Controllable electrical, magnetoelectric and optical properties of BiFeO3 via domain engineering
Introduction
Multiferroic materials possess two or more ferroic orders (ferroelectricity, ferromagnetism, ferroelasticity, ferrotoroidicity, etc.) simultaneously [1], [2], [3], [4], [5], in which the couplings between various ferroic orders bring in abundant physical effects. For example, in the most intensively investigated multiferroic category, i.e., magnetoelectrics (MEs), the coupling between electric and magnetic orders enables the mutual control of magnetism and ferroelectricity, i.e., manipulation of electric polarization by magnetic field, or the other way around, magnetism by electric field [6]. These phenomena not only initiate a new research branch in condensed matter physics that incorporates charge, spin, orbit and lattice, but also generate huge promises for practical applications such as new memory technology, spintronics, electromagnetic transducing and sensing, etc. [7], [8] Of all multiferroics, bismuth ferrite (BiFeO3, denoted as BFO) is undoubtedly the most known and important one, since it is one of the very few room-temperature single-phase multiferroic materials that exhibits both ferroelectricity (FE) and antiferromagnetism (AF) [2], [9], [10].
BFO was discovered as a FE in the last century, but the measured polarization was only 6.1 μC cm−2, one order of magnitude lower than the values of atomic coordinate calculation and empirical estimation, due to the low quality and large leakage current of the fabricated bulk BFO [11]. Ramesh group observed a large ferroelectric polarization of ∼ 60 μC cm−2 normal to (0 0 1) plane in epitaxial BFO thin films grown on SrTiO3 (STO) substrates in 2003 [2], which opened up a new era of BFO research. Further experimental work confirmed the robust polarization in high-quality unstrained polycrystalline films (∼55 μC cm−2) [12], ceramics (∼40 μC cm−2) [13] and single crystals (60 μC cm−2 along [0 1 0]pseudo-cubic and ∼ 100 μC cm−2 along [109]pseudo-cubic) [14], manifesting the strong and intrinsic ferroelectricity. At room temperature, BFO has a rhombohedral (R) phase with an R3c space group, with the lattice parameter of a = 3.965 Å and α = 89.4° in a perovskite pseudo-cubic (PC) unit cell (PC indexing will be used in the following text unless specified) [15], [16]. The commonly used tolerance factor is 0.954 for BFO [17], where 12-fold coordinated Bi3+ (1.36 Å) extrapolated according to ref. [18], [19], 6-fold coordinated high-spin Fe3+ (0.645 Å) [9] and 6-fold coordinated O2– (1.4 Å) are used [20]. This means the FeO6 octahedra must buckle to fit the rather small unit cell and therefore presents antiphase rotation [21]. It is reported that the FeO6 octahedra is tilted about ∼ 14° along 〈1 1 1〉 axis, which consequently leads to the Fe-O-Fe angle of 154°–156° (Fig. 1a) [9]. The coupling of Bi 6s2 lone electron pairs with the O 2p orbital results in off-centered Bi3+ and hence FE [22], [23], [24]. As a robust FE whose Curie temperature (Tc) is 1103 K, it has one of the highest spontaneous polarization Ps of ∼ 100 μC cm−2 [25], [26] among lead-free FEs.
The rhombohedral nature of BFO makes its spontaneous polarization along the 〈1 1 1〉 orientation (cube diagonals) with 8 possible directions (Fig. 1b). Several energetically equivalent order parameter states, whose magnitudes are the same while directions different, would coexist regionally in one phase. These regions are defined as ferroelectric domains. Within a single domain, all the local Ps vectors align in the same orientation. The atomically thin boundary of adjacent domains are named domain walls (DWs) [27]. Equilibrium domain structures are formed by the competition of electrostatic energy and elastic energy that favor smaller polydomains with interfacial energy of DWs that favors larger monodomains. There are 3 types of DWs in BFO: 71°, 109° and 180°, according to the angle between the Ps in adjacent domains. In general, the 180° DW is purely ferroelectric because the antiparallel polarizations have identical spontaneous strain; the 71° and 109° DWs are both ferroelectric and ferroelastic. Since domains are formed to reduce elastic and electric energy, elastic and electric boundary conditions must be considered to illuminate the crystallographic preferences of different DWs. To ensure mechanical compatibility (the difference of spontaneous strain across the DW should be perpendicular to the wall) and charge neutrality (the difference of spontaneous polarization across the DW should be parallel to the wall), the 71° domains are separated by {1 1 0} twin boundaries, the 109° domains by {1 0 0} twin boundaries [28]. 180° domains that have identical spontaneous strains naturally satisfy the mechanical boundary condition, so they are separated by any crystallographic planes parallel to the polarization so as to be charge neutral [29]. These DW crystallographic relations are schematically displayed in Fig. 1c. The coercivities of the three types of domain switching are different; in epitaxial BFO films, 180° domain switching dominates at lower bias voltages, while 71° and 109° switching at higher voltages [30].
