铋系锰氧化物多铁薄膜的结构设计及性能研究

随着大数据与人工智能时代的来临，数据存量以指数型方式不断增长，实现更高速率、更高密度以及更低功耗的数据存储已经迫在眉睫。然而，当前主流的磁存储技术在存储密度以及写入效率等方面仍然较低。尽管铁电存储技术相对应地克服了以上缺陷，但其读取数据方式却具有破坏性。集合磁序和电序于一体的多铁性材料恰好为实现以上二者的兼容提供了独特的平台。作为研究最为广泛的环境友好型无铅多铁性材料，铋系多铁性钙钛矿氧化物薄膜材料在信息存储技术的应用领域展现出了诱人的前景。

本文围绕BiMnO3薄膜材料的多铁性与外部因素的关联性进行了相关实验设计。通过选择具有不同晶格常数的单晶衬底与BiMnO3产生不同晶格失配，实验验证了外延应力对BiMnO3薄膜多铁性的调控作用。利用水溶性牺牲层以释放衬底应力，实验获得了结构稳定的多铁性柔性自支撑BiMnO3薄膜。通过调整超晶格的周期厚度参数，实验在基于BiMnO3的超晶格体系中实现了较强铁电性和铁磁性的共存。具体而言，包括如下的研究：

在单相BiMnO3薄膜材料中，探究了外延应力对薄膜铁电性与铁磁性的调控作用。实验采用脉冲激光沉积技术在与BiMnO3具有不同晶格失配度的四种单晶衬底上生长了具有应变大小差异的高质量BiMnO3薄膜。研究结果表明，压缩应力提高了薄膜面外方向的结构不对称性，使BiMnO3薄膜产生更大的晶格四方性，从而更有效地增强了BiMnO3薄膜的铁电压电特性。与此同时，研究发现，BiMnO3薄膜的铁磁态对外延应力的敏感度较低。但通过提高Mn2+离子占比，改变薄膜Mn离子eg轨道电子占据，影响BiMnO3薄膜的磁交换行为，拉伸应力具有增大BiMnO3薄膜磁矫顽场的作用。因此，相较而言，压缩应力造成的晶格失配更有利于实现BiMnO3薄膜潜在的磁电耦合效应，这对于探索高性能多铁性材料的实现具有重要的指导意义。

通过选用晶格常数更小的LaAlO3单晶作为衬底，进一步探究了更大的压缩应力对BiMnO3薄膜的结构、压电以及磁学特性的影响。研究结果表明，-3.65%的晶格失配度使BiMnO3薄膜失去典型的钙钛矿结构，形成特殊的层状超晶胞结构。通过调节生长氧气分压，实验在基于同等化学计量比的BiMnO3材料中获得了具有两种不同Bi-O结构的超晶胞薄膜。两种不同结构的超晶胞薄膜都具有典型压电响应特性。由于两种超晶胞薄膜面外方向的Bi-O结构不同，导致其Mn离子间超交换作用不同，两种超晶胞薄膜具有饱和磁化强度的差异性。

基于拓宽材料的应用空间，探究了实现柔性自支撑结构对BiMnO3薄膜多铁性的影响。利用水溶性的Sr3Al2O6薄膜作为中间牺牲层，实验成功实现了BiMnO3薄膜从刚性单晶衬底的转移。研究结果表明，自支撑BiMnO3薄膜是具有稳定结构的高质量单晶薄膜。由于转移前后薄膜晶格稳定性高，自支撑BiMnO3薄膜的铁电压电性和强铁磁特性都呈现良好的保持性。此外，由于BiMnO3薄膜的铁电畴结构在外力作用下具有动态演变特性，自支撑BiMnO3薄膜呈现新奇的超柔性。

在基于BiMnO3薄膜的复合材料中，探究了超晶格周期参数对BiFeO3/BiMnO3超晶格多铁性的调控作用。实验利用采用脉冲激光沉积技术制备了纯相且界面结构清晰的系列高质量BiFeO3/BiMnO3超晶格。通过调整超晶格周期厚度参数，实验发现较厚BiFeO3层的强铁电极化场有利于诱导BiMnO3层产生强极化，从而增强超晶格的铁电性。与此同时，反铁磁性BiFeO3层的引入使超晶格界面处产生了Fe-O-Mn的反铁磁超交换作用，极大地削弱了来自BiMnO3层的净磁矩，造成BiFeO3/BiMnO3超晶格呈现弱铁磁性。通过调整其中BiMnO3层周期厚度，实验有效地调控了超晶格中Fe-O-Mn的反铁磁超交换耦合作用强度，从而在BiFeO3/BiMnO3超晶格体系中实现了较强铁电态与铁磁态的共存。

