First principles studies of multiferroic materials

Abstract

Multiferroics, materials where spontaneous long-range magnetic and dipolar orders coexist, represent an attractive class of compounds, which combine rich and fascinating fundamental physics with a technologically appealing potential for applications in the general area of spintronics. Ab-initio calculations have significantly contributed to recent progress in this area, by elucidating different mechanisms for multiferroicity and providing essential information on various compounds where these effects are manifestly at play. In particular, here we present examples of density-functional theory investigations for two main classes of materials: a) proper multiferroics (where ferroelectricity is driven by hybridization or purely structural effects), with BiFeO3 as prototype material, and b) improper multiferroics (where ferroelectricity is driven by correlation effects and is strongly linked to electronic degrees of freedom such as spin, charge, or orbital ordering), with rare-earth manganites as prototypes. As for proper multiferroics, first-principles calculations are shown to provide an accurate qualitative and quantitative description of the physics in BiFeO3, ranging from the prediction of large ferroelectric polarization and weak ferromagnetism, over the effect of epitaxial strain, to the identification of possible scenarios for coupling between ferroelectric and magnetic order. For the class of improper multiferroics, ab-initio calculations have shown that, in those cases where spin-ordering breaks inversion symmetry (i.e. in antiferromagnetic E-type HoMnO3), the magnetically-induced ferroelectric polarization can be as large as a few _C/cm2. The presented examples point the way to several possible avenues for future research: On the technological side, first-principles simulations can contribute to a rational materials design, aimed at identifying spintronic materials that exhibit ferromagnetism and ferroelectricity at or above room-temperature. On the fundamental side, ab-initio approaches can be used to explore new mechanisms for ferroelectricity by exploiting electronic correlations that are at play in transition metal oxides, and by suggesting ways to maximize the strength of these effects as well as the corresponding ordering temperatures.