22 Search Results

Thin-film ferroelectrics have been pursued for capacitive and nonvolatile memory devices. They rely on polarizations that are oriented in an out-of-plane direction to facilitate integration and addressability with complementary metal-oxide semiconductor architectures. The internal depolarization field, however, formed by surface charges can suppress the out-of-plane polarization in ultrathin ferroelectric films that could otherwise exhibit lower coercive fields and operate with lower power. Here, we unveil stabilization of a polar longitudinal optical (LO) mode in the n = 2 Ruddlesden–Popper family that produces out-of-plane ferroelectricity, persists under open-circuit boundary conditions, and is distinct from hyperferroelectricity. Our first-principles calculations show the stabilizationmore » of the LO mode is ubiquitous in chalcogenides and halides and relies on anharmonic trilinear mode coupling. We further show that the out-of-plane ferroelectricity can be predicted with a crystallographic tolerance factor, and we use these insights to design a room-temperature multiferroic with strong magnetoelectric coupling suitable for magneto-electric spin-orbit transistors.« less

Hexagonal LuFe₂O₄ is a promising charge order (CO) driven multiferroic material with high charge and spin-ordering temperatures. The coexisting charge and spin orders on Fe³⁺/Fe²⁺ sites result in magnetoelectric behaviors, but the coupling mechanism between the charge and spin orders remains elusive. Here, by tuning external pressure, we reveal three charge-ordered phases with suggested correlation to magnetic orders in LuFe₂O₄: (i) a centrosymmetric incommensurate three-dimensional CO with ferrimagnetism, (ii) a non-centrosymmetric incommensurate quasi-two-dimensional CO with ferrimagnetism, and (iii) a centrosymmetric commensurate CO with antiferromagnetism. Experimental in situ single-crystal X-ray diffraction and X-ray magnetic circular dichroism measurements combined with density functionalmore » theory calculations suggest that the charge density redistribution caused by pressure-induced compression in the frustrated double-layer [Fe₂O₄] cluster is responsible for the correlated spin-charge phase transitions. The pressure-enhanced effective Coulomb interactions among Fe-Fe bonds drive the frustrated (1/3, 1/3) CO to a less frustrated (1/4, 1/4) CO, which induces the ferrimagnetic to antiferromagnetic transition. Our results not only elucidate the coupling mechanism among charge, spin, and lattice degrees of freedom in LuFe₂O₄, but also provide a new way to tune the spin-charge orders in a highly controlled manner.« less

While nature provides a plethora of perovskite materials, only a few exhibit large ferroelectricity and possibly multiferroicity. The majority of perovskite materials have the nonpolar CaTiO₃(CTO) structure, limiting the scope of their applications. Based on the effective Hamiltonian model as well as first-principles calculations, we propose a general thin-film design method to stabilize the functional BiFeO₃(BFO)- type structure, which is a common metastable structure in widespread CTO-type perovskite oxides. It is found that the improper antiferroelectricity in CTO-type perovskite and ferroelectricity in BFO-type perovskite have distinct dependences on mechanical and electric boundary conditions, both of which involve oxygen octahedral rotationmore » and tilt. Thus the above difference can be used to stabilize the highly polar BFO-type structure in many CTO-type perovskite materials.« less

In this work, first-principles methods are employed to understand the existence of magnetic-domain-wall-induced electric polarization observed in rare-earth iron garnets. In contrast with previous beliefs, it is found that the occurrence of such polarization neither requires the local magnetic moments of the rare-earth ions nor noncollinear magnetism. It can rather be understood as originating from a magnetoelectric effect arising from ferromagnetic interactions between octahedral and tetrahedral Fe ions at the domain walls, and the mechanism behind is found to be a symmetric exchange-striction mechanism.

Topological magnetic states are promising for ultradense memory and logic devices. Recent progress in two-dimensional magnets encourages the idea to realize topological states, such as skyrmions and merons, in freestanding monolayers. However, monolayers such as CrI₃ lack Dzyaloshinskii-Moriya interactions (DMIs) and thus do not naturally exhibit skyrmions/merons but rather a ferromagnetic state. Here we propose the fabrication of Cr(I, $$X)_3$$ Janus monolayers, in which the Cr atoms are covalently bonded to the underlying I ions and top-layer Br or Cl atoms. By performing first-principles calculations and Monte Carlo simulations, we identify strong enough DMIs, which leads to not only helicalmore » cycloid phases, but also to topologically nontrivial states, such as the intrinsic domain wall skyrmions in Cr(I, Br)₃ and the magnetic-field-induced bimerons in Cr(I, Cl)₃. Finally, microscopic origins of such spin textures are revealed as well.« less

