本征空位缺陷态Ca2Nb2O7薄膜的可控制备及d0磁性研究

Ferroelectric material with high Curie temperature is considered as potential multiferroics at room temperature, thus attracts considerable attention in recent years. However, the origin of ferromagnetism observed in undoped ferroelectric film is still unclear, the effect of cation vacancy on magnetism cannot be verified by experiment, the interface between film and substrate as well as the cooperative effect of cation and anion vacancy is unconsidered in theoretical calculation. To solve these problems, this project takes Ca2Nb2O7 as the object of research, which is a typical A2Nb2O7 niobate ferroelectric material with high Curie temperature. Controllable fabrication of Ca2Nb2O7 film on perovskite-type substrate with Ca/Nb/O single vacancy and Ca+O or Nb+O double vacancies is achieved through the preparation of ceramic target with cation defect and the adjustment of oxygen pressure during deposition using pulsed laser deposition technique. The effect of single vacancy and cooperative effect of Ca+O or Nb+O double vacancies are investigated by magnetic measurement in combination with related characterization and first principle calculations. This project will clarify the origin of magnetism in undoped niobate film on perovskite-type substrate, and provide experimental and theoretical reference for the study of d0 magnetism on A2Nb2O7-type niobate ferroelectric film with high Curie temperature.

近年来，具有高铁电居里温度的铁电材料因是一种潜在的室温多铁材料而受到广泛的关注，但实验和理论上对无掺杂铁电薄膜中观察到的铁磁性来源都存在一定的分歧，实验上无法验证阳离子空位缺陷对磁性的影响，理论上没有考虑薄膜与衬底组成的界面以及阳离子和氧空位协同作用对薄膜磁性的影响。针对这些问题，本项目选择具有高铁电居里温度的A2Nb2O7型铌酸盐铁电材料Ca2Nb2O7为研究对象，利用脉冲激光沉积技术，通过阳离子缺陷靶材的制备和氧气压强的调节，在钙钛矿结构衬底上实现具有Ca/Nb/O单空位及Ca+O或Nb+O双空位缺陷Ca2Nb2O7薄膜的可控制备；磁性测量结合各种表征手段和第一性原理计算，探讨研究薄膜表面及界面处阳离子空位和氧空位缺陷单独作用及协同作用机制。本项目的开展有利于明确钙钛矿结构衬底上无掺杂铌酸盐薄膜磁性的来源，为A2Nb2O7型层状钙钛矿结构铌酸盐薄膜的磁性研究提供实验和理论依据。

针对实验和理论上对无掺杂铁电材料中观察到的铁磁性来源存在争议和分歧，实验上无法验证阳离子空位缺陷对磁性的影响，理论上没有考虑阳离子和氧空位协同作用对薄膜磁性影响这些问题，本项目以具有高铁电居里温度的A2Nb2O7型层状钙钛矿结构铌酸盐Ca2Nb2O7为研究对象，主要研究PLD制备过程中各种参数对本征空位缺陷态Ca2Nb2O7单相薄膜质量的影响规律及薄膜表面Ca/Nb/O单空位和Ca+O或Nb+O复合空位缺陷对薄膜表面磁性的影响规律。XRD结果显示：靶材中Ca:Nb为1:1、0.95:1或1:0.95、衬底STO（110）、衬底靶材距离60mm、衬底温度650-800℃、脉冲激光能量200mJ和频率3Hz、氧气压强1mTorr可以满足[010]方向Ca2Nb2O7单相薄膜的生长条件，其中对氧气压强的要求最高，0/0.5/5.0/10.0mTorr的氧气压强下或产生CaNb2O6杂相或得到非晶薄膜，无法通过调节氧气压强制备只含O空位缺陷的Ca2Nb2O7单相薄膜。EDS显示薄膜主要元素为Ca、Nb和O；元素mapping显示Ca和Nb均匀地分布在薄膜中；AFM显示薄膜均为垂直于衬底表面柱状生长；XPS显示Ca和Nb分别为+2和+5价，证明利用Ca:Nb为1:1、0.95:1靶材制备的薄膜中Ca+O复合空位的存在，5k和300k温度下MH测试显示薄膜均无铁磁性，而利用Ca:Nb为1:0.95靶材制备的薄膜中存在Nb+O复合空位，5k和300k温度下MH测试显示铁磁性，且MT测试显示该磁性5-300k的温度区间内具有温度稳定性。第一性原理计算显示：等化学计量比Ca2Nb2O7（010）表面无磁性，表层Ca/Nb/O单空位通过附近O 2p或Nb 3d电子的自旋极化可引入磁性；Ca+O复合空位不能在Ca2Nb2O7表面引入磁性，而Nb+O复合空位通过Nb空位附近剩余O原子2p电子的自旋极化可引入磁性，且最稳定系统铁磁性耦合相对反铁磁性耦合更加稳定。本项目给出了利用PLD技术制备层状钙钛矿结构Ca2Nb2O7单相薄膜的参数条件，从实验和理论上验证了Nb+O复合空位缺陷在Ca2Nb2O7薄膜表面引入铁磁性的可能性，可以为其他A2Nb2O7型层状钙钛矿结构铌酸盐薄膜的PLD制备和磁性研究提供实验和理论参考。