Unlike FE that stems from the 6s2 lone electron pairs of Bi3+, the AF of BFO originates from the Fe-O-Fe bonding. The Fe3+ ion has five electrons on the 3d orbital, where the spin moments are parallel. The exchange interaction between adjacent Fe ions is indirect, assisted by oxygen 2p orbitals. The half-filled d5 interaction leads to AF, according to the Goodenough-Kanamori-Anderson rules [31]. A cycloidal spin structure with a period of ∼ 62 nm is sustained below the Néel temperature of 643 K [21]. The slightly canted, partially uncompensated AF moment also leads to weak ferromagnetism (FM) of ∼ 0.05 μB per unit cell in BFO [22], [32]. The magnetic moment couples ferromagnetically in {1 1 1} planes, which lies perpendicularly to the spontaneous polarization, and antiferromagnetically in adjacent planes. Therefore, the 180° switching of polarization does not change magnetic configurations while the 71° and 109° switching does [33]. However, since the ferroelectricity and magnetism originate from different elements, the intrinsic ME coupling of pure BFO is rather weak. A major research object for multiferroic BFO is to enhance the ME coupling coefficient [34].
Moreover, BFO itself as a strong FE [35] extends the research interests beyond the realm of ME. The large polarization and high Curie temperature as well as lead-free nature of BFO galvanizes extensive research of FE-related properties and applications, such as dielectrics [36], piezoelectrics [37], [38], [39], FeRAM [40], FeFET [40], [41], etc. In addition, BFO as a charge-transfer insulator [42] possesses a bandgap of 2.2–2.8 eV, smaller than that of most conventional FEs [43], [44], [45]. Its conduction band (CB) predominantly comprises Fe 3d orbitals while valence band (VB), O 2p orbitals [31], [46]. The overlap integral of 2p and 3d orbitals depends on the Fe-O bond length and angle, thus the bandgap is controlled by the oxygen octahedra tilting. The small bandgap of BFO results in non-negligible and usable conduction [47] and potential optical-related applications such as photovoltaic (PV) effect [35], [48], [49], photocatalysis [50] and photostriction [51], which enrich both the physical phenomena and functionalities of BFO.
Since most of the properties of BFO are related to, or coupled with, its electrical behaviors, FE domains are sitting at a central position in BFO research. For example, ME coupling is dependent on the interplay of FE and AF orders via ferroelastic domain switching [52]. Domain polymorphs, symmetry change and ferroelastic switching are critical to piezoelectric materials [38], [53]. To realize high dielectric performance, nanodomains are expected, and hence several attempts have been conducted to break the long-range FE order in BFO [54]. On the other hand, long-range periodic domains in BFO enables photonic applications [55]. Besides, DWs have emerged as a rising research spotlight [56], because polarization discontinuity at DWs allows for the arising of charge accumulation, non-equilibrium polar states, topology and chirality, ensuing rich functionalities and some unexpected exotic phenomena. It is believed that the PV behavior of BFO is closely linked to both the anomalous PV effect at DWs and bulk effects within domains [57], [58]. At DWs, unusually high conductivity [59], [60], enhanced piezoelectricity [61] and anisotropic magnetoresistance [62] are also observed, which can be utilized for microwave devices [63] and DW logics [56], [64].
