The orbital selective external potential (OSEP) method, recently developed by the authors, allows the energy level of a specific atomic orbital to be shifted, thus allowing for the identification of the role of this orbital in the chemical and physical properties of the system. Using OSEP, the origins of ferroelectricity in two classic ferroelectric perovskites, BaTiO 3 and PbTiO 3 , are systematically revisited. The fact that the hybridization between the Ti 3d state and the O 2p state is essential for the formation of ferroelectricity in both BaTiO 3 and PbTiO 3 is reproduced, which validates this method. Particularly, for PbTiO 3 , the Pb 6s lone‐pair electron states can also be tuned using the OSEP method, and its influence on the ferroelectricity is unveiled in detail. Surprisingly, it is found that the electric dipoles formed by lone‐pair lobes contribute only slightly to the overall ferroelectric polarization, whereas the concomitant ionic displacements and lattice distortions are prominently favorable for ferroelectricity. Indeed, it is found that the Pb–O hybridization plays an important role in the ferroelectricity of PbTiO 3 , which makes the polarization of PbTiO 3 significantly larger than that of BaTiO 3 . This study provides a simple yet straightforward way to demonstrate the mechanisms of ferroelectricity in perovskite oxides, which can be applied to the study of ferroelectric mechanism in other relevant materials.

Multiferroics, where two or more ferroic order parameters coexist, is one of the hottest fields in condensed matter physics and materials science. To search multiferroics, currently most researches are focused on frustrated magnets, which usually have complicated magnetic structure and low magnetic ordering temperature. Here, we argue that actually simple interatomic magnetic exchange interaction already contains a driving force for ferroelectricity, thus providing a new microscopic mechanism for the coexistence and strong coupling between ferroelectricity and magnetism. We demonstrate this mechanism by showing that even the simplest antiferromagnetic insulator like MnO, could display a magnetically induced ferroelectricity under a biaxial strain. In addition, we show that such mechanism also exists in the most important single phase multiferroics, i.e. BiFeO3, suggesting that this mechanism is ubiquitous in systems with superexchange interaction.

The control of magnetism via an electric field has attracted substantial attention because of potential applications in magnetoelectronics, spintronics and high-frequency devices. In this study, we demonstrate a new approach to enhance and control the magnetization of multiferroic thin film by an electric stimulus. First, to reduce the strength of the antiferromagnetic superexchange interaction in BiFeO3, we applied strain engineering to stabilize a highly strained phase. Second, the direction of the ferroelectric polarization was controlled by an electric field to enhance the Dzyaloshinskii–Moriya interaction in the highly strained BiFeO3 phase. Because of the magnetoelectric coupling in BiFeO3, a strong correlation between the modulated ferroelectricity and enhanced magnetization was observed. The tunability of this strong correlation by an electric field provides an intriguing route to control ferromagnetism in a single-phase multiferroic. A highly strained thin film that switches magnetic spins using electricity may advance low-power spintronic technology. Multiferroics are magnets that also have permanent electric polarization — a combination that can enable spins to be used in logic or memory devices if the electric and magnetic coupling forces are strong enough. Now, Ying-Hao Chu of National Chiao Tung University in Taiwan and co-workers have boosted the naturally weak magnetism of the multiferroic material bismuth ferrite (BiFeO3) by depositing it as a thin film on neodymium-based crystals. The significant epitaxial strain between the lattice-mismatched interfaces induced spontaneous magnetization, which could be enhanced by applying electric fields. Perfectly aligned nanoscale magnetic patterns were generated when a modest voltage was applied to the highly distorted BiFeO3 thin films via an atomic force microscope tip. Electrical control of the spin degree of freedom has enabled various vital applications in modern electronic devices, ranging from magnetoelectronics and spintronics to high-frequency devices. We demonstrate a new approach to ‘write’ enhanced magnetization in a single-phase multiferroic thin film by the application of an electric field. The induced magnetization can be controlled with nanoscopic precision via the assistance of scanning probe techniques, thus offering high potential for use in multifunctional, low power consumption and green nanoelectronics.

