Magnetic‐Field Controllable Displacement‐Type Ferroelectricity Driven by Off‐Center Fe2+ Ions in CaFe3Ti4O12 Perovskite
Dabiao LuDenis SheptyakovYingying CaoHaoting ZhaoJie ZhangMaocai PiXubin YeZhehong LiuXueqiang ZhangZhao PanXingxing JiangZhiwei HuYi‐feng YangPu YuYouwen Long
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Abstract Displacement‐type ferroelectrics usually exclude magnetic d ‐electron contribution. Applying a magnetic field thus can little change the electric polarization. Herein, a magnetic ionic driven displacement‐type perovskite ferroelectric CaFe 3 Ti 4 O 12 is reported. In this compound, magnetic Fe 2+ ions contribute to both ferroelectric and antiferromagnetic orders respectively at T C ≈107 and T N ≈ 3.1 K, resulting in coupled electric and magnetic domains. A moderate magnetic field can induce a metamagnetic transition toward ferromagnetic correlations. External magnetic fields can thus readily tune the magnetic and the joint ferroelectric domains, giving rise to exceptional magnetic‐field controllable displacement‐type polarization with a large magnetoelectric (ME) coupling coefficient. This study opens up a new avenue to find unprecedented ME effects in displacement‐type ferroelectrics for numerous applications.Ferrimagnetism
Magnetoelectric effect
Magnetism
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A balancing act between antiferromagnetism and ferromagnetism, as shown in the cover artwork, occurs in A2MRu5B2 (A=Zr/Hf; M=Fe/Mn) as a result of highly field-dependent magnetic interactions between ferromagnetic Fe/Mn chains. DFT calculations predict antiferromagnetism (AFM) in the Fe-based phases but competing ferromagnetism (FM) and antiferromagnetism in Mn-based ones. Experiments subsequently found a strong field-dependence of the magnetic transitions in all compounds. More information can be found in the Full Paper by B. P. T. Fokwa, et al. on page 1979.
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Exchange interaction
Organic polymer
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BiFeO3 simultaneously shows antiferromagnetic and ferroelectric order with high transition temperatures, i.e. T N ∼ 370°C and T C ∼ 830°C, respectively. Naturally, it has been inferred that coupling exists between the magnetic and ferroelectric order parameters like in the multiferroic manganites with low transition temperatures. A thorough investigation of the ferroelectric properties of BiFeO3 is therefore in line with the understanding of its multiferroic behaviour. Here, we review the ferroelectric properties of epitaxial (001) oriented BiFeO3 films grown by different techniques on several substrates. Structural characterization along with ferroelectric quantitative analysis point at the high quality of the films. Emphasis is put on identifying the various polarization variants and domain dynamics under an applied bias. In these studies, to unravel the intricate ferroelectric domain structure, piezo-force microscopy scans have been taken along the principal crystallographic directions. Two cases have been analysed. First, a 600 nm thick film grown on SrTiO3 (001) with a thin SrRuO3 underlayer exhibits a mosaic domain pattern due to the presence of both up and down polarization domains. Mainly four polarization domains have been identified in this case, which correspond to two structural domains. Second, epitaxial BiFeO3 films grown on DyScO3 (110) and miscut SrTiO3 (001) with a thin SrRuO3 underlayer show stripe patterns, with mainly two down polarization domains. A single structural domain of orthorhombic SrRuO3 epitaxial underlayer induces this changes in the domain structure of BiFeO3. The suppression of up domains by changing the substrate conditions prove the possibility of ferroelectric domain engineering. The three possible polarization switching mechanisms, namely 71 and 109° rotations, as well as 180° rotation, have been identified by following the domain dynamics in a two-domain epitaxial BiFeO3 film. Interestingly, 180° polarization reversal seems to be the most favorable switching mechanism in epitaxial films under an applied bias along [001]. The observation of both ferroelastic and ferroelectric switching processes open exciting possibilities for the optimization of BiFeO3's ferroelectric properties and investigation of magnetoelectric coupling in epitaxial films. A recent photoemission study using linearly polarized X-rays proved the coupling between the ferroelectric and antiferromagnetic domain structures.
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Control over ferroelectric polarization variants in BiFeO3 films through the use of various vicinal SrTiO3 substrates is demonstrated. The revolution of domain formation as a function of vicinality is characterized, and the ferroelectric polarization variants in these films and the corresponding structural variants are carefully analyzed. The piezo/ferroelectric properties of the BiFeO3 films, in turn, can be tailored through this approach.
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Perovskite ferroelectric (FE) materials have attracted considerable attention for a wide range of applications, such as dynamic random access memories (DRAM), microwave tunable phase shifters and second harmonic generators (SHGs). [1–3] Moreover, materials that have coupled electric, magnetic, and structural order parameters that result in simultaneous ferroelectricity, ferromagnetism, and ferroelasticity are known as multiferroics. [4–6] These multiferroics materials have attracted a lot of attention in recent years because they can potentially offer a whole range of new applications, including nonvolatile ferroelectric memories, novel multiple state memories, and devices based on magnetoeletric effects. Although there are some reports on the electrical and magnetic properties of perovskite-type ferroelectric and multiferroics materials, optical properties and electronic transitions have not been well investigated up to now. On the other hand, phase transition is one of the important characteristics for the ferroelectric/multiferroics system. As we know, the phase transition is strongly related to the structural variation, which certainly can result in the electronic band modifications. Therefore, one can study the phase transition of the above material systems by the corresponding spectral response behavior at different temperatures.
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Ferroelasticity
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Even a century after the discovery of ferroelectricity, the quest for the novel multifunctionalities in ferroelectric and multiferroics continues unbounded. Vertically aligned nanocomposites (VANs) offer a new avenue toward improved (multi)functionality, both for fundamental understanding and for real-world applications. In these systems, vertical strain effects, interfaces, and defects serve as key driving forces to tune properties in very positive ways. In this Perspective, the twists and turns in the development of ferroelectric/multiferroics oxide–oxide and unconventional metal–oxide VANs are highlighted. In addition, the future trends and challenges to improve classic ferroelectric/multiferroic VANs are presented, with emphasis on the enhanced functionalities offered by existing VANs, as well as those in emerging systems.
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We report here on the preservation of ferroelectricity down to 2 nm in BiFeO3 ultrathin films. The electric polarization can be switched reversibly and is stable over several days. Our findings bring insight on the fundamental problem of ferroelectricity at low thickness and confirm the potential of BiFeO3 as a lead-free ferroelectric and multiferroic material for nanoscale devices.
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