【作者】 赵旭；

【导师】 陈伟；

【作者基本信息】 河北师范大学 ， 凝聚态物理， 2010， 博士

【摘要】 本论文系统研究了双层钙钛矿结构锰氧化物La_1.4Sr_1.6Mn₂O₇中Sr位Ca²⁺、Mg²⁺和K+离子的替代效应,研究了不同半径离子的替代对材料的结构、磁、磁熵变以及磁电阻的影响。主要工作可以概括为:1.研究了名义组分La_1.4Sr_1.6-xCa_xMn₂O₇（x=0.0- 1.6）系列样品的结构、磁和磁热效应。0≤x≤0.8的样品均为Sr3Ti2O7型四方晶系的钙钛矿结构,空间群为I4/mmm,而1.0≤x≤1.6样品为Pbnm空间群正交的ABO3型钙钛矿锰氧化物与少量CaO的混合物,晶体结构相变发生在0.8≤x≤1.0的掺杂范围。对于x=0.2- 0.8的样品,随着Ca²⁺离子含量的增加,三维铁磁有序转变温度逐渐降低直至消失,而二维铁磁短程有序仍然存在,在x=0.4样品中甚至增大。表明Ca²⁺的替代抑制了三维的交换相互作用,而eg电子轨道的变化造成了MnO6八面体的Jahn–Teller扭曲,是x = 0.4样品中二维铁磁短程有序增强的主要驱动力。在1T外加磁场下,名义组分La1.4Sr1.6-xCaxMn2O（7x= 1.6）样品在居里温度215K附近得到了2.28J kg-1K-1的磁熵变,由此可知该材料可以作为亚室温磁制冷材料的候选者。2.研究了离子半径比Ca²⁺更小的Mg²⁺离子对钙钛矿结构锰氧化物La_1.4Sr_1.6Mn₂O₇中Sr位的替代效应,发现Mg²⁺离子实际进入了钙钛矿结构的Mn位,形成了La1.4Sr1.6Mn2-2yMg2yO7/La0.67Sr0.33Mn1-yMgyO3（327/113）复合材料。并且随着Mg²⁺离子含量的增加,327相的百分含量逐渐降低,113相的百分含量逐渐增加,x=0.4样品精修后得到其实际分子式为La0.6Sr0.4Mn0.83Mg0.17O3。说明在离子替代过程中,离子半径起到了至关重要的作用（Mn离子半径与Mg更接近）。Mn位Mg²⁺离子的替代抑制了复合材料中两相的铁磁性和导电性,对于轻掺杂样品,材料在低温处的奈尔温度和高温处的居里温度均随Mg²⁺含量的增加而降低;而对于重掺杂的样品,其磁化强度曲线表现出了自旋玻璃的行为,导致样品中的电阻率比轻掺杂样品增长了近4个数量级,电阻率表现为绝缘性。同时发现在低温和高温两个温区,Mg²⁺的掺杂均使复合材料低场磁电阻有很大的增加,5K时x=0.1,0.2样品1T下的磁电阻分别为40%和39%,200K时分别为8%和11%,从而扩大了材料低场磁电阻的使用温区,有利于其应用。3.系统研究了La_1.4Sr_1.6Mn₂O₇/La0.67Sr0.33MnO3复合材料的制备方法,分别合成了两相与三相的系列复合材料（1-x）La_1.4Sr_1.6Mn₂O₇/xLa0.67Sr0.33MnO3与（1-x′-y）La_1.4Sr_1.6Mn₂O₇/x′La0.67Sr0.33MnO3/yLa2O3。在对复合材料（1-x）La_1.4Sr_1.6Mn₂O₇/xLa0.67Sr0.33MnO3的磁性研究中发现,由于两相含量的变化和两相之间的反铁磁耦合,在高温和低温处出现了两个磁相变温度（TC1和TC2）,并且在TC1与TC2之间,磁化强度形成了一个平台。随着掺杂量x的增加,平台处的磁化强度逐渐增大。随x逐渐增加,x≤0.67样品的电阻率逐渐变低,金属-绝缘转变温度虽然存在,但开始变得更加的平缓和宽化。当x增加到x=0.93和1.0,材料中电阻率进一步降低,在整个测量的温区内表现为金属性。而三相系列样品中La2O3的掺入,对113相的居里温度（360K）之上的顺磁态的磁化强度和电阻率影响最小,对327相的居里温度（90K）之下的铁磁-反铁磁共存态的磁化强度与电阻率影响最大,分别达到8%和30%。同时在x′=0.465样品中发现了有利于应用的宽温区、低场磁熵变,其在90K温度附近,1、2以及7T下的最大磁熵变分别达到0.45,0.9和1.55J kg-1K-1。更多还原

