Origin of a Tetragonal BiFeO3 Phase with a Giant c/a Ratio on SrTiO3 Substrates

A tetragonal BiFeO₃ phase with giant c/a of approximately 1.25 has been of great interest recently as it potentially possesses a giant polarization and much enhanced electromechanical response. This super‐tetragonal phase is known to be a stable phase only under high compressive strains of above approximately 4.5%, according to first principle calculations. However, in previous work, this super‐tetragonal BiFeO₃ phase was obtained in films deposited at high growth rate on SrTiO₃ substrates with compressive strain of only around 1.5%. By detailed structure analysis using high resolution synchrotron X‐ray diffraction, atomic force microscopy, and transmission electron microscopy, the parasitic β‐Bi₂O₃ phase is identified as the origin inducing the formation of super‐tetragonal BiFeO₃ phase on SrTiO₃ substrates. In addition, ab initio calculations also confirm that this super‐tetragonal phase is more stable than monoclinic phase when Bi₂O₃ is present. Using Bi₂O₃ as a buffer layer, an alternative route, not involving strain engineering, is proposed to stabilize this promising super‐tetragonal BiFeO₃ phase at low growth rates.     

BiFeO3多铁性低维纳米结构研究进展

BiFeO3多铁性低维纳米结构（如纳米晶、纳米线、纳米管、纳米岛等）因其出色的室温多铁性能以及纳尺度下的新型尺寸效应特性,在新型多态存储器及自旋电子学器件方面受到广泛关注。近年来,人们在BiFeO3多铁性低维纳米结构的制备与表征（电、磁性能以及微结构）方面取得了相当进展,本文对此进行了评述。首先,对高质量的BiFeO3多铁性低维纳米结构的制备方法进行了简短评述,然后介绍了BiFeO3多铁性低维纳米结构的纳尺度电性能与磁性能表征以及磁电耦合效应。最后,综述了BiFeO3多铁性低维纳米结构的微结构研究进展以及BiFeO3多铁性低维纳米结构的理论研究结果,并指出了未来BiFeO3多铁性低维纳米结构研究需要重点解决的一些问题。     

Room Temperature Ferrimagnetism and Ferroelectricity in Strained,Thin Films of BiFe0.5Mn0.5O3

Highly strained films of BiFe_0.5Mn_0.5O₃ (BFMO) grown at very low rates by pulsed laser deposition were demonstrated to exhibit both ferrimagnetism and ferroelectricity at room temperature and above. Magnetisation measurements demonstrated ferrimagnetism (T_C ～ 600K), with a room temperature saturation moment (M_S) of up to 90 emu/cc (～ 0.58 μ_B/f.u) on high quality (001) SrTiO₃. X‐ray magnetic circular dichroism showed that the ferrimagnetism arose from antiferromagnetically coupled Fe³⁺ and Mn³⁺. While scanning transmission electron microscope studies showed there was no long range ordering of Fe and Mn, the magnetic properties were found to be strongly dependent on the strain state in the films. The magnetism is explained to arise from one of three possible mechanisms with Bi polarization playing a key role. A signature of room temperature ferroelectricity in the films was measured by piezoresponse force microscopy and was confirmed using angular dark field scanning transmission electron microscopy. The demonstration of strain induced, high temperature multiferroism is a promising development for future spintronic and memory applications at room temperature and above.     

Low‐Symmetry Monoclinic Phases and Polarization Rotation Path Mediated by Epitaxial Strain in Multiferroic BiFeO3 Thin Films

A morphotropic phase boundary driven by epitaxial strain has been observed in lead‐free multiferroic BiFeO₃ thin films and the strain‐driven phase transitions have been widely reported as iso‐symmetric Cc‐Cc by recent works. In this paper, it is suggested that the tetragonal‐like BiFeO₃ phase identified in epitaxial films on (001) LaAlO₃ single crystal substrates is monoclinic M_C. This M_C phase is different from the M_A type monoclinic phase reported in BiFeO₃ films grown on low mismatch substrates, such as SrTiO₃. This is confirmed not only by synchrotron X‐ray studies but also by piezoresponse force microscopy measurements. The polarization vectors of the tetragonal‐like phase lie in the (100) plane, not the (11 0) plane as previously reported. A phenomenological analysis is proposed to explain the formation of M_C Phase. Such a low‐symmetry M_C phase, with its linkage to M_A phase and the multiphase coexistence open an avenue for large piezoelectric response in BiFeO₃ films and shed light on a complete understanding of possible polarization rotation paths and enhanced multiferroicity in BiFeO₃ films mediated by epitaxial strain. This work may also aid the understanding of developing new lead‐free strain‐driven morphotropic phase boundary in other ferroic systems.     

