论文标题:钙钛矿结构锰氧化物的合成及性质研究
Study on Preparations and Magnetic Properties of Manganites with Perovskite Structure
论文作者
论文导师 张密林,论文学位 博士,论文专业 材料学
论文单位 哈尔滨工程大学,点击次数 199,论文页数 131页File Size6245K
2006-08-01论文网 http://www.lw23.com/lunwen_38057352/
Magnetoresistance; Perovskite; Double Exchange; Superexchange
近年来，随着金属多层膜，磁隧道结和钙钛矿锰氧化物等材料中磁阻现象的发现，以研究磁阻效应的机理和应用为目的的磁电子学迅速发展。这其中钙钛矿结构的稀土锰氧化物以其超大的磁阻值和丰富的物理内涵而备受瞩目。尽管人们对此已做了大量的工作，但是对这类氧化物的深入认识直至超大磁电阻效应物理机制的合理解释仍需做艰苦细致的努力。本论文选择锰基钙钛矿稀土氧化物作为研究对象，系统地研究了与Mn相邻的过渡金属取代Mn离子后产生的B位的变化和B位原子之间的相互作用对氧化物的结构、磁性和磁阻性质的影响，以及A位的取代对性质的影响，希望对该系列化合物中电、磁性质的变化规律和相互作用机理作出合理的解释。 我们选择了La_(0.7)Ca_(0.3)MnO_3作为母体化合物。并通过过渡金属离子Cr，Ti，Ni，Fe对Mn离子取代来研究B位原子的变化对性质的影响。结果发现，Cr~(3+)因为与Mn~(4+)具有相同的电子形态而能够参与双交换作用，使掺杂Cr~(3+)的系列样品的磁化强度随Cr含量的增加而增加。而掺杂Ti~(4+)，Ni(2+)，Fe~(3+)离子的化合物虽然与Mn离子之间的相互作用各不相同，但引起的磁性变化却是相同的。这四种元素的掺杂都提高了体系的电阻率。通过对这几种过渡金属取代的比较，发现在La_(0.7Ca_(0.2O_3中对Mn离子进行取代的离子和Mn离子之间的交换作用对磁性质的影响并不起主要作用，掺杂引起的主要作用是致使Mn位的无序度增加和对双交换作用的稀释和阻碍。 目前n=3的层状钙钛矿研究较少，但是由于该化合物具有结构可变性和理论上可以解释磁转换机理，我们对Ca_(4-x)Bi_xMn_3O_(10-δ)和A位缺失的La_(3-x)CaMn_3O_(10-δ))进行了深入的研究。在Ca_(4-x)Bi_xMn_3O~(10+δ)(0=x=0.4)中随Bi~(3+)含量的增加，我们发现Ca_(4-x)Bi_xMn_3O_(10-δ)中磁化强度在x＜0.2的范围内随x增加而增强，在高于0.2的掺杂范围后随x的增加而逐渐降低。这一区间的磁结构是由基态x=0时的G—AFM向x=0.4时的C—AFM转变的过程。这可以由Mn离子之间较强的超交换作用来解释。在研究La_xCa_(4-x)Mn_3O_(10-δ)(x=0～0.3)的电性和磁性时，发现A位离子失配对磁性的影响与二价金属离子取代La~(3+)产生的结果相似，都使得铁磁性增加，但在三层钙钛矿中的结构有相对的弹性，因此磁电阻并没有随A位失配度的增加而增加。 我们利用简单的预制基底法合成了长度达到50uμm的La_(0.7)(Sr_(0.3)MnO_3纳米棒，发现其形态的改变使得居里温度由原来的360K降低到240K。我们利用固相法和熔盐法结合合成了结晶良好的Lao_(0.07)Sr_(0.3)MnO_3纳米颗粒。并发现加入盐的比例对居里温度有很大影响。
The discovery of magnetoresistance in metal multiplayer, magnetic tunneling junction and manganese perovskite oxides has aroused many attentions recent years. Many works focus on the doped rare-earth manganate for their huge value of magnetoresistance and various magnetic and electric properties. However, there are still a lot of problems need to clarify about the magnetoresistance mechanism. In this thesis, the main research works focused on the influence of B-site substituted by various transition metal neighbors with Mn ionic on the magnetic and electrical properties for the perovskite manganate. Also, the effect of A-site doping on the physic magnetic properties is studied. We hope to understand the action mechanism and influence factors on physical properties in the family compounds more clearly through ions doping. We choose La_(0.7)Ca_(0.3)MnO_3 as parent compound to investigate the effect on the magnetic and transport properties of Mn replaced by Cr, Ti, Fe and Ni ions. In the case of Cr doping, the magnetization progressively increases with x. It indicates that the Cr~(3+) ion has the same electronic configuration with Mn~(4+)(t_(2g)~3). In others case of doping Ti~(4+), Ni~(2+) and Fe~(3+), there is the same change trend in the magnetic properties although their exchange interaction between Mn ion is difference. It is found that the resistivity were enlarged in all doping ions for La_(0.7)Ca_(0.3)MnO_3 (M= Cr, Ti, Fe and Ni). The fact was found that the crucial effect of Mn-site doping is the increase of Mn-site disorder and the dilution of the double exchange by comparing with their difference of electrical and magnetic properties. Therefore, the exchange interaction between doping ions and Mn ion doesn"t play an important role. There are few of reports about the n=3 layered perovskite compounds. But because of the changeable in crystal structure and the need of studying magnetic transport mechanism in theory, the magnetic and electrical properties of Bi~(3+) doped Ca_(4-x)Bi_xMn_3O_(10-d) and A-sit vacancy La_(3-x)CaMn_3O_(10-d) family were investigated in detail. It is found that the magnetization increase with x in x=0.2, and then decrease with x in x>0.2. This is the process of the magnetic structure vary from G-type AFM for x=0 to C-type AFM for x=0.4. this can be ascribe to the powerful superexchange between Mn ionics. In the family of La_xCa_(4-x)Mn_3O_(10 -d) (x=0～0.3), it is found that the results is similar between A-site vacancy doping and other bivalent metal doping. Both can lead to ferromagnetic (FM) increase. Meanwhile, there is slight flexibility in three-layer perovskite, so MR do not inceasing with the degree of A-site vacany. We synthesize the La_(0.7)Sr_(0.3)MnO_3 nanorod with length 50μm through a simple solid state method. It is found that the Tc decrease from 360K of tradition solid state way to 240K. We obtain the La_(0.7)Sr_(0.3)MnO_3 nanoparticle, which is crystal well, by combining the solid state method and molten salts way.
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