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 fascinatingfundamental physics with a technologically appealing potential for applications in thegeneral area of spintronics. Ab initio calculations have significantly contributed to recentprogress in this area, by elucidating different mechanisms for multiferroicity andproviding essential information on various compounds where these effects aremanifestly at play. In particular, here we present examples of density-functionaltheory investigations for two main classes of materials: (a) multiferroics whereferroelectricity is driven by hybridization or purely structural effects, withBiFeO3 as the prototype material, and (b) multiferroics where ferroelectricity is driven bycorrelation effects and is strongly linked to electronic degrees of freedom such asspin-, charge-, or orbital-ordering, with rare-earth manganites as prototypes.As for the first class of multiferroics, first principles calculations are shown toprovide an accurate qualitative and quantitative description of the physics inBiFeO3, ranging from the prediction of large ferroelectric polarization and weak ferromagnetism,over the effect of epitaxial strain, to the identification of possible scenarios forcoupling between ferroelectric and magnetic order. For the second class ofmultiferroics, ab initio calculations have shown that, in those cases wherespin-ordering breaks inversion symmetry (e.g. in antiferromagnetic E-typeHoMnO3), the magnetically induced ferroelectric polarization can be as large as a fewµC cm−2. The examples presented point the way to several possible avenues for future research: onthe technological side, first principles simulations can contribute to a rational materialsdesign, aimed at identifying spintronic materials that exhibit ferromagnetism andferroelectricity at or above room temperature. On the fundamental side, ab initioapproaches can be used to explore new mechanisms for ferroelectricity by exploitingelectronic correlations that are at play in transition metal oxides, and by suggesting waysto maximize the strength of these effects as well as the corresponding orderingtemperatures.