Continuous Control of Charge Transport in Bi‐Deficient BiFeO3 Films Through Local Ferroelectric Switching

It is demonstrated that electric transport in Bi‐deficient Bi_1‐δFeO₃ ferroelectric thin films, which act as a p‐type semiconductor, can be continuously and reversibly controlled by manipulating ferroelectric domains. Ferroelectric domain configuration is modified by applying a weak voltage stress to Pt/Bi_1‐δFeO₃/SrRuO₃ thin‐film capacitors. This results in diode behavior in macroscopic charge‐transport properties as well as shrinkage of polarization‐voltage hysteresis loops. The forward current density depends on the voltage stress time controlling the domain configuration in the Bi_1‐δFeO₃ film. Piezoresponse force microscopy shows that the density of head‐to‐head/tail‐to‐tail unpenetrating local domains created by the voltage stress is directly related to the continuous modification of the charge transport and the diode effect. The control of charge transport is discussed in conjunction with polarization‐dependent interfacial barriers and charge trapping at the non‐neutral domain walls of unpenetrating tail‐to‐tail domains. Because domain walls in Bi_1‐δFeO₃ act as local conducting paths for charge transport, the domain‐wall‐mediated charge transport can be extended to ferroelectric resistive nonvolatile memories and nanochannel field‐effect transistors with high performances conceptually.     

外延BiFeO3薄膜的TEM显微结构表征

外延BiFeO3薄膜中丰富的结构与特殊的性能一直是近年来研究的热点.显微结构的研究不仅可以帮助人们进一步认识BiFeO3的结构信息,还可以帮助人们深入了解BiFeO3结构与性能间的关系,开拓新的应用领域.本文利用球差校正高分辨透射电子显微镜对外延在LaAlO3过渡层/Si基底上的BiFeO3薄膜进行研究.通过原子尺度的定量分析,在应力状态复杂区域观察到类菱方相、应力释放后恢复的菱方相以及拉应力状态下c/a值小于1的类菱方相,并在该区域观察到109°铁电畴,且畴间存在4.4°的畸变夹角.还观察到比较大的c/a比.     

Y和Cr共掺杂BiFeO_3陶瓷的多铁性能研究

采用固相反应法制备了Y_2O_3和Cr_2O_3共掺杂BiFeO3陶瓷,研究了Bi_(0.9)Y_(0.1)Fe_(1–x)Cr_xO_3(BYFC_x,x=0,0.002,0.004,0.006,0.008)陶瓷的多铁性能。XRD分析表明,经850℃烧结的BYFC_x陶瓷形成了三方钙钛矿结构固溶体。随着Cr掺杂量增加,BYFC_x陶瓷在室温下的铁磁性能和铁电性能提高明显。当x为0.004时,所制陶瓷的铁磁性能最好,剩余磁化强度Mr为0.23A·m~2/kg,饱和磁化强度Ms为3.15A·m~2/kg,矫顽力Hc为2.3kA/m。Mr、Ms和Hc随着Cr掺杂量的增加先增大后减小。     

Boosting Photocurrent via Heating BiFeO3 Materials for Enhanced Self‐Powered UV Photodetectors