Multiferroics are materials where two or more ferroic orders coexist owing to the interplay between spin, charge, lattice and orbital degrees of freedom. The explosive expansion of multiferroics literature in recent years demonstrates the fast growing interest in this field. In these studies, the first-principles calculation has played a pioneer role in the experiment explanation, mechanism discovery and prediction of novel multiferroics or magnetoelectric materials. In this review, we discuss, by no means comprehensively, the extensive applications and successful achievements of first-principles approach in the study of multiferroicity, magnetoelectric effect and tunnel junctions. In particular, we introduce some our recently developed methods, e.g., the orbital selective external potential method, which prove to be powerful tools in the finding of mechanisms responsible for the intriguing phenomena occurred in multiferroics or magnetoelectric materials. We also summarize first-principles studies on three types of electric control of magnetism, which is the common goal of both spintronics and multiferroics. Our review offers in depth understanding on the origin of ferroelectricity in transition metal oxides, and the coexistence of ferroelectricity and ordered magnetism, and might be helpful to explore novel multiferroic or magnetoelectric materials in the future.

Magnetic ordering could have significant influence on band structures, spin-dependent transport, and other important properties of materials. Its measurement, especially for the case of antiferromagnetic (AFM) ordering, however, is generally difficult to be achieved. Here we demonstrate the feasibility of magnetic ordering detection using a noncontact and nondestructive optical method. Taking the tetragonal BiFeO3 (BFO) as an example and combining density functional theory calculations with tight-binding models, we find that when BFO changes from C1-type to G-type AFM phase, the top of valance band shifts from the Z point to Γ point, which makes the original direct band gap become indirect. This can be explained by Slater-Koster parameters using the Harrison approach. The impact of magnetic ordering on band dispersion dramatically changes the optical properties. For the linear ones, the energy shift of the optical band gap could be as large as 0.4 eV. As for the nonlinear ones, the change is even larger. The second-harmonic generation coefficient d33 of G-AFM becomes more than 13 times smaller than that of C1-AFM case. Finally, we propose a practical way to distinguish the two AFM phases of BFO using the optical method, which is of great importance in next-generation information storage technologies.

Atomic-scale magnetic nanostructures are promising candidates for future information processing devices. Utilizing external electric field to manipulate their magnetic properties is an especially thrilling project. Here, by carefully identifying the different contributions of each atomic orbital to the magnetic anisotropy energy (MAE) of the ferromagnetic metal films, we argue that it is possible to engineer both the MAE and the magnetic response to the electric field of atomic-scale magnetic nanostructures. Taking the iron monolayer as a matrix, we propose several interesting iron nanostructures with dramatically different magnetic properties. Such nanostructures could exhibit a strong magnetoelectric effect. Our work may open new avenues to the artificial design of electrically controlled magnetic devices.

We report density functional calculations of the electronic structure, Fermi surface, phonon spectrum and electron–phonon coupling for the newly discovered superconductor LaO0.5F0.5BiSe2. It is confirmed that there is a strong Fermi surface nesting at (π,π,0), which results in unstable phonon branches. Combining the frozen phonon total energy calculations and an anharmonic oscillator model, we find that the quantum fluctuation prevents the appearance of static long–range order. The calculation shows that LaO0.5F0.5BiSe2 is highly anisotropic, and same as its cousin LaO0.5F0.5BiS2, this compound is also a conventional electron-phonon coupling induced superconductor.

Using relativistic density-functional theory calculations, we investigate magnetocrystalline anisotropy energy (MAE) of the poly(vinylidene fluoride)(PVDF)/Fe/Cu/Ag heterostructure. We find that MAE of this heterostructure can be flexibly manipulated by the ferroelectric polarization of PVDF. In particular, by carefully designing the interface structure, we demonstrate that the ferroelectric polarization reversal can switch the easy axis of the Fe layer from in-plane to out-of-plane, due to the surface/interface magnetoelectric effect. We expect such strategy would be beneficial to electric-field controlled magnetic data storage.

The nature of the stereochemically active lone pair has long been a matter for debate. Here, by application of our recently developed orbital selective external potential (OSEP) method, we have studied the microscopic mechanism of stereochemically active lone pairs in various compounds. The OSEP method allows us to shift the energy level of a specific atomic orbital, therefore is helpful to identify unambiguously the role of this orbital in the chemical and physical properties of the system we are interested in. Our numerical results, with compelling proofs, demonstrate that the on-site mixing of the cation valence s orbital with the nominally empty p orbitals of the same subshell is crucial to the formation of a lone pair, whereas the anion p orbital has only a small effect. Our detailed investigation of Sn and Pb monochalcogenides shows that structures of these systems have significant effects on lone pairs. In return, the formation of lone pairs, which can be controlled by our OSEP method, could result in structural instabilities of Sn and Pb monochalcogenides.