With the advent of the era of big data and artificial intelligence, the exponential growth of data has put forward higher demands on storage technology. The realization of higher speed, higher density and lower power consumption in data storage is imminent. However, the storage density and writing efficiency of current mainstream magnetic storage are still low. Although ferroelectric storage overcomes the above defects correspondingly, they are limited by the need for a destructive read and reset operation. Multiferroic materials, which show ferromagnetism and ferroelectricity simultaneously, provide a unique platform for realizing the compatibility of the two. As the most widely studied lead-free multiferroic materials, bismuth-based multiferroic perovskite oxide thin films show attractive prospects of the applications of storage technology.

In this thesis, the correlations between the multiferroic properties and external factors such as strain, special freestanding structure and superlattice interface of multiferroic BiMnO3 thin films are studied. Firstly, by adjusting the lattice mismatch of films and substrates, the multiferroicity of single-phase BiMnO3 thin films is tuned. Then, the flexible freestanding BiMnO3 membranes with stable structure and multiferroic properties are prepared using water-soluble sacrificial layer to release the strains of the substrate. Finally, by tunning the periodic thickness parameters, the coexistence of strong ferroelectricity and ferromagnetism have realized in BiMnO3-based superlattice systems. Our researches mainly include the following three parts:

Single-phase BiMnO3 thin films with different epitaxial strains are designed to tune their ferroelectric and ferromagnetic properties. High-quality BiMnO3 thin films with different epitaxial strains were fabricated using pulsed laser deposition by selecting different single crystal substrates. Through improving the structural asymmetry in the out-of-plane direction of the films, compressive strains produce larger lattices tetragonality in BiMnO3 films, which effectively enhance the ferroelectric properties of the films. Meanwhile, it is found that the ferromagnetic state of BiMnO3 films is less sensitive to the epitaxial strains. By increasing the proportion of Mn2+ ions, tensile strains change the Mn ion eg orbital electron occupancy and magnetic exchange to increase the magnetic coercive field of BiMnO3 films. Overall, compressive strains are more conducive to realizing the potential magnetoelectric coupling of BiMnO3 films. This work has important guiding significance of exploring the realization of high-performance multiferroic materials.

By choosing single crystal LaAlO3 with smaller lattice constant as the substrate, the effect of larger compressive strain on the structure, piezoelectric and magnetic properties of BiMnO3 films are further explored. The results show that the lattice mismatch of -3.65% contributes to form the special layered supercell structure of BiMnO3. By adjusting the growth oxygen partial pressure, layered supercells with two different Bi-O structures were fabricated. Both the supercells show typical piezoelectric responses. Due to the different Bi-O structures of the out-of-plane direction, two supercells show difference in the magnetic super-exchange interaction between Mn ions, resulting in different saturated magnetizations of the two supercells.

Based on broadening the applications of materials, BiMnO3 films with flexible freestanding structures are designed to explore its multiferroic properties. Using pulsed laser deposition, water-soluble Sr3Al2O6 thin films were fabricated to serve as the intermediate sacrificial layers for achieving the flexibility of BiMnO3 films. The freestanding BiMnO3 membranes are verified to show high quality with stable structure. Due to the high stability of lattices, the ferroelectricity and ferromagnetism of the transferred freestanding BiMnO3 membranes are preserved well. Moreover, the freestanding BiMnO3 membranes exhibit novel super-flexibility, which originates from the dynamic evolution of ferroelectric nanodomains of the BiMnO3 membranes.

In BiMnO3-based composite system, the superlattice structure of BiFeO3/BiMnO3 is designed with different periodic thickness parameters to tune its multiferroicity. By precisely adjusting the growth parameters using pulsed laser deposition, high-quality BiFeO3/BiMnO3 superlattices with pure phase and clear interfaces were obtained. Through adjusting the periodic parameters of the superlattices, we find that the polarization of BiMnO3 layers is induced by the strong ferroelectric polarization field of the thicker BiFeO3 layers. Moreover, the introduction of antiferromagnetic BiFeO3 leads to the interfacial antiferromagnetic super-exchange coupling of Fe-O-Mn, which tends to kill the net magnetic moment from BiMnO3 layers. By means of obtaining an appropriate ratio of periodic thickness between the BiFeO3 layers and the BiMnO3 layers, the coexistence of strong ferroelectricity and ferromagnetism have achieved finally at low temperature in BiFeO3/BiMnO3 superlattices.