BiFeO₃ (BFO) is a potentially important Pb‐free ferroelectric with a narrow bandgap and is expected to become a novel photodetector. The photocurrent in BFO₃ strongly depends on the temperature but only a few studies have investigated in detail the relationships between photocurrent and temperature. Here, the temperature‐dependent photocurrent and the corresponding photosensing properties of a Ag/BFO/indiumtin oxide (ITO) photodetector based on an optimized planar‐structured electrode configuration are investigated. The photocurrent and responsivity of the BFO₃‐based photodetector can first be increased and then be decreased with increasing temperature. The largest photocurrent and responsivity can reach 51.5 µA and 6.56 × 10^?4 A W^?1 at 66.1 °C, which is enhanced 126.3% as compared with that at room temperature. This may be caused by the temperature‐modulated bandgap and barrier height in Ag/BFO/ITO device. This study clarifies the relationship between photosensing performance and the operating temperature of BFO‐based photodetector and will push forward the application of ferroelectric materials in photoelectric field.     

Interfacial-Strain-Controlled Ferroelectricity in Self-Assembled BiFeO3 Nanostructures

Self-assembled BiFeO₃-CoFe₂O₄ (BFO-CFO) vertically aligned nanocomposites are promising for logic, memory, and multiferroic applications, primarily due to the tunability enabled by strain engineering at the prodigious epitaxial vertical interfaces. However, local investigations directly revealing functional properties in the vicinity of such critical interfaces are often hampered by the size, geometry, microstructure, and concomitant experimental artifacts. Ferroelectric switching in the presence of lateral distributions of vertical strain thus remains relatively unexplored, with broader implications for all strain-engineered functional devices. By implementing tomographic atomic force microscopy, 3D domain orientation mapping, and spatially-resolved ferroelectric switching movies, local tensile strain significantly impacts the ferroelectric switching, principally by retarding domain nucleation in the BFO nearest to the vertically epitaxial tensile-strained interfaces. The relaxed centers of the BFO pillars become preferred domain nucleation and growth sites for low biases, with up to an order of magnitude change in the edge:center switching ratio for high biases. The new, multi-dimensional imaging approach—and its corresponding insights especially for directly strained interface effects on local properties—thereby advances the fundamental understanding of polarization switching and provides design principles for optimizing functional response in confined nanoferroic systems.     

Deterministic Manipulation of Multi-State Polarization Switching in Multiferroic Thin Films

Deterministically controllable multi-state polarizations in ferroelectric materials are promising for the application of next-generation non-volatile multi-state memory devices. However, the achievement of multi-state polarizations has been inhibited by the challenge of selective control of switching pathways. Herein, an approach to selectively control 71° ferroelastic and 180° ferroelectric switching paths by combining the out-of-plane electric field and in-plane trailing field in multiferroic BiFeO₃ thin films with periodically ordered 71° domain wall is reported. Four-state polarization states can be deterministically achieved and reversibly controlled through precisely selecting different switching paths. These studies reveal the ability to obtain multiple polarization states for the realization of multi-state memories and magnetoelectric coupling-based devices.     

Readdressing of Magnetoelectric Effect in Bulk BiFeO3

After decades of study, BiFeO₃ is still the most promising single‐phase multiferroic material due to its large polarization and high operating temperature, drawing much attention. As a typical type‐I multiferroic material, the magnetoelectric coupling in BiFeO₃ is deemed to be weak due to the different origins of its ferroelectricity and magnetism. Here, the magnetoelectric effect in bulk BiFeO₃ is readdressed both theoretically and experimentally. Based on the Dzyaloshinsky–Moriya interaction scenario, the magnetoelectric effect in BiFeO₃ is actually strong, with a coupling energy of about 1.25 meV and a magnetism‐coupled parasitic polarization comparable to that of the type‐II multiferroics. However, such strong magnetoelectric coupling also causes the cycloidal spin structure, which inhibits the observation of linear magnetoelectric coupling in bulk BiFeO₃. To resolve this contradiction, Sm‐substitution is utilized to suppress the magnetoelectric effect and unlocks the weak ferromagnetism. At an optimized composition, such a weak ferromagnetic state can be switched back to the cycloidal state by an electric field, thus realizing electrical control of the magnetism. It has been argued that field‐controlled phase transition is a promising path to colossal magnetoelectric effect. It is of pioneering significance for further investigations down this road.     