【Abstract】 The structural, magnetic, magnetocaloric and magnetoresistance properties have been investigated in bilayered manganites La_1.4Sr_1.6Mn₂O₇ doping with Ca²⁺, Mg²⁺ and K+ at Sr site. The main results are as follows:1. The structure, magnetic and magnetiocaloric properties of the nominal compositions La_1.4Sr_1.6-xCa_xMn₂O₇ （x=0.0-1.6） have been investigated. The samples are tetragonal bilayered perovskite with space group I4/mmm in the 0≤x≤0.8 dopant regimes, while they are composed of a major phase of ABO3-type perovskite structure with space group Pbnm and a slight calcium oxide in the 1.0≤x≤1.6 dopant regimes. The structural transition takes place in the range of 0.8≤x≤1.0. For the x=0.2-0.8 samples, 3D FM ordering temperature decreases at first and then disappears with the increase of Ca²⁺-doping level, while the 2D FM short-range ordering remains and even increases in the x = 0.4 sample. This result implies that the exchange interaction of 3D （Jc） is depressed with increasing Ca²⁺ content. The enhancement of 2D FM short-range ordering in the x=0.4 sample is mainly derived from a sufficiently large Jahn–Teller distortion of MnO6 octahedra triggered by a variation in the nature of the orbital state of the eg electrons. A large magnetic entropy change of 2.28 J kg-1 K-1 is obtained in the nominal composition La1.4Sr1.6?xCaxMn2O7 （x = 1.6） upon 1 T applied magnetic field near its Tc=215 K. It indicates that this compound could be used as a magnetic refrigerant in sub-room temperature magnetic refrigerator.2. The doping effect of Mg²⁺ ions at Sr site in bilayered manganite La_1.4Sr_1.6Mn₂O₇ has been investigated. The Mg²⁺ ions occupy Mn sites in the perovskite structure actually, and formed La1.4Sr1.6Mn2-2yMg2yO7/La0.67Sr0.33Mn1-yMgyO3 （327/113） composites. The content of 327 phase decreases and the fraction of 113 phase increases gradually with increasing the content of Mg²⁺ ions. The practical molecular formula at x=0.4 sample is La0.6Sr0.4Mn0.83Mg0.17O3 using the Rietveld method. FM ordering and conductivity of the two phases in the composites are depressed by Mg²⁺ ions doping at Mn site. For lightly doped samples, the Neel and Curie temperature decreases with increasing the content of Mg²⁺ ions. For heavily doped samples, the magnetization exhibits the spin-glass behaveior, which may cause a insulating behavior. The resistivity in heavily doped samples is as large as four orders of that in lightly doped samples. At lower and higher temperature, the low-field magnetoresistance have been increased by doping Mg²⁺ at Mn site. For x=0.1, 0.2 samples, the MR values are as large as 40% and 39%, respectively, at 5K and 1T, and 8% and 11%, respectively, at 200 K and 1T.3. The two and three phases composites of （1-x）La_1.4Sr_1.6Mn₂O₇/xLa0.67Sr0.33MnO3 and （1-x′-y）La_1.4Sr_1.6Mn₂O₇/x′La0.67Sr0.33MnO3/yLa2O3 have been synthesized. Two magnetic transition temperatures （TC1 and TC2） at low and high temperature are observed in （1-x）La_1.4Sr_1.6Mn₂O₇/xLa0.67Sr0.33MnO3 composites because the variation of the phase content and anti-ferromagnetic coupling between two phases. The magnetization formes a plateau between TC1 and TC2, and the magnitude of magnetization increases with increasing x when TC1 >T >TC2. For the x≤0.67 samples, the magnitude of resistivity decreases gradually with increasing x, and the peak of metal-insulator transition becomes smoother and broader. For the x=0.93 and 1.0 samples, the resistivity further decreases and shows metallic behavior in the measuring temperature range. In three-phase system, the influence of La2O3 on magnetization and resistivity is quite slight above the Curie temperature of 113 phase （360K）, while it is significant under the Curie temperature of 327 phase （90K）. At the same time, the large magnetic entropy change of 0.45, 0.9 and 1.55J kg-1K-1 in 1, 2, and 7T is obtained in the x′=0.465 sample at 90K.更多还原