With the advent of the era of big data and artificial intelligence, the exponential growth of data has put forward higher demands on storage technology. The realization of higher speed, higher density and lower power consumption in data storage is imminent. However, the storage density and writing efficiency of current mainstream magnetic storage are still low. Although ferroelectric storage overcomes the above defects correspondingly, they are limited by the need for a destructive read and reset operation. Multiferroic materials, which show ferromagnetism and ferroelectricity simultaneously, provide a unique platform for realizing the compatibility of the two. As the most widely studied lead-free multiferroic materials, bismuth-based multiferroic perovskite oxide thin films show attractive prospects of the applications of storage technology.

In this thesis, the correlations between the multiferroic properties and external factors such as strain, special freestanding structure and superlattice interface of multiferroic BiMnO3 thin films are studied. Firstly, by adjusting the lattice mismatch of films and substrates, the multiferroicity of single-phase BiMnO3 thin films is tuned. Then, the flexible freestanding BiMnO3 membranes with stable structure and multiferroic properties are prepared using water-soluble sacrificial layer to release the strains of the substrate. Finally, by tunning the periodic thickness parameters, the coexistence of strong ferroelectricity and ferromagnetism have realized in BiMnO3-based superlattice systems. Our researches mainly include the following three parts:

Single-phase BiMnO3 thin films with different epitaxial strains are designed to tune their ferroelectric and ferromagnetic properties. High-quality BiMnO3 thin films with different epitaxial strains were fabricated using pulsed laser deposition by selecting different single crystal substrates. Through improving the structural asymmetry in the out-of-plane direction of the films, compressive strains produce larger lattices tetragonality in BiMnO3 films, which effectively enhance the ferroelectric properties of the films. Meanwhile, it is found that the ferromagnetic state of BiMnO3 films is less sensitive to the epitaxial strains. By increasing the proportion of Mn2+ ions, tensile strains change the Mn ion eg orbital electron occupancy and magnetic exchange to increase the magnetic coercive field of BiMnO3 films. Overall, compressive strains are more conducive to realizing the potential magnetoelectric coupling of BiMnO3 films. This work has important guiding significance of exploring the realization of high-performance multiferroic materials.

By choosing single crystal LaAlO3 with smaller lattice constant as the substrate, the effect of larger compressive strain on the structure, piezoelectric and magnetic properties of BiMnO3 films are further explored. The results show that the lattice mismatch of -3.65% contributes to form the special layered supercell structure of BiMnO3. By adjusting the growth oxygen partial pressure, layered supercells with two different Bi-O structures were fabricated. Both the supercells show typical piezoelectric responses. Due to the different Bi-O structures of the out-of-plane direction, two supercells show difference in the magnetic super-exchange interaction between Mn ions, resulting in different saturated magnetizations of the two supercells.

Based on broadening the applications of materials, BiMnO3 films with flexible freestanding structures are designed to explore its multiferroic properties. Using pulsed laser deposition, water-soluble Sr3Al2O6 thin films were fabricated to serve as the intermediate sacrificial layers for achieving the flexibility of BiMnO3 films. The freestanding BiMnO3 membranes are verified to show high quality with stable structure. Due to the high stability of lattices, the ferroelectricity and ferromagnetism of the transferred freestanding BiMnO3 membranes are preserved well. Moreover, the freestanding BiMnO3 membranes exhibit novel super-flexibility, which originates from the dynamic evolution of ferroelectric nanodomains of the BiMnO3 membranes.

In BiMnO3-based composite system, the superlattice structure of BiFeO3/BiMnO3 is designed with different periodic thickness parameters to tune its multiferroicity. By precisely adjusting the growth parameters using pulsed laser deposition, high-quality BiFeO3/BiMnO3 superlattices with pure phase and clear interfaces were obtained. Through adjusting the periodic parameters of the superlattices, we find that the polarization of BiMnO3 layers is induced by the strong ferroelectric polarization field of the thicker BiFeO3 layers. Moreover, the introduction of antiferromagnetic BiFeO3 leads to the interfacial antiferromagnetic super-exchange coupling of Fe-O-Mn, which tends to kill the net magnetic moment from BiMnO3 layers. By means of obtaining an appropriate ratio of periodic thickness between the BiFeO3 layers and the BiMnO3 layers, the coexistence of strong ferroelectricity and ferromagnetism have achieved finally at low temperature in BiFeO3/BiMnO3 superlattices.

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