外延BiFeO_3薄膜的温度特性

基于Landau-Devonshire唯象理论,采用等效衬底晶格常数的方法,并拟合了磁刚度系数的温度函数,对多铁性外延BiFeO3薄膜的铁电性和磁性进行了研究。结果表明,70nm厚薄膜的磁化强度在10～731℃先增加后减小,在371℃时有最大值64569A/m,随膜厚的增加磁化强度的极值减小;自发极化、c轴晶格常数随膜厚的增加而减小,同一厚度薄膜随温度的升高自发极化强度减小;压电常数、相对介电常数随温度升高而增大,随膜厚增加而增加。     

3D Reconstruction of Sawtooth 180° Tail-to-Tail Domain Walls in Single Crystal BiFeO3

Previous studies of single crystal BiFeO₃ have found a dense domain structure with alternating sawtooth and flat domain walls (DWs). The nature of these domains and their 3D structure has remained elusive to date. Herein, several sections taken at different orientations are used to examine the structure in detail, concentrating here on the sawtooth DWs using diffraction contrast transmission electron microscopy, electron diffraction, and aberration-corrected scanning transmission electron microscopy (STEM). All DWs are found to be 180° type; the flat walls have head-to-head polarity while the sawtooth DWs are tail-to-tail with peaks elongated along the polar [111] axis, formed by neutral ($11\overline{2}$) DW facets and slightly charged facets with orientations close to ($3\overline{2}1$) and ($\overline{2}31$). The neutral DW facets are Ising type and very abrupt, while the charged DW facets have mixed Néel/Bloch/Ising character with a chiral nature and a width of about 2 nm.     

Interfacial Strain Gradients Control Nanoscale Domain Morphology in Epitaxial BiFeO3 Multiferroic Films

Domain switching pathways fundamentally control performance in ferroelectric thin film devices. In epitaxial bismuth ferrite (BiFeO₃) films, the domain morphology is known to influence the multiferroic orders. While both striped and mosaic domains have been observed, the origins of the latter have remained unclear. Here, it is shown that domain morphology is defined by the strain profile across the film–substrate interface. In samples with mosaic domains, X‐ray diffraction analysis reveals strong strain gradients, while geometric phase analysis using scanning transmission electron microscopy finds that within 5 nm of the film–substrate interface, the out‐of‐plane strain shows an anomalous dip while the in‐plane strain is constant. Conversely, if uniform strain is maintained across the interface with zero strain gradient, striped domains are formed. Critically, an ex situ thermal treatment, which eliminates the interfacial strain gradient, converts the domains from mosaic to striped. The antiferromagnetic state of the BiFeO₃ is also influenced by the domain structure, whereby the mosaic domains disrupt the long‐range spin cycloid. This work demonstrates that atomic scale tuning of interfacial strain gradients is a powerful route to manipulate the global multiferroic orders in epitaxial films.     

(1–x)BiFeO3-xBaTiO3陶瓷的结构与电学性能研究

采用固相反应法制备了(1–x)BiFeO3-xBaTiO3多晶陶瓷样品,研究了 BaTiO3添加对 BiFeO3陶瓷结构、电学性能的影响。结果表明:当 x 由 0 增加到 0.4 时,样品的相结构由三方钙钛矿结构逐渐转变为立方结构,杂相有效消除;漏电流密度从 1.1×10–5A·cm–2下降至 1.1×10–7A·cm–2,相对介电常数提高了 2.6 倍,剩余极化强度增加了近 20 倍。     

(1-x)BiFeO3-xCoFe2O4复合陶瓷介电性能的研究

采用传统的固相烧结工艺制备3种不同摩尔比的(1-x)BiFeO3-xCoFe2O4(简称(1-x)BFO-xCFO,x为摩尔分数,且x=0.1,0.3,0.5)复合陶瓷样品,并分析了其在室温和变温下的介电性能。研究结果表明,室温下陶瓷样品的介电常数和介电损耗均随CFO含量的增加而降低;当频率大于100kHz时,样品的介电常数随CFO含量的增加而变化不大;当x(CFO)=0.1时,样品在高频时介电损耗变小,而当x(CFO)=0.3或0.5时,陶瓷样品的介电损耗明显大于纯BFO陶瓷的介电损耗。在高温情况下,由于CFO的添加使陶瓷样品存在更多的缺陷活化及CFO所造成的漏电流,使样品在低频时的介电常数与介电损耗都变得很大。在100kHz频率下的交流导电率均随着温度的增加而减少;且相同温度下样品中包含的CFO含量越大其交流导电率也越大。当x(CFO)=0.3和0.5时,复合陶瓷样品的交流导电率随温度的变化规律几乎相同,且它们在相同温度下的交流导电率比纯BFO的大5～6个数量级。     

