施主-受主“离子对”掺杂层状非本征铁电体Ca3Mn2O7薄膜的极化与磁化行为及机理研究

The multiferroics has been widely used in storages, sensors and microwave devices. How to design and optimized the multiferroic property is a research hotspot in this area. In the present study, the improper ferroelectric Ca3Mn2O7 film with Ruddlesden-Popper（RP）structure will be investigated systematically in order to optimize its multiferroic properties. The phases transitions and properties of the RP structure improper ferroelectrics is very sensitive with the stress. The “iron pairs” could induce a large local lattice distortion, and the electric dipole moment has the electric polarization effect on the neighbour area. Based on these, Ca3Mn2O7 films could be doped by the “iron pairs”, in order to modify the electric, magnetic and magnetoelectric coupling properties, and also improve the applicable temperature. Finally, the multiferroics with excellent performance could be fabricated for single-phase. On the basis of “iron pairs” doping to Ca3Mn2O7 films for the adjustment of structures, phase transformation behaviors and electromagnetic properties, in this study, the effects of “iron pairs” on phase transitions will be studied. In addition, the “iron pairs” mechanism of action on the polarization and magnetization behaviors will be also investigated. According to both experimental and theoretical studies, the optimization of the “iron pairs” doped Ca3Mn2O7 films performance will be explored. The results given in the present study are significant for the improvement of properties and applicable temperature of improper ferroelectrics, and also provide a research approach and theoretical basis for enlarging the application range of the improper ferroelectrics.

多铁材料在众多领域中具有重要应用，设计和优化非本征铁电体的多铁性质是目前的研究热点之一。本项目以Ruddlesden-Popper结构非本征铁电体Ca3Mn2O7薄膜为研究对象，探索改善其多铁性能的途径。鉴于该结构非本征铁电体的相变及性能对应力十分敏感，而“离子对”不仅可以产生较大的局域晶格畸变，其电偶极矩又对近邻区域有极化作用，本项目提出利用施主-受主共掺所形成的“离子对”实现Ca3Mn2O7薄膜的改性，以期在改善薄膜电、磁及磁电耦合性能的基础上，提高其使用温度，进而获得综合性能优异的单相多铁薄膜。通过不同种类和含量“离子对”掺杂薄膜的结构、相变行为、电磁性能等的研究，阐明“离子对”掺杂对薄膜相变行为的影响规律与机制；明确“离子对”掺杂对薄膜极化和磁化行为的作用机理；探索利用“离子对”掺杂对薄膜改性的优化途径。本研究将为设计非本征铁电体的多铁性能、进而扩大其应用范围提供思路和研究基础。

多铁材料能实现多重物理量之间的交叉调控，因此多铁材料在微波、存储器、传感器等领域中有重要的应用价值。随着对多铁材料中钙钛矿体系研究的深入，研究者的注意力从简单钙钛矿结构转向复杂的层状类钙钛矿体系，其丰富而复杂的结构为更优化的设计与调控提供了可行性。然而至今为止，同时具有铁电性、铁磁性以及较大的磁电耦合系数的稳定单相多铁材料仍然很少。层状非本征铁电体由于其理论上具有强铁磁、铁电耦合效应，被当作多铁材料的重要候选人。Ca3Mn2O7是一种n=2的Ruddlesden-Popper(RP)结构层状非本征铁电体。通过第一性原理计算发现，其铁电性和铁磁性来源于氧八面体的倾转，但是所需的能量很高。.本工作过施主-受主 “离子对”共掺，在Ca3Mn2O7中引入晶格畸变，降低八面体转动势垒实现层状非本征铁电陶瓷的改性，对于多铁性器件的研究非常重要。首先利用固相反应法制备了不同工艺参数与不同“离子对”掺杂的单相Ca3Mn2O7陶瓷。但是，该类陶瓷的致密性较差，不利于后续研究。后续，利用溶胶凝胶法制备了Ca3-x(Al0.5Li0.5)xMn2O7陶瓷。陶瓷致密性得到大幅提高。Al3+-Li+“离子对”掺杂后，相比纯相Ca3Mn2O7陶瓷，Mn2+峰完全消失，Mn3+峰强度减弱，表明“离子对”掺杂可能会有效抑制Ca3Mn2O7陶瓷氧空位的产生。.相较未掺杂Ca3Mn2O7陶瓷，Al3+-Li+“离子对”掺杂同时降低了Ca3Mn2O7陶瓷的介电常数与介电损耗（介电损耗由222降至9，降低了约23倍）。同时室温I-V测试结果确认了掺杂陶瓷耐压性的明显提高。由于“离子对”掺杂产生的电偶极矩在陶瓷中形成了局域电场的作用下，在Ca2.98(Al0.5Li0.5)0.02Mn2O7陶瓷中出现了单向导通的现象。.掺杂Ca3Mn2O7陶瓷中实现了室温铁磁性，Ca3.95(Al0.5Li0.5)0.05Mn2O7陶瓷的磁滞回线表现出明显铁磁性。但“离子对”掺杂并没有改变陶瓷的尼尔温度。说明“离子对”掺杂降低了Ca3Mn2O7陶瓷八面体转动的势垒，八面体的转动为陶瓷带来了铁磁性。同时，掺杂陶瓷表现出良好的室温磁电耦合性能。纯相陶瓷在9 kOe下的阻抗变化率为4.2%。，Ca2.985(Al0.5Li0.5)0.015Mn2O7陶瓷在9 kOe下的阻抗变化率最大为14.5%，是纯相陶瓷的3倍。