【作者】 汪川惠；

【导师】 袁松柳；

【作者基本信息】 华中科技大学 ， 材料物理与化学， 2011， 博士

【摘要】 具有两种或者两种以上的铁性性质的材料被称为多铁材料,其中铁性包括铁磁性、铁电性和铁弹性。多铁材料可能具有的磁电耦合效应有希望通过外加磁场来控制电荷或者通过外加电场来控制自旋。这种行为在设计多功能器件上具有非常诱人的应用前景,它为传统的传感器、激励器、存储器件提供了一个额外的自由度。例如,目前常用的存储材料有磁存储器和铁电存储器,磁存储器采用磁写磁读,其读取速度快但是写入慢,而铁电存储器采用电写电读,读取复杂而写入快,如果使用磁电耦合材料为记录介质,做成电写磁读,则可能同时实现超高速率的读写过程。此外,磁电材料还有可能应用在多态记忆元,电场控制的磁共振装置、磁场控制的压电传感器、电场控制的压磁传感器、磁力成像技术和检测,磁场屏蔽等各种领域中。因此对于多铁材料的研究,不仅在理论上具有重要的意义,同时在技术和应用层面上有着广阔的应用前景。本文对单相多铁材料的研究做了以下几个方面的工作：(1)使用溶胶凝胶法制备了Bi,Mn共掺杂BaTi03名义组分为Ba1-xBixTi1-yMny03(0≤y≤0.08)块材样品,成功的在不破坏铁电性的情况下,在纯的铁电体BaTi03中引入了铁磁性,形成了一个单相多铁体系。并对Bi和Mn掺杂浓度对样品磁性的影响进行了研究。研究表明,固定Mn掺杂量的样品,Bi离子的掺杂组分增加,样品的磁性增加；固定Bi的掺杂量,在Mn的掺杂量较低时(x≤0.04),样品的铁磁性较为明显。对于样品室温铁磁性的起因,我们采用晶界缺陷诱导的束缚磁极化子理论来进行解释。(2)研究了Ba1-xBixTi1-yMnyO3 (0≤y≤0.08)块材样品的介电性质和铁电性质。它们的介电转变峰Tm均在室温以上,具有弛豫铁电性；随着Mn掺杂组分的增加介电常数峰值向高温移动,样品的介电损耗增大。所有的样品都具有铁电性；对于Bi掺杂量为0.07和0.1的样品,当Mn的掺杂量从0增加到0.02时矫顽力和剩余极化先减小,此后随着Mn的掺杂量的增加,矫顽力和剩余极化又单调增加。分析认为以上结果均是由于Bi和Mn掺杂导致的结构变化以及氧空位浓度的变化两者共同作用所造成的。(3)使用溶胶凝胶法制备了名义组分为Bi5-xLaxTi3Fe0.5Co0.5O15(0≤x≤0.4)的陶瓷块材。研究了烧结温度和掺杂组分对体系磁性的影响。在不掺La的情况下,不同烧结温度时,Bi5Ti3Fe0.5Co0.5O15体系在室温下都表现出弱铁磁性；随着烧结温度的升高,10000e下的样品的ZFC和FC开始出现多个交叉点,以最高温度1000℃烧结的样品最为明显；掺La以后样品室温的磁化强度有所增强,ZFC和FC并没有出现异常的行为。这种现象认为是两种相互作用竞争作用的结果。从后面的研究可以知道,这两种相互作用分别是单离子各向异性和反对称的DM相互作用。(4)研究了烧结温度和La掺杂浓度对Bi5-xLaxTi3Fe0.5Co0.5O15体系的介电性以及铁电性的影响。所有的样品的介电常数和介电损耗都是随着频率的增加而减小；烧结温度增加介电性变差,剩余极化和矫顽力先增大后减小；La掺杂使得介电性增强,电滞回线矫顽力减小；对于烧结温度为850℃的样品,其剩余极化随着La掺杂量的增加而增加,而对于烧结温度为1000℃的样品,剩余极化随着La掺杂量的增加而减少。介电性能主要是受氧空位浓度的影响,铁电性的变化规律就是颗粒尺寸效应和Bi挥发两种效应竞争的结果。(5)在Bi5Co0.5Fe0.5Ti3O15材料体系发现了负磁化现象,研究了烧结温度、外加场大小以及B位掺杂量的变化对(0.4≤y≤0.6)材料体系的负磁化现象的影响。烧结温度达到9000C样品开始出现负磁化效应,并且烧结温度越高,负磁化的绝对值越大。对于1000℃烧结的样品,随着外加场的增加,负磁化的补偿温度向低温移动,当外加场足够大时,负磁化现象消失；随着Co掺杂量的增加,补偿温度向高温移动。结合前面的研究结果以及相关文献报道,我们认为负磁化行为是反对称的DM相互作用与单离子各向异性的相互竞争造成的,而Bi挥发导致的晶格畸变增大是这两种作用可以达到相互竞争程度的诱因。更多还原