As domain structures are of great importance to the properties of BFO, especially those related to its FE properties (both the static polarization configuration and the dynamic polarization switching), a large proportion of BFO researches spanning bulks, thin films and nanostructures, are focused on the relationships among synthesis, domain structures (domain patterns and phase symmetries), as well as corresponding properties and applications, which is termed as domain engineering. The rapid development of domain engineering in BFO is largely attributed to the advances of material synthesis, probing and characterization techniques. So far, the domain structures of BFO have been manipulated via chemical doping [31], [65], [66] that leads to shrink of lattice and antiferroelectricity (AFE); solid-solution [36], [67] that induces nanodomains and relaxor FE (RFE); substrate engineering in which domain variants are altered by substrate miscut, symmetry and orientation [68], [69]; strain engineering that distorts domains or creates new domain symmetries [38], [52], [70], [71]; flexoelectric effect that flips domains through strain gradient [72], [73]; electric boundary conditions where charge distribution at the interface changes the depolarization field and thus domain structures [74]; spatial-confined nanostructure design that induces topological domains and DWs [75], [76]. The typical strategies of domain engineering in BFO are schematically illustrated in Fig. 2 and will be discussed in section 2.3 in detail.
Undoubtedly, domain engineering has become an effective and fundamental tool in the study of BFO to explore fundamental physical phenomena and to manipulate properties towards practical applications. Nevertheless, as research kept evolving in the past decades, a review that systematically summarizes the relationships between intricate domain structures and versatile properties of BFO has been absent. In this review, we offer a most up-to-date overview of the intensive research advances in BFO in the framework of domain engineering. To begin with, basic knowledge of domain structures of BFO and representative domain engineering strategies are introduced. The electrical properties such as ferroelectricity, piezoelectricity and conduction, ME couplings, as well as optical behaviors such as photovoltaic effect, photocatalysis and mechanical-optical effects are then discussed in sequence, focusing on their manipulation via domain engineering. Exotic electrical, magnetic and optical phenomena at the DWs of BFO are also summarized. We finally conclude by bringing up with the remaining challenges and prospects for this rising field.
Section snippets
Characterization of ferroelectric domains in BFO
Precise characterization is the prerequisite for investigation and manipulation of FE domains in BFO. In this subsection, commonly employed strategies for domain characterization are introduced, including polarizing optical microscopy, transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and piezoresponse force microscopy (PFM).
Polarizing optical microscopy is based on the birefringence effect of FEs, in which domains with different polarization directions
Controllable electrical properties of BFO via domain engineering
The initial research surge in BFO was driven by its potential as a room-temperature single-phase multiferroic and ME material. Nevertheless, it was gradually realized that in BFO the electrical properties alone are also outstanding and intriguing. The strong FE polarization and high Curie temperature make BFO a promising alternative of Pb-based FEs and piezoelectrics. The phase polymorphism and rich domain structures of BFO offer a fertile ground for both the exploration of emergent physical
Control of ferroic domain architectures in BFO thin films
The renaissance of ME multiferroics originates from the observation of coexistence of large FE polarization and magnetization in BFO film [2]. The FE or ferroelastic domain architectures as well as their control have been well demonstrated with the rapid development of advanced SPM techniques, which reveals unique domain structures and DW functionalities. By contrast, the direct observation and control of the corresponding magnetic domain architectures in BFO remain great challenges due to the
Controllable optical-related properties of BFO via domain engineering
In comparison to the large optical gap (∼2.8–3.5 eV) of most conventional FEs (e.g., LiNbO3, PTO, BTO, etc.), BFO shows a considerably smaller bandgap in the visible region around ∼ 2.2–2.8 eV [44], [45], [281], [282]. Owing to the suitable bandgap and unique multiferroic properties, the interactions of electronic, ferroelectric, magnetic and mechanical characteristics of BFO with visible light have emerged as an exciting research field. A variety of intriguing phenomena such as non-linear
Exotic electrical, magnetic and optical phenomena at BFO domain walls
DW is the boundary between adjacent domains. It is a two-dimensional topological defect as well as an interface in materials, which has a much smaller thickness and exhibits interesting functionalities fundamentally different from the bulk domains [7], [56], [349], [350], [351], [352]. As mentioned, BFO has three types of FE DWs: 71°, 109° and 180°. Usually, these different types of DWs are neutral and coexist in the film. However, through delicate control of the strain and electrostatic
Summary of the current status
In summary, we provide a comprehensive review on the electrical, magnetoelectric and optical behaviors of BFO, focusing on the intensive research efforts in domain engineering towards the modulation of functional properties for practical applications. The remarkable progress in BFO over the last two decades has benefitted from the high-quality thin film deposition technique as well as the development of microscopic characterization techniques, which are extremely helpful for the study of the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key Research and Development Program of China [grant number 2021YFB3800601], the Basic Science Center Project of the National Natural Science Foundation of China [grant number 51788104] and the National Natural Science Foundation of China [grant numbers 51729201, 92066203, 51872009, 22075126, 52102130].
