The technological appeal of multiferroics is the ability to control magnetism with electric field. For devices to be useful, such control must be achieved at room temperature. The only single-phase multiferroic material exhibiting unambiguous magnetoelectric coupling at room temperature is BiFeO3 (refs 4 and 5). Its weak ferromagnetism arises from the canting of the antiferromagnetically aligned spins by the Dzyaloshinskii-Moriya (DM) interaction. Prior theory considered the symmetry of the thermodynamic ground state and concluded that direct 180-degree switching of the DM vector by the ferroelectric polarization was forbidden. Instead, we examined the kinetics of the switching process, something not considered previously in theoretical work. Here we show a deterministic reversal of the DM vector and canted moment using an electric field at room temperature. First-principles calculations reveal that the switching kinetics favours a two-step switching process. In each step the DM vector and polarization are coupled and 180-degree deterministic switching of magnetization hence becomes possible, in agreement with experimental observation. We exploit this switching to demonstrate energy-efficient control of a spin-valve device at room temperature. The energy per unit area required is approximately an order of magnitude less than that needed for spin-transfer torque switching. Given that the DM interaction is fundamental to single-phase multiferroics and magnetoelectrics, our results suggest ways to engineer magnetoelectric switching and tailor technologically pertinent functionality for nanometre-scale, low-energy-consumption, non-volatile magnetoelectronics.

After decades of searching for robust nanoscale ferroelectricity that could enable integration into the next generation memory and logic devices, hafnia-based thin films have appeared as the ultimate candidate because their ferroelectric (FE) polarization becomes more robust as the size is reduced. This exposes a new kind of ferroelectricity, whose mechanism still needs to be understood. Towards this end, thin films with increased crystal quality are needed. We report the epitaxial growth of Hf0.5Zr0.5O2 (HZO) thin films on (001)-oriented La0.7Sr0.3MnO3/SrTiO3 (STO) substrates. The films, which are under epitaxial compressive strain and are predominantly (111)-oriented, display large FE polarization values up to 34 μC/cm 2 and do not need wake-up cycling. Structural characterization reveals a rhombohedral phase, different from the commonly reported polar orthorhombic phase. This unexpected finding allows us to propose a compelling model for the formation of the FE phase. In addition, these results point towards nanoparticles of simple oxides as a vastly unexplored class of nanoscale ferroelectrics.

The wide spectrum of exotic properties exhibited by transition-metal oxides stems from the complex competition between several quantum interactions. The capacity to select the emergence of specific phases at will is nowadays extensively recognized as key for the design of diverse new devices with tailored functionalities. In this context, interface engineering in complex oxide heterostructures has developed into a flourishing field, enabling not only further tuning of the exceptional properties of these materials, but also giving access to hidden phases and emergent physical phenomena. Here we demonstrate how interfacial interactions can induce a complex magnetic structure in a non-magnetic material. We specifically show that exchange bias can unexpectedly emerge in heterostructures consisting of paramagnetic LaNiO3 (LNO) and ferromagnetic LaMnO3 (LMO). The observation of exchange bias in (111)-oriented LNO-LMO superlattices, manifested as a shift of the magnetization-field loop, not only implies the development of interface-induced magnetism in the paramagnetic LNO layers, but also provides us with a very subtle tool for probing the interfacial coupling between the LNO and LMO layers. First-principles calculations indicate that this interfacial interaction may give rise to an unusual spin order, resembling a spin-density wave, within the LNO layers.

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We describe a method for computing the response of an insulator to a static, homogeneous electric field. It consists of iteratively minimizing an electric enthalpy functional expressed in terms of occupied Bloch-like states on a uniform grid of k points. The functional has equivalent local minima below a critical field E(c) that depends inversely on the density of k points; the disappearance of the minima at E(c) signals the onset of Zener breakdown. We illustrate the procedure by computing the piezoelectric and nonlinear dielectric susceptibility tensors of III-V semiconductors.

