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Breaking Inversion Symmetry in Magnets: Understand via THeory (BISMUTH)
Start date: May 1, 2008, End date: Apr 30, 2012 PROJECT  FINISHED 

Multiferroics (i.e. materials where ferroelectricity and magnetism coexist) are presently drawing enormous interests, due to their technologically-relevant multifunctional character and to the astoundingly rich playground for fundamental condensed-matter physics they constitute. Here, we put forward several concepts on how to break inversion symmetry and achieve sizable ferroelectricity in collinear magnets; our approach is corroborated via first-principles calculations as tools to quantitatively estimate relevant ferroelectric and magnetic properties as well as to reveal ab-initio the main mechanisms behind the dipolar and magnetic orders. In closer detail, we focus on the interplay between ferroelectricity and electronic degrees of freedom in magnets, i.e. on those cases where spin- or orbital- or charge-ordering can be the driving force for a spontaneous polarization to develop. Antiferromagnetism will be considered as a primary mechanism for lifting inversion symmetry; however, the effects of charge disproportionation and orbital ordering will also be studied by examining a wide class of materials, including ortho-manganites with E-type spin-arrangement, non-E-type antiferromagnets, nickelates, etc. Finally, as an example of materials-design accessible to our ab-initio approach, we use “chemistry” to break inversion symmetry by artificially constructing an oxide superlattice and propose a way to switch, via an electric field, from antiferromagnetism to ferrimagnetism. To our knowledge, the link between electronic degrees of freedom and ferroelectricity in collinear magnets is an almost totally unexplored field by ab-initio methods; indeed, its clear understanding and optimization would lead to a scientific breakthrough in the multiferroics area. Technologically, it would pave the way to materials design of magnetic ferroelectrics with properties persisting above room temperature and, therefore, to a novel generation of electrically-controlled spintronic devices
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