【Abstract】 Such materials, which combine two or more "ferroic" properties in the same phase, are known as multiferroics. These materials may exhibit magnetoelectric coupling effect, which shows the prospect of controlling charges by applied magnetic fields and spins by applied voltages, and using this to construct new forms of multifunctional devices. It can provide a more degree of freedom for conventional sensor, actuator and memory. For example, the memories we already used are magnetic storage and ferroelectric RAM (FRAM). Magnetic storage adopt magnetic read and write with low reading speed and high writing speed, while FRAM adopt electric read and write with high the reading speed and low writing speed. If we use the magnetoelectric material as storage medium and carry out magnetic read and electric write, the super-high speed of read and write could be achieved. Besides, magnetoelectric materials still have the application potential in multi-state storage medium, spin magnetic resonance controlled by applied electric field, Piezoelectric sensors controlled by applied magnetic field, magneto-elastic transducer controlled by applied electric field, magnetic shield and so on. Therefore, the research of multiferroic materials has a very important significance both in theory and in technology and commerciality.In this thesis, we have done the following work:(1) We synthesized Bi and Mn co-doped BaTiO3 ceramics with the nominal composition of Ba1-xBixTi1-yMnyO3(0<y<0.08) by sol-gel method, successfully produced ferromagnetism into pure ferroelectric BaTiO3 without destroying its ferroelectricity. And then effects of doping content of Bi and Mn on the magnetism have been studied. When we fixed the Mn content, as increasing of the Bi content, the magnetic property are enhanced; while we fixed Bi content, low Mn content (x≤0.04) show obvious ferromagnetic property. We adopt bound magnetic polaron which induced by the oxygen vacancies at the grain boundaries of particles as activation of the room temperature ferromagnetism.(2) We studied the dielectric and ferroelectric properties of the Ba1-xBixTi1-yMnyO3 (0≤y≤0.08), the dielectric peak of all the samples are above the room temperature, and all the samples show relaxor ferroelectric behavior. As the increasing of Mn content, the dielectric peak move to high temperature, the dielectric loss is increased. All the samples show ferroelectric properties; as for the 0.07 and 0.1 Bi content, as the Mn content increasing from 0 to 0.02, the coercive force and remnant polarization is decreased, as the Mn content further increasing, the coercive force and remnant polarization increase monotonously. We believe that all the experiment results are result from both the structure distortion and the concentration of oxygen vacancies which induced by Bi and Mn dopant.(3) The nominal composition of Bi5-xLaxTi3Fe0.5Co0.5015(0<x<0.4) ceramics were synthesized by sol-gel method. And then effects of sintering temperature and doping content on the magnetism have been studied. As for the samples without La doped, the Bi5Ti3Fe0.5Co0.5O15 ceramics sintered at all the temperature show weak ferromagnetic property at room temperature; As the sintering temperature increased, the M-T curves measure at 1000 Oe under ZFC process have more than one cross point with the FC processed one; The room temperature ferromagnetic property are enhanced by La doping, and no abnormal ZFC and FC appear. It is believe that the abnormal phenomena is caused by the competition of two mechanism, from the following research, the two mechanism is single-ion magnetocrystalline anisotropy and antisymmetric Dzyaloshinsky-Moriya interactions, respectively.(4) The effects of sintering temperature and doping content on the magnetism in Bi5-xLaxTi3Fe0.5Co0.5015(0<x<0.4) ceramics have been studied. The dielectric constant and loss are monotonously decreased as the frequency increase in all the samples; the dielectric properties are weaken by the increasing sintering temperature, the coercive force and remnant polarization increase at first and then decrease; La doping enhanced the dielectric properties, As for the sample sintered at 850℃, the remnant polarization decreased as the La content increasing, while for the sample sintered at 1000℃, the remnant polarization show the opposite effect. The dielectric property is affected by the concentration of oxygen vacancies, the ferroelectric property is the result of the competition of grain size effect and the Bi volatilization effect.(5) We found negative magnetization effect in the Bi5Co0.5Fe0.5Ti3O15 ceramics, the effects of sintering temperature, applied magnetic field and doping content of B sites on negative magnetization effect have been studied. The negative magnetization effect appear when the sintering temperature achieved 900℃. As the sintering temperature increasing, the absolute value of the negative magnetization increases. As for the sample sintered at 1000℃, with increasing (positive) magnetic field, the compensation temperature decreases and the magnetization changes from negative to positive value at high-enough fields; With the Co content increasing, the compensation temperature move to high temperature.Consider with the magnetic effect we mentioned before and some literature, we believe the negative magnetization effect is the competition between single-ion magnetocrystalline anisotropy and antisymmetric Dzyaloshinsky-Moriya interactions. The lattice distortion induced by Bi volatilization is the important reason to make the energy of two mechanisms comparable.更多还原