Using first-principles density functional calculations, electronic and optical properties of ferromagnetic semiconductor EuO are investigated. In particular, we have developed a way to obtain the spin-dependent optical response of the magnetic materials, which is helpful to verify the spin-dependent band structure of EuO. Significantly different optical responses from spin-up and spin-down channels are obtained in both linear and nonlinear cases, making it possible to distinguish contributions from different spin channels in the optical absorption spectra if the spin-flip process can be neglected. In addition, the red-shift of the absorption edge from paramagnetic to ferromagnetic ordering is explained by exchange interactions. Using such a method, we have also compared the optical properties of multiferroic EuO which is induced by strong epitaxial strain. Our results show that from tensile to compressive strain, the blue-shift of the leading absorption peaks in the optical spectra, the red-shift of the optical band gap in the spin-up state can be observed, consistent to the energy difference between spin-splitting orbits. The spin-dependent nonlinear optical properties reveal that in the infrared and visible light regions, the contributions to second-harmonic generation (SHG) susceptibilities are mainly from spin-majority channels. In addition, the strain effect is also discussed. With the increase of epitaxial strain, the larger energy shift of the leading absorption peaks and the more remarkable nonlinear optical response can be obtained.

Complex oxide heterointerfaces, which play host to an incredible variety of interface physical phenomena, are of great current interest in introducing new functionalities to systems. Here, coherent super‐tetragonal BiFeO3/LaAlO3 and rhombohedral BiFeO3/LaAlO3 heterointerfaces are investigated by using a combination of high‐angle annular dark‐field (HAADF) imaging and annular bright‐field (ABF) imaging in a spherical aberration (Cs) corrected scanning transmission electron microscope (STEM), and first‐principles calculations. The complicated ferroelectric polarization pinning and relaxation that occurs at both interfaces is revealed with atomic resolution, with a dramatic change in structure of BiFeO3, from cubic to super‐tetragonal‐like. The results enable a detailed explanation to be given of how non‐bulk phase structures are stabilized in thin films of this material.

Using the first-principles calculation based on density functional theory and Monte Carlo simulation within the Heisenberg Hamiltonian, we have investigated the magnetic stability at finite pressure and temperature of CaC and some other selected hypothetical IIA–IVA compounds with the zinc-blende crystal structure. The results from simulated external pressure and temperature test indicate that the magnetic stability is dependent on the variation in atomic number of anion or cation element. Additionally, the origin of sp magnetism is mentioned briefly in this work and the dependence of Curie temperature on the volume is also discussed. First-principles computational charge-injection test show that the magnetization of these sp compounds originates in the spin polarization of the p shell of anions. For CaC, the exchange parameter J1 of the Heisenberg Hamiltonian depends strongly on the lattice constant, while the J2 and J3 are weakly dependent on the lattice constant. Moreover, discussion of volume-conserving deformations for CaC further demonstrates the stability of ferromagnetism and half metallicity for the compounds.

Using density-functional theory calculations, we investigate the magnetic as well as the dynamical properties of tetragonal SrRuO3 (SRO) under the influence of epitaxial strain. It is found that both tensile and compressive strain in the xy-plane can induce an abrupt change in the magnetic moment of the Ru atom. In particular, under an in-plane compressive strain of ~4%, a ferromagnetic to nonmagnetic transition is induced, whereas for a tensile strain larger than 3%, the magnetic moment of Ru drops gradually with increase of the strain, exhibiting a weak ferromagnetic state. We find that these magnetic transitions can be qualitatively explained by the Stoner model. In addition, frozen-phonon calculations at the Γ point and phonon dispersion calculations reveal that structural instabilities can occur under both compressive and tensile strain. These instabilities are very similar to those of the ferroelectric perovskite oxides, even though SRO remains metallic in the range we studied. This might have an influence on the physical properties of oxide supercells having SRO as a constituent.