Domain Selectivity in BiFeO3 Thin Films by Modified Substrate Termination

Ferroelectric domain formation is an essential feature in ferroelectric thin films. These domains and domain walls can be manipulated depending on the growth conditions. In rhombohedral BiFeO₃ thin films, the ordering of the domains and the presence of specific types of domain walls play a crucial role in attaining unique ferroelectric and magnetic properties. In this study, controlled ordering of domains in BiFeO₃ film is presented, as well as a controlled selectivity between two types of domain walls is presented, i.e., 71° and 109°, by modifying the substrate termination. The experiments on two different substrates, namely SrTiO₃ and TbScO₃, strongly indicate that the domain selectivity is determined by the growth kinetics of the initial BiFeO₃ layers.     

Boosted Photocurrent via Heating BiFeo3 Thin Film for UV Photodetector at Wide Temperature Range

The current research on ferroelectric photovoltaic materials is concentrated on enhancing the output photocurrent. As solar cells operate at high temperatures, it is crucial to take into account the effect of increasing temperatures on ferroelectric photovoltaics. In this study, an LNO (lanthanum nickelate, LaNiO₃)/BFO (bismuth ferrate, BiFeO₃)/ITO (indium tin oxide) device is constructed on a mica substrate by sol–gel method. The device achieves output photocurrent enhancement at a wide temperature range (33–183 °C), with the largest photocurrent enhancement at 130 °C, which is 178% relative to room temperature, and the output power is also increased by 9.88 times. At the same time, compared with BFO bulk, it is found that the performance of BFO film is always higher than that of bulk in the test temperature range, and the output photocurrent of BFO film at room temperature is 104 times higher than that of bulk. This article investigates the effect of high temperatures on ferroelectric photovoltaics and also provides a strategy for enhancing the photovoltaic performance of ferroelectric films, providing guidance for future applications of ferroelectric films in flexible solar cells and other applications.     

Neutron Diffraction Investigations of Magnetism in BiFeO3 Epitaxial Films

The recovery of a modulated magnetic structure in epitaxial BiFeO₃ thin films as revealed by neutron diffraction is reported. The magnetic structure in thin films is found to strongly depend on substrate orientation. The substrate orientation causes different strain–relaxation processes resulting in different thin‐film crystal structures. The (110) oriented film with a monoclinic structural phase has a single‐domain modulated magnetic structure where the magnetic moment lies in the HHL plane. For the (111) oriented film that has a rhombohedral structure, a modulated structure superimposed on the G‐type antiferromagnetic order is found. These results indicate that slight structural modifications in the BiFeO₃ thin film cause drastic changes in the magnetic structure.     

Mechanically Driven Reversible Polarization Switching in Imprinted BiFeO3 Thin Films

Mechanically driven polarization switching via scanning probe microscopy provides a valuable voltage-free strategy for designing ferroelectric nanodomain structures. However, it is still challenging to realize reversible polarization switching with mechanical forces. Here, the mechanically driven reversible polarization switching observed in imprinted ferroelectric BiFeO₃ thin films is reported, i.e., up-to-down switching by a sharp scanning tip and down-to-up switching by a blunt tip. Free energy calculations, phase-field simulations, and piezoresponse force microscopy reveal that reversible mechanical switching arises from the interplay among the flexoelectric effect, the piezoelectric effect, and the internal upward built-in field in BiFeO₃ films. This study gains a deeper insight into the mechanism and control of mechanically driven polarization switching, and provides guidance for exploring potential ferroelectric-based electro-mechanical microelectronics.