Yiqian Liu received his B.E degree in Materials Science and Engineering from Tsinghua University in 2020. He is now a graduate researcher at Tsinghua University under the supervision of Prof. Yuan-Hua Lin. His research focus lies in oxide films and multilayers for energy storage.
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Yiqian Liu received his B.E degree in Materials Science and Engineering from Tsinghua University in 2020. He is now a graduate researcher at Tsinghua University under the supervision of Prof. Yuan-Hua Lin. His research focus lies in oxide films and multilayers for energy storage.
Yao Wang is Associate Professor in Materials Physics and Chemistry at Beihang University. She received her Ph.D. degree in Materials Science and Engineering from Tsinghua University and joined Beihang University in 2009, and worked as a visiting scholar in University of Houston and University of Texas, San Antonio from 2013 to 2014. Her research work focuses on developing high-performance functional nanomaterials, in particular, polymer-based piezoelectric and thermoelectric nanocomposites, and exploring their applications in flexible electronics. She is the author and coauthor of more 90 peer-reviewed research papers.
Ji Ma is Associate Professor at Kunming University of Science and Technology. He received his Ph.D. degree in School of Materials Science and Engineering, Tsinghua University in 2019 under the supervision of Prof. Ce-Wen Nan. His present research interests mainly focus on the self-assembled growth of BiFeO3 (and other multiferroics or ferroelectrics) nano-islands by pulsed laser deposition, and the novel properties of quadrant domains and topologically confined domain walls in these islands.
Shun Li is Jinshan Distinguished Professor at Institute of Quantum and Sustainable Technology (IQST), School of Chemistry and Chemical Engineering of Jiangsu University, China. He received his B.E. Degree from Tianjin University in 2007 and his Ph.D. degree from Institut national de la recherche scientifique (INRS) in Canada in 2015. His research interests include piezo-/ferroelectric nanomaterials and thin films for energy conversion and environmental remediation.
Hao Pan received his B.E. degree in 2015 and Ph.D. degree in 2020, respectively, in Materials Science and Engineering from Tsinghua University, under the supervision of Prof. Yuan-Hua Lin. His research focuses on functional complex oxide films, dielectrics and ferroelectrics for energy storage and conversion.
Ce-Wen Nan is Professor in School of Materials Science and Engineering at Tsinghua University. Before joining the faculty of Tsinghua University in 1999, he had worked in Wuhan University of Technology, China, since 1985. His recent research focuses on multiferroic materials, thermoelectric oxides, polymer-based composites, and solid-state electrolytes. He is Immediate Past President of the International Ceramic Federation. He was elected as Fellow of the Chinese Academy of Sciences in 2011 and Fellow of The World Academy of Sciences in 2012.
Yuan-Hua Lin is Changjiang Scholar Distinguished Professor of Materials Science in School of Materials Science and Engineering, Tsinghua University. He received his B.S. degree from the East China Institute of Technology, M.S. degree from the Institute of Process and Engineering, Chinese Academy of Sciences, and Ph.D. degree from Tsinghua University. His main research interests include high-k ceramics and thin films for high energy density capacitor applications, oxide-based optical functional materials, and high-temperature oxide thermoelectric materials and devices for energy conversion.
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These authors contributed equally.