Quantum spin liquids (QSLs) form an extremely unusual magnetic state in which the spins are highly correlated and fluctuate coherently down to the lowest temperatures, but without symmetry breaking and without the formation of any static long-range-ordered magnetism. Such intriguing phenomena are not only of great fundamental relevance in themselves, but also hold promise for quantum computing and quantum information. Further, among different types of QSLs, the exactly solvable Kitaev model is attracting much attention, with most proposed candidate materials, e.g., RuCl₃ and Na₂ IrO₃, having an effective $$\textit{S}$$ = 1/2 spin value. Here, via extensive first-principles-based simulations, we reportmore » the investigation of the Kitaev physics and possible Kitaev QSL state in epitaxially strained Cr-based monolayers, such as CrSiTe₃, that rather possess a $$\textit{S}$$ = 3/2 spin value. Our study thus extends the playground of Kitaev physics and QSLs to 3$$\textit{d}$$ transition metal compounds.« less

The electron reflectivity from NiO thin films grown on Ag(001) has been systematically studied as a function of film thickness and electron energy. A strong electron quantum interference effect was observed from the NiO film, which is used to derive the unoccupied band dispersion above the Fermi surface along the Γ-X direction using the phase accumulation model. The experimental bands agree well with first-principles calculations. A weaker electron quantum interference effect was also observed from the CoO film.

Half-metallic ferromagnets (HMFs) that possess intriguing physical properties with completely spin-polarized current are key candidates for high-efficiency spintronic devices. However, HMFs that could simultaneously have high Curie temperature (T_c), wide half-metallic gap (Δ_HM), and large bulk magnetocrystalline anisotropy energy (MAE) are very rare, which significantly restrict their room-temperature (RT) applications. In this article, through materials screening in layered halide double perovskites (LHDPs), we have theoretically identified that Cs₄FePb₂Cl₁₂, which has good crystallographic, dynamic and thermal stabilities, possesses an intrinsic half-metallic ground-state with a highT_c ~ 450 K. Interestingly, the long-range ferromagnetic ordering in bulk Cs₄FePb₂Cl₁₂is contributed by the strong super-superexchangemore » interactions between the neighboring Fedorbitals mediated by different anionic Clporbitals. The highT_cof layered Cs₄FePb₂Cl₁₂can be well maintained even in the monolayer limitation, i.e.,T_c ~ 370 K for Cs₄FePb₂Cl₁₂monolayer, which is critical for nanoscale device applications. Moreover, both bulk and monolayer Cs₄FePb₂Cl₁₂can exhibit wide Δ_HM ~ 0.55 eV and large MAE >320 μeV/Fe, comparable to that of the best HMFs reported in the literature. Our findings can significantly extend the potentials of LHDPs for high-temperature spintronic applications.« less

Bulk LiOsO₃ was experimentally identified as a “ferroelectric” metal where polar distortions coexist with metallicity [Shi et al., Nat. Mater. 12, 1024 (2013)]. It is generally believed that polar displacements in a ferroelectric metal cannot be switched by an external electric field. Here, via comprehensive density functional theory calculations, we demonstrate that a two-unit cell-thick LiOsO₃ thin film exhibits a ferroelectric ground state having an out-of-plane electric dipole moment that can be switched by an external electric field. Moreover, its dipole moment-versus-electric field hysteresis loop is asymmetric because only surface Li ions’ displacements are reversed by the external electric fieldmore » whereas the field-induced force on inner Li atoms is nearly fully screened by itinerant electrons. Here, as a relevant by-product of our study, we also extend the concept of “Born effective charge” to finite metallic systems, and show its usefulness to rationalize the observed effects.« less

Controlling the direction of the magnetization by an electric field in multiferroics that are both ferroelectric and strongly ferromagnetic will open the door to the design of the next generation of spintronics and memory devices. Using first-principles simulations, we report that the discovery that the PbTiO₃/LaTiO₃ (PTO/LTO) superlattice possesses such highly desired control, as evidenced by the electric-field-induced rotation of 90° and even a possible full reversal of its magnetization in some cases. Moreover, such systems also exhibit Jahn-Teller distortions, as well as orbital orderings, that are switchable by the electric field, therefore making PTO/LTO of importance for the tuningmore » of electronic properties too. Here, the origin for such striking electric-field controls of magnetization, Jahn-Teller deformations, and orbital orderings resides in the existence of three different types of energetic coupling: one coupling polarization with antiphase and in-phase oxygen octahedral tiltings, a second one coupling polarization with antiphase oxygen octahedra tilting and Jahn-Teller distortions, and finally a biquadratic coupling between antiphase oxygen octahedral tilting and magnetization.« less