Ab initio Evidence for Giant Magnetoelectric Responses Driven by Structural Softness

We show that inducing structural softness in regular magnetoelectric (ME) multiferroics -- i.e., tuning the materials to make their structure strongly reactive to applied fields -- makes it possible to obtain very large ME effects. We present illustrative first-principles results for BiFeO3 thin films.Comment: 4 pages with 3 figures embedded. More information at http://www.icmab.es/dmmis/leem/jorg

Magnetoelectric response of multiferroic BiFeO3 and related materials

We present a first-principles scheme for computing the magnetoelectric response of multiferroics. We apply our method to BiFeO3 (BFO) and related compounds in which Fe is substituted by other magnetic species. We show that under certain relevant conditions -- i.e., in absence of incommensurate spin modulation, as in BFO thin films and some BFO-based solid solutions -- these materials display a large linear magnetoelectric response. Our calculations reveal the atomistic origin of the coupling and allow us to identify the most promising strategies to enhance it.Comment: 4 pages with 1 figure embedded. More information at http://www.icmab.es/dmmis/leem/jorg

First-principles predictions of low-energy phases of multiferroic BiFeO3

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

Tuning the atomic and domain structure of epitaxial films of multiferroic BiFeO3

Recent works have shown that the domain walls of room-temperature multiferroic BiFeO3 (BFO) thin films can display distinct and promising functionalities. It is thus important to understand the mechanisms underlying domain formation in these films. High-resolution x-ray diffraction and piezo-force microscopy, combined with first-principles simulations, have allowed us to characterize both the atomic and domain structure of BFO films grown under compressive strain on (001)-SrTiO3, as a function of thickness. We derive a twining model that describes the experimental observations and explains why the 71o domain walls are the ones commonly observed in these films. This understanding provides us with a new degree of freedom to control the structure and, thus, the properties of BiFeO3 thin films.Comment: RevTeX; 4 two-column pages; 4 color figures. Figure 2b does not seem to display well. A proper version can be found in the source fil

Negative capacitance in multidomain ferroelectric superlattices

The stability of spontaneous electrical polarization in ferroelectrics is fundamental to many of their current applications, which range from the simple electric cigarette lighter to non-volatile random access memories1. Research on nanoscale ferroelectrics reveals that their behaviour is profoundly different from that in bulk ferroelectrics, which could lead to new phenomena with potential for future devices2, 3, 4. As ferroelectrics become thinner, maintaining a stable polarization becomes increasingly challenging. On the other hand, intentionally destabilizing this polarization can cause the effective electric permittivity of a ferroelectric to become negative5, enabling it to behave as a negative capacitance when integrated in a heterostructure. Negative capacitance has been proposed as a way of overcoming fundamental limitations on the power consumption of field-effect transistors6. However, experimental demonstrations of this phenomenon remain contentious7. The prevalent interpretations based on homogeneous polarization models are difficult to reconcile with the expected strong tendency for domain formation8, 9, but the effect of domains on negative capacitance has received little attention5, 10, 11, 12. Here we report 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. Our first-principles-based atomistic simulations provide detailed microscopic insight into the origin of this phenomenon, identifying the dominant contribution of near-interface layers and paving the way for its future exploitation

Stabilization of weak ferromagnetism by strong magnetic response to epitaxial strain in multiferroic BiFeO3

Multiferroic BiFeO3 exhibits excellent magnetoelectric coupling critical for magnetic information processing with minimal power consumption. However, the degenerate nature of the easy spin axis in the (111) plane presents roadblocks for real world applications. Here, we explore the stabilization and switchability of the weak ferromagnetic moments under applied epitaxial strain using a combination of first-principles calculations and group-theoretic analyses. We demonstrate that the antiferromagnetic moment vector can be stabilized along unique crystallographic directions ([110] and [-110]) under compressive and tensile strains. A direct coupling between the anisotropic antiferrodistortive rotations and the Dzyaloshinskii-Moria interactions drives the stabilization of the weak ferromagnetism. Furthermore, energetically competing C- and G-type magnetic orderings are observed at high compressive strains, suggesting that it may be possible to switch the weak ferromagnetism "on" and "off" under the application of strain. These findings emphasize the importance of strain and antiferrodistortive rotations as routes to enhancing induced weak ferromagnetism in multiferroic oxides.ope

Second-principles method for materials simulations including electron and lattice degrees of freedom

We present a first-principles-based (second-principles) scheme that permits large-scale materials simulations including both atomic and electronic degrees of freedom on the same footing. The method is based on a predictive quantum-mechanical theory - e.g., density functional theory - and its accuracy can be systematically improved at a very modest computational cost. Our approach is based on dividing the electron density of the system into a reference part - typically corresponding to the system's neutral, geometry-dependent ground state - and a deformation part - defined as the difference between the actual and reference densities. We then take advantage of the fact that the bulk part of the system's energy depends on the reference density alone; this part can be efficiently and accurately described by a force field, thus avoiding explicit consideration of the electrons. Then, the effects associated to the difference density can be treated perturbatively with good precision by working in a suitably chosen Wannier function basis. Further, the electronic model can be restricted to the bands of interest. All these features combined yield a very flexible and computationally very efficient scheme. Here we present the basic formulation of this approach, as well as a practical strategy to compute model parameters for realistic materials. We illustrate the accuracy and scope of the proposed method with two case studies, namely, the relative stability of various spin arrangements in NiO (featuring complex magnetic interactions in a strongly-correlated oxide) and the formation of a two-dimensional electron gas at the interface between band insulators LaAlO3 and SrTiO3 (featuring subtle electron-lattice couplings and screening effects). We conclude by discussing ways to overcome the limitations of the present approach (most notably, the assumption of a fixed bonding topology), as well as its many envisioned possibilities and future extensions.We thank M. Moreno and J. A. Aramburu for use-ful discussions. P.G.F. and J.J. acknowledge financial sup-port from the Spanish Ministry of Economy and Competitiveness through the MINECO Grant No. FIS2012-37549-C05-04. P.G.F. also acknowledges funding from the Ram ́on y Cajal FellowshipRYC-2013-12515. J.I. is funded by MINECO-Spain Grant MAT2013-40581-P and Fonds National de la Recherche (FNR) Luxembourg Grant FNR/P12/4853155/Kreise