Manipulation of spin states via purely electric means forms the research branch “all-electric spintronics”. In this paper, we briefly review recent progress relating to the all-electric spintronics, including electric-field control of Rashba spin-orbit coupling, magnetic anisotropy, exchange bias, ferromagnetism, and other forms of magnetoelectric coupling. Special focus is given to surface/interface systems, including semiconductor (oxide) heterostructures, magnetic/nonmagnetic surfaces, semiconductor-metal interfaces, and other nanostructures, which can be good candidates for functional materials for spintronic.

The nature of the stereochemically active lone pair has long been in debate. Here, by application of our recently developed orbital selective external potential (OSEP) method, we have studied the microscopic mechanism of stereochemically active lone pairs in various compounds. The OSEP method allows us to shift the energy level of specific atomic orbital, therefore is helpful to identify unambiguously the role of this orbital to the chemical and physical properties of the system we are interested in. Our numerical results, with compelling proofs, demonstrate that the on-site mixing of cation valence s orbital with the nominally empty p orbitals of the same subshell is crucial to the formation of lone pair, whereas the anion p orbital has only small effect. Our detailed investigation of Sn and Pb monochalcogenides show that structures of these systems have significant effects on lone pairs. In return, the formation of lone pair, which can be controlled by our OSEP method, could result in structural instabilities of Sn and Pb monochalcogenides.

采用广义梯度近似和哈伯德U的第一性原理计算方法,对YzCo1-xFe2O4(Y=Mg,Zn,Mn,x=0.00,0.25,0.50,0.75,1.00)系列体材料的电学与磁学性质进行了研究.研究发现:①八面体占位的CoB2+高/低自旋态随原子掺杂比的不同可能发生转换;②高自旋态CoB2+的磁矩随反转比的不同出现较大波动,其变化范围约为2.54～2.85μB;③对YxCo1-xFe2O4而言,当反转比x在0.75～1.00范围变化时,材料将发生半金属-绝缘体相变.

We discuss the electronic structure, lattice dynamics, and electron-phonon interaction of the newly discovered superconductor LaO0.5F0.5BiS2 using density-functional-based calculations. A strong Fermi surface nesting at k = (pi, pi, 0) suggests a proximity to charge-density-wave instability and leads to imaginary harmonic phonons at this k point associated with in-plane displacements of S atoms. Total energy analysis resolves only a shallow double-well potential well preventing the appearance of static long-range order. Both harmonic and anharmonic contributions to electron-phonon coupling are evaluated and give a total coupling constant lambda similar or equal to 0.85, prompting this material to be a conventional superconductor contrary to structurally similar FeAs materials. DOI: 10.1103/PhysRevB.87.115124

Using first-principles density-functional theory calculations, we systematically investigate the magnetic anisotropy of the multilayer system Cu/(FePt)n/MgO, a promising spintronics structure. Particularly, we have studied the influence of the epitaxial strain, thickness of the ferromagnetic layer, and different interfaces on the magnetic anisotropy energy (MAE) of the system. It is found that the thickness of FePt has slight influence on the MAE, while the increase of the in-plane lattice constant a, or tensile strain, can significantly reduce and even change the sign of the MAE. The calculated density of states shows that the occupation number of the minority spin channel of Fe dx(2)-y(2) orbital decreases with the increase of a, which leads to the reduction of the orbital moment anisotropy of the Fe atom and therefore the decrease of MAE. We also consider the influence of the Cu/FePt and FePt/MgO interfaces on the MAE, and find that both interfaces can reduce the MAE. Especially, the effect of the Cu/FePt interface is more pronounced due to the increased occupation number of the minority spin channel of Fe dz(2) orbital.

We report a systematic study on the structural, electronic, magnetic, and ferroelectric properties of [111]-oriented BiFeO3/BiAlO3 (BFO/BAO) superlattice using density-functional calculations. It is found that the Fe-O-Fe superexchange interactions in BFO/BAO superlattice are greatly suppressed by the inserted BAO layers, with the antiferromagnetic-ferromagnetic transition energy decreasing from around 280 meV per BFO formula unit (five atoms) to 11.6 meV per BFO/BAO formula unit (ten atoms). The tensile strain can further decrease this energy, making the magnetic transition more plausible. In addition, we find that BFO/BAO superlattice preserves the large ferroelectric polarization as well as energy gap of bulk BFO. Therefore, BAO may be a good candidate for constructing the BFO-based superlattice with improved multiferroicity.