Hybrid exchange-correlation functional for accurate prediction of the electronic anD structural properties of ferroelectric oxides. / D. I. Bilc; R. Orlando; R. Shaltaf; G. M. Rignanese; J. Íñiguez; Ph. Ghosez. -In: PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS. -ISSN 1098-ISSN -0121. -77:16(2008, pp. 165107-1-165107-13. Original Citation:Hybrid exchange-correlation functional for accurate prediction of the electronic anD structural properties of ferroelectric oxides. Published version:DOI:10.1103/PhysRevB.77.165107 Terms of use:Open Access (Article begins on next page) Anyone can freely access the full text of works made available as "Open Access". Works made available under a Creative Commons license can be used according to the terms and conditions of said license. Use of all other works requires consent of the right holder (author or publisher) if not exempted from copyright protection by the applicable law. Using a linear combination of atomic orbitals approach, we report a systematic comparison of various density functional theory ͑DFT͒ and hybrid exchange-correlation functionals for the prediction of the electronic and structural properties of prototypical ferroelectric oxides. It is found that none of the available functionals is able to provide, at the same time, accurate electronic and structural properties of the cubic and tetragonal phases of BaTiO 3 and PbTiO 3 . Some, although not all, usual DFT functionals predict the structure with acceptable accuracy, but always underestimate the electronic band gaps. Conversely, common hybrid functionals yield an improved description of the band gaps, but overestimate the volume and atomic distortions associated with ferroelectricity, giving rise to an unacceptably large c / a ratio for the tetragonal phases of both compounds. This supertetragonality is found to be induced mainly by the exchange energy corresponding to the generalized gradient approximation ͑GGA͒ and, to a lesser extent, by the exact exchange term of the hybrid functional. We thus propose an alternative functional that mixes exact exchange with the recently proposed GGA of Wu and Cohen ͓Phys. Rev. B 73, 235116 ͑2006͔͒ which, for solids, improves over the treatment of exchange of the most usual GGA's. The new functional renders an accurate description of both the structural and electronic properties of typical ferroelectric oxides. Availability: This is the author's manuscript

*These authors contributed equally to this work.The stability of the spontaneous electrical polarisation characteristic of ferroelectrics is fundamental to a multitude of their current applications, ranging from the simple electrical cigarette lighter to non-volatile random access memories 1 . Yet, the technological potential of these materials is far from being exhausted as research on nanoscale ferroelectrics reveals their properties to be profoundly different from those in bulk, giving rise to fascinating new phenomena with exciting prospects for future 2 devices 2-4 . As ferroelectrics become thinner, maintaining a stable polarisation becomes increasingly challenging. On the other hand, intentionally destabilising this polarisation can cause the effective electrical permittivity of a ferroelectric to become negative 5 , enabling it to behave as a negative capacitance when integrated in a heterostructure.Negative capacitance has been garnering increasing attention following the realisation that it could be exploited to overcome fundamental limitations on the power consumption of field effect transistors 6 . Experimentally, however, demonstrations of this phenomenon are still contentious 7 . The prevalent interpretations based on homogeneous polarisation models are difficult to reconcile with the expected strong tendency for domain formation 8,9 , while the effect of domains on negative capacitance has received surprisingly little attention 5,10-12 . Here we report the observation of negative capacitance in a model system of multidomain ferroelectric-dielectric superlattices across a wide range of temperatures, in both the ferroelectric and paraelectric phases. Using a phenomenological model we show that domain-wall motion not only gives rise to negative permittivity but can also enhance, rather than limit, its temperature range. Furthermore, our first-principles-based atomistic simulations provide detailed microscopic insight on the origin of this phenomenon, identifying the dominant contribution of near-interface layers and paving the way for its future exploitation.Negative capacitance (NC) has its origins in the imperfect screening of the spontaneous polarisation 5,10,13,14 . Imperfect screening is intrinsic to any semiconductor-ferroelectric or even metal-ferroelectric interfaces because of their finite effective screening lengths 15,16 .Alternatively, it can be engineered in a controlled manner by deliberately inserting a dielectric layer of relative permittivity ߳ ௗ between the ferroelectric and the electrodes as suggested by Salahuddin and Data 6 and shown in Fig. 1a. The physical separation of the 3 ferroelectric bound charge from the metallic screening charges creates a depolarizing field inside the ferroelectric, destabilizing the polarisation and lowering the ferroelectric transition temperature. The effect of the dielectric layer can be understood by considering the free energy of the bilayer capacitor with the usual assumption of a uniform polarisation ܲ (see Methods). Below the bulk transitio...

We used first-principles methods to perform a systematic search for potentially-stable phases of multiferroic BiFeO3. We considered a simulation cell compatible with the atomic distortions that are most common among perovskite oxides, and found a large number of local minima of the energy within 100 meV/f.u. of the ferroelectric ground state. We discuss the variety of low-symmetry structures discovered, as well as the implications of these findings as regards current experimental (e.g., on thin films displaying {\em super-tetragonal} phases) and theoretical (on models for BiFeO3's structural phase transitions) work on this compound.Comment: 14 pages, 9 figures, accepted in PRB (contains small changes in the text with respect to the first version