。发现在10K时,随着尺寸的减小,该化合物的磁化强度逐渐减小。对于一般上母体为反铁磁的化合物而言,磁化强度随着尺寸的减小逐渐变大,而我们的母体为反铁磁的Y0.1Ca0.9MnO3纳米体系表现出正好相反的结果。由于在Y0.1Ca0.9MnO3纳米体系中,低温存在弱铁磁行为,故我们可以采用核壳结构模型很好的解释该反常行为。更多还原

【Abstract】 The pervoskite manganese oxide compounds have attracted an intense interest in recent years since the GMR effect, which have strong potention technological application, found in La0.67Ca0.33MnO3thin film. Many works were carried out about the properties of magnetic and electrical transport, and the interaction among spin, orbital, lattice and electron for this system. It was found that the Seebeck coefficient of CaMnO3is very high at room temperature (about350μV/K), so, it can be regarded as the candidate for the n-type oxide thermoelectric material. In this dissertation, first, we investigated the thermoelectric performance of Yb0.1Ca0.9Mn1-xNbxO3at high temperature; second, we studied the magnetic behavior of YxCa1-xMnO3by ESR between140K and470K; third, we studied the influence of the nanoparticle size to the magnetic behavior of Y0.1Ca0.9MnO3; finally, we investigate the EB effect of the Ln0.2Ca0.8MnO3(Ln=Rare earth). Details are as follows:In chapter one, we briefly introduce the structure of crystalline and electrical, the concepts of double exchange and superexchange interaction, the magnetic structure and CMR effect, and so on. Besides, the properties and developments of electron-doped CaMnO3are presented particularly.In chapter two, the thermoelectric performance of Yb0.1Ca0.9MnO3doped with Nb5+at B-site is investigated in this chapter. It is found that there is a phase transition from O-type to O*-type orthorhombic structure with increasing of Nb doping content, which indicates that the structure distortion becomes more seriously. Since the electron-phonon interaction can be enhanced by the structure distortion, the small polaron formation is promoted in Yb0.1Ca0.9Mn1-xNbxO3with increasing Nb content. In the whole measured temperature range, the electrical conductivity can be fitted very well by the adiabatic small polaron hopping model. The activated energy Ea is ascending with increasing Nb content. The temperature dependence of Seebeck coefficient S of Yb1.1Ca0.9Mn1-xNbxO3shows that the S is basically inversive to the charge carrier concentration. S(T) can be fitted well by Cutler and Mott model which indicates that the density of state around the Fermi level is strongly affected by Nb-doping at B-site. It is contrary to those of CaMnO3and RE0.1Ca0.9MnO3, when Nb content x>0.05, the|S|and σ show a same tendency of the temperature dependence. In chapter three, the magnetic property of YxCa1-xMnO3was investigated by electron-spin resonance (ESR) under the temperature from140K to470K. It is found that with temperature increasing, the ESR linewidth reaches the maximum at Tmax (=200K,220K and270K for x=0.08,0.10and0.12, respectively) and then decreases in the temperature range from140K to470K for x≥0.08, while it decreases with temperature increasing in the whole temperature range for x=0.05. The spin-lattice relaxation plays the main role for x≥0.08at the temperature range from140K to Tmax in which the ESR linewidth can be described very well by the adiabatic small polaron hopping model. The ESR line intensity decreases with temperature increasing for all the samples between140K and470K except300K≤T≤330K in which it increases with temperature increasing. Meanwhile, the g-factor reaches the minimum value at about T=300K, and then increases with temperature increasing for all the samples between140K and470K. The behaviors of the ESR linewidth and intensity versus temperature indicate the appearance of the short range magnetic correlations. In addition, the electron transport properties of YxCa1-xMnO3reveal a metal-insulator transition at TMI which varies from254K to293K with Y3+content increasing. The resistivity versus temperature between140K and TMI can be fitted well by the adiabatic small polaron hopping model. It is suggested that both the metal-insulator transition of YxCa1-xMnO3and the change of magnetic interactions are closely related with the small polaron collapses at high temperature.In chapter four, the Griffith phase of YxCa1-xMnO3was investigated. It can be found that the1/χ-T curve follows the Curie-Weiss law at high-temperature, but exhibits a Griffith phaselike downturn below a certain temperature. The onset of this downturn is denoted as TG below which the ferromagnetic clusters emerge in the paramagnetism matrix, as is described in a Griffith phase system. The downturn tendency can be depressed by doped content or magnetic field. And the1/χ-T curve can be well described by the theory of Griffith phase between Tc and TG, The appearance of Griffith phase in this system even doped by2%Y3+ions, indicates furtherly that the carriers in this system play a important roles.In chapter five, different size nanoparticles of Y0.1Ca0.9MnO3were prepared successfully by sol-gel method. The size dependence of the magnetic was investigated. It is found that the magnetization decreases with the nanoparticle size increasing at10K, which is generally contrary to the behaviors of the compound with antiferromagnetic matrix. Since weak ferromagnetic embedded in Y0.1Ca0.9MnO2 antiferromagnetic matrix at low temperature, so, we can understand the abnormal behavior by the core-shell model easily.更多还原