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

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