The magnetoelectric behavior of BiFeO$_3$ has been explored on the basis of accurate density functional calculations. The structural, electronic, magnetic, and ferroelectric properties of BiFeO$_3$ are predicted correctly without including strong correlation effect in the calculation. Moreover, the experimentally-observed elongation of cubic perovskite-like lattice along the [111] direction is correctly reproduced. At high pressure we predicted a pressure-induced structural transition and the total energy calculations at expanded lattice show two lower energy ferroelectric phases, closer in energy to the ground state phase. Band-structure calculations show that BiFeO$_3$ will be an insulator in A- and G-type antiferromagnetic phases and a metal in other magnetic configurations. Chemical bonding in BiFeO$_3$ has been analyzed using various tools and electron localization function analysis shows that stereochemically active lone-pair electrons at the Bi sites are responsible for displacements of the Bi atoms from the centro-symmetric to the noncentrosymmetric structure and hence the ferroelectricity. A large ferroelectric polarization (88.7 $μ$C/cm$^{2}$) is predicted in accordance with recent experimental findings. The net polarization is found to mainly ($>$ 98%) originate from Bi atoms. Moreover the large scatter in experimentally reported polarization values is due to the large anisotropy in the spontaneous polarization.
Understanding the mechanisms of magnetoelectric (ME) coupling within multiferroic structures is paramount from a fundamental as well as an applied point of view. We report here that the magnetoelectric properties, as well as the magnetization, of BaTiO$_3$-BiFeO$_3$ superlattices can be tuned by varying the BiFeO$_3$ layer thickness. The magnetoelectric voltage coefficient ($α_{ME}$) reaches its maximum of 60.2 Vcm$^{-1}$Oe$^{-1}$ at 300 K, one of the highest values reported so far, for a sample with a BiFeO$_3$ thickness of 5 nm and a BaTiO$_3$ thickness of 10 nm. To gain deeper insight into …
Phase transitions in multiferroic BiFeO3 crystals, thin-layers, and ceramics: Enduring potential for a single phase, room-temperature magnetoelectric 'holy grail'
Clear anomalies in the lattice thermal expansion (deviation from linear variation) and elastic properties (softening of the sound velocity) at the antiferromagnetic-to-paramagnetic transition are observed in the prototypical multiferroic BiFeO3 using a combination of picosecond acoustic pump-probe and high-temperature X-ray diffraction experiments. Similar anomalies are also evidenced using first-principles calculations supporting our experimental findings. Those calculations in addition to a simple Landau-like model we also developed allow to understand the elastic softening and lattice chang…
Using high resolution powder x-ray and neutron diffraction experiments, we determined the off-centered displacement of the ions within a unit cell and magnetoelectric coupling in nanoscale BiFeO$_3$ ($\approx$20-200 nm). We found that both the off-centered displacement of the ions and magnetoelectric coupling exhibit nonmonotonic variation with particle size. They increase as the particle size reduces from bulk and reach maximum around 30 nm. With further decrease in particle size, they decrease precipitously. The magnetoelectric coupling is determined by the anomaly in off-centering of ions a…
Identifying the Magnetoelectric Modes of Multiferroic BiFeO$_3$
我们已经确定了四个磁电模式的多铁性BiFeO $_3$使用太赫兹光谱测量。通过包括沿电极化的强度方向的易轴各向异性的影响，获得了与观察到的峰的极好的协议。通过扭曲摆线自旋状态，各向异性将$Ψ_{\pm 1}$模式分裂成20和21.5 cm $^{-1}$处的峰，并激活27 cm $^{-1}$处的较低$Φ_{\pm 2}$模式（T=200 K）。在21.5 cm $^{-1}$处，用上$Ψ_{\pm 1}$模式识别电磁振子。我们的结果也解释了最近的拉曼和非弹性中子散射测量。
Magnetic fields have been regarded as an additional stimulus for electro- and photocatalytic reactions, but not as a direct trigger for catalytic processes. Multiferroic/magnetoelectric materials, whose electrical polarization and surface charges can be magnetically altered, are especially suitable for triggering and control of catalytic reactions solely with magnetic fields. Here, we demonstrate that magnetic fields can be employed as an independent input energy source for hydrogen harvesting by means of the magnetoelectric effect. Composite multiferroic CoFe2O4-BiFeO3 core-shell nanoparticle…
Two-dimensional (2D) magnets have broad application prospects in the spintronics, but how to effectively control them with a small electric field is still an issue. Here we propose that 2D magnets can be efficiently controlled in a multiferroic heterostructure composed of 2D magnetic material and perovskite oxide ferroelectric (POF) whose dielectric polarization is easily flipped under a small electric field. We illustrate the feasibility of such strategy in the bilayer CrI3/BiFeO3(001) heterostructure by using the first-principles calculations. Different from the traditional POF multiferroic …
