This award supports theoretical and computational research that builds on recent advances in the fields of magnetoelectric multiferroics and thin film ferroelectrics, to design and create novel magnetoelectric multiferroics and understand their behavior in thin film form. The PI seeks to help elucidate the interplay between chemistry, ordering and strain in determining the electronic and magnetic properties of thin film magnetoelectric multiferroics. There are two sub-projects: (i) The design of novel thin film multiferroics by exploiting compositional inversion symmetrybreaking to engineer ferroelectricity into otherwise non-ferroelectric magnetic materials. This strategy enables the PI to circumvent the contraindication between the conventional mechanism for ferroelectricity and the existence of magnetism. (ii) The determination of the effects of strain on the ferroelectric behavior of multiferroics. The PI aims to identify or design multiferroics in which either, strain can be exploited to enhance the polarization, or excessive strain dependence of the polarization can be avoided. The research will also test the appropriateness of the various beyond-LDA approaches for describing strongly-correlated magnetic insulators. This award also supports the extension of a senior undergraduate / beginning graduate level Access Grid-based cyber course on Magnetism and magnetic materials developed under the previous Information Technology Research award. In an associated outreach effort, the PI plans to develop a hands-on classroom kit for teachers on Materials Chemistry for 5th Grade to address topics in the California Science Standards. The cyberinfrastructure aspects of this award include: advanced computational research, cyber course development, and fundamental materials research that contributes to the foundations of future cyberinfrastructure. NON-TECHNICAL SUMMARY: This award supports theoretical and computational research on magnetoelectric multiferroics. These are materials that simultaneously display magnetism and ferroelectricity, the electrical analog of magnetism, in the same phase. The PI aims to use theory and computation to design and create novel magnetoelectric multiferroic materials and understand their behavior in thin film form. The research will help to elucidate how the interplay between chemistry, ordering and strain determines the electronic and magnetic properties of thin film magnetoelectric multiferroics. Few multiferroic materials are currently known. They have potential technological applications in information technology and the next generation of cyberinfrastructure. In addition to possessing the combined functionalities of their parent ferromagnets and ferroelectrics, coupling between the two phenomena opens new device paradigms, in which electronic behavior is controlled by a magnetic field and magnetic behavior is controlled by an electric field. A strong collaboration with synthetic materials scientists and device physicists, aims to ensure synergy between this theoretical effort, the experimental growth and characterization of new materials, and the incorporation of newly discovered materials into devices. This award also supports the extension of a senior undergraduate / beginning graduate level Access Grid-based cyber course on Magnetism and magnetic materials developed under the previous Information Technology Research award. In an associated outreach effort, the PI plans to develop a hands-on classroom kit for teachers on Materials Chemistry for 5th Grade to address topics in the California Science Standards. The cyberinfrastructure aspects of this award include: advanced computational research, cyber course development, and fundamental materials research that contributes to the foundations of future cyberinfrastructure.
Recent reports on epitaxial BiFeO3 films show that the crystal structure changes from nearly rhombohedral ('R like') to nearly tetragonal ('T like') at strains exceeding approximate to-4.5%, with the T-like structure being characterized by a highly enhanced c/a ratio. While both the R-like and the T-like phases are monoclinic, our detailed x-ray diffraction results reveal a symmetry change from M-A and M-C type, respectively, at this R-like-to-T-like transition. Therefore, the ferroelectric polarization is confined to different (pseudocubic) planes in the two phases. By applying additional strain or by modifying the unit-cell volume of the film by substituting Ba for Bi, the monoclinic distortion in the T-like MC phase is reduced, i.e., the system approaches a true tetragonal symmetry. Therefore, in going from bulk to highly strained films, a phase sequence of rhombohedral (R)-to-monoclinic (R-like M-A)-to-monoclinic (T-like M-C)-to-tetragonal (T) is observed. This sequence is otherwise seen only near morphotropic phase boundaries in lead-based solid-solution perovskites (i.e., near a compositionally induced phase instability), where it can be controlled by electric field, temperature, or composition. Our results now show that this evolution can occur in a lead-free, stoichiometric material and can be induced by stress alone. Our first manuscript on this topic was published in Physical Review B and presented in an invited talk at an APS meeting. 16 additional publications resulted from this research. Our findings have shed light on the fundamental mechanisms driving ferroelectricity, and have suggested new routes to ferroelectricity that are compatible with the simultaneous existence of magnetism. We have introduced the term 'toroidization' to describe the toroidal moment per unit volume in bulk systems. Our analysis of the toroidization in bulk periodic systems, including a system for calculating it within a density functional formalism, has provided a framework for further developments in understanding ferrotoroidic ordering and magnetoelectric response. We have used the concepts of toroidization to design new linear magnetoelectric materials from first-principles. Our review of thin film multiferroics provides an introduction to the large number of researchers moving into the field.