Origin of Large Dielectric Constant with Large Remnant Polarization and Evidence of Magnetoelectric Coupling in Multiferroic La modified BiFeO3-PbTiO3 Solid Solution
Over the years attempts have been made to compensate for the inherent weaknesses in the bulk state of the multiferroic BiFeO3, such as high leakage current and the absence of ferromagnetic correlation, and exploit its magnetoelectric potential by forming solid solutions with other perovskites. Studies in the recent few years have shown that alloying of BiFeO3 with BaTiO3, both with and without additives, can induce both ferroelectric and ferromagnetic switching. While the coexistence of both the ferroic orders is encouraging from the view point of technological applications, the origin of ferr…
In a nanocomposite of reduced graphene oxide (RGO) and BiFeO$_3$ (BFO), the remanent ferroelectric polarization is found to follow nonmonotonic magnetic field dependence at room temperature as the applied magnetic field is swept across 0-20 kOe on a pristine sample. The remanent ferroelectric polarization is determined both from direct electrical measurements on an assembly of nanoparticles and powder neutron diffraction patterns recorded under 0-20 kOe field. The nanosized ($\sim$20 nm) particles of BFO are anchored onto the graphene sheets of RGO via Fe-C bonds with concomitant rise in coval…
The spin-driven component of electric polarization in a single crystal of multiferroic BiFeO$_{3}$ was experimentally investigated in pulsed high magnetic fields up to 41 T. Sequential measurements of electric polarization for various magnetic field directions provide clear evidence of electric polarization normal to the hexagonal $c$ axis (${\bf P}_{\rm t}$) in not only the cycloidal phase, but also the field-induced canted antiferromagnetic phase. The direction of ${\bf P}_{\rm t}$ is directly coupled with the ferromagnetic moment in the canted antiferromagnetic phase, and thus controlled by…
Bismuth ferrite (BiFeO$_3$) is one of the very few known single-phase multiferroic materials. While the bulk compound is rhombohedral (R), the discovery of an epitaxial strain-induced structural transition into a so-called 'super tetragonal-phase' (T-phase) in this material incited a flurry of research activity focused on gaining an understanding of this phase transition and its possible functionalities. This metastable phase of BiFeO$_3$ is also multiferroic, with giant ferroelectric polarization and coexisting antiferromagnetic order, but above all it is the strain relaxation-induced phase m…
The effects of high magnetic fields applied perpendicular to the spontaneous ferroelectric polarization on single crystals of BiFeO$_3$ were investigated through magnetization, magnetostriction, and neutron diffraction measurements. The magnetostriction measurements revealed lattice distortion of $2\times 10^{-5}$, during the reorientation process of the cycloidal spin order by applied magnetic fields. Furthermore, anomalous changes in magnetostriction and electric polarization at a larger field demonstrate an intermediate phase between cycloidal and canted antiferromagnetic states, where a la…
Although several multiferroic materials/heterostructures have been extensively studied, finding strong magnetoelectric couplings for the electric field control of the magnetization remains challenging. Here, a novel interfacial magnetoelectric coupling based on three components (ferroelectric dipole, magnetic moment, and antiferromagnetic order) is analytically formulated. As an extension of carrier-mediated magnetoelectricity, the new coupling is shown to induce an electric-magnetic hysteresis loop. Realizations employing BiFeO$_3$ bilayers grown along the [$111$] axis are proposed. Without i…
Bismuth ferrite (BiFeO3) is one of the most promising multiferroics with a sufficiently high ferroelectric (FE) and antiferromagnetic transition temperatures, and magnetoelectric (ME) coupling coefficient at room temperature, and thus it is highly sensitive to the impact of cross-influence of applied electric and magnetic fields. According to the urgent demands of nanotechnology miniaturization for ultra-high density data storage in advanced nonvolatile memory cells, it is very important to reduce the sizes of multiferroic nanoparticles in the self-assembled arrays without serious deterioratio…
The perovskite (1-x)YMnO$_3$-xBiFeO$_3$ binary system is very promising because of its multiferroic end members. Nanocrystalline phases have been recently obtained by mechanosynthesis across the system, and the perovskite structural evolution has been described. Two continuous solid solutions with orthorhombic Pnma and rhombohedral R3c symmetries were found, which coexist within a broad compositional interval of 0.5 < x < 0.9. This might be a polar-nonpolar morphotropic phase boundary region, at which strong phase-change magnetoelectric responses can be expected. A major issue is phase decompo…
In this contribution to the special issue on magnetoelectrics and their applications, we focus on some single phase multiferroics theoretically predicted and/or experimentally discovered by the authors in recent years. In these materials, iron is the common core element. However, these materials are conceptually different from the mostly-studied BiFeO$_3$, since their ferroelectricity is improper. Our reviewed materials are not simply repeating one magnetoelectric mechanism, but cover multiple branches of improper ferroelectricity, including the magnetism-driven ferroelectrics, geometric ferro…
University of Duisburg-Essen（杜伊斯堡-埃森大学）；European Organization for Nuclear Research (CERN)（欧洲核子研究组织（CERN））；Universidade de Aveiro（阿威罗大学）；Institute for Research in Fundamental Sciences (IPM)（基础科学研究所（IPM））
Antiferromagnets (AFMs) are presently considered as promising materials for applications in spintronics and random access memories due to the robustness of information stored in AFM state against perturbing magnetic fields (P. Wadley et al., Science 351, 587 (2016)). In this respect, AFM multiferroics maybe attractive alternatives for conventional AFMs as the coupling of magnetism with ferroelectricity (magnetoelectric effect) offers an elegant possibility of electric field control and switching of AFM domains. Here we report the results of comprehensive experimental and theoretical investigat…
The complex dielectric permittivity of a sintered ceramic tablet consisting of 70.5 % $BiFeO_{3}$ and 27.7% $Bi_{2}Fe_{4}O_{9}$ was analyzed as a function of temperature from -120 °C to 230 °C. The results reveal a complicated dielectric response with temperature activated relaxation features. They also reveal a ferroelectric phase transition that decayed with repeated heating cycles of the tablet. The source of the behaviour is assigned to relaxation processes happening along the grain boundaries of differing compositions in the tablet. The origin of the phase transition is traced to locally …
Orbital degrees of freedom as origin of magnetoelectric coupling in magnetite
对磁电性的微观理解，即磁（电）性质和外部电（磁）场之间的耦合，是未来几代电控自旋电子器件的重要里程碑。在这里，我们关注人类已知的第一个磁电：磁铁矿。通过基于唯象朗道理论和密度泛函模拟的联合方法，我们发现非中心对称Cc结构中电荷/轨道有序Fe 3 O 4的磁电性是由特殊的轨道有序和现场自旋轨道耦合之间的相互作用驱动的.与现有实验的良好协议证实了我们的理论图片，指向磁铁矿作为一个新的类别的磁电的原型，铁电极化可以通过磁场诱导，调谐或切换。
We report a theoretical study of the non-linear magnetoelectric response of GdFeO$_3$ through an analytical approach combined with a Heisenberg model which is fitted against first-principles calculations. Our theory reproduces the non-linear change of polarization under applied magnetic field reported experimentally such that it allows to analyze the origin of the large responses in the different directions. We show that the non-linear character of the response in these materials originates from the fact that the antiferromagnetic order of Gd atoms changes non-linearly with respect to the appl…
For disordered Heisenberg systems with small single ion anisotropy, two spin glass transitions below the long range ordered phase transition temperature has been predicted theoretically for compositions close to the percolation threshold. Experimental verification of these predictions is still controversial for conventional spin glasses. We show that multiferroic spin glass systems can provide a unique platform for verifying these theoretical predictions via a study of change in magnetoelastic and magnetoelectric couplings, obtained from an analysis of diffraction data, at the spin glass trans…
Origin of the asymmetric exchange bias in BiFeO$_3$-Bi$_2$Fe$_4$O$_9$ nanocomposite
我们表明，从详细的磁测量在2-300 K的BiFeO $_3$ -Bi $_2$ Fe $_4$ O $_9$纳米复合材料提供了一个独特的自旋形态，其中超自旋玻璃（SSG）和稀反铁磁场（DAFF）共存的铁磁性Bi $_2$ Fe $_4$ O $_9$和反铁磁性BiFeO $_3$之间的界面。共存的SSG和DAFF结合形成局部自旋织构，其在自旋冻结温度（$\sim$ 29 K）以下引起路径依赖的交换偏置。交换偏置根据跟踪滞后回线时遵循的协议或路径而变化。在超顺磁性Bi $_2$ Fe $_4$ O $_9$的阻断温度（T $_B$ $\sim$ 60 K）以下观察到交换偏置。常规交换偏置（CEB）随着温度的降低而非单调增加。交换偏置（H $_E$）和介电常数（H $_C$）的大小随温度的降低而增加，并发现在20 K以下是不对称的，这取决于跟踪磁滞回线和偏置场所遵循的路径。在铁磁和反铁磁粒子之间的界面处的局部自旋织构沿着所施加的场的负方向产生不可切换的单向各向异性。这种纹理的影响也表现在“不对称”的滞后回线在2 K的涂抹在更高的温度下的跳跃。界面自旋织构的作用，从而产生的交换偏置的路径依赖性清楚地划定。
