Ferroelectricity Emerging from Antisite Defects in Complex Oxides

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Nontechnical Description: Ferroelectric materials can store electric charge, and have applications in energy storage, memory devices, and actuators. This project develops a new class of ferroelectric materials that also have magnetic properties. This combination of properties makes them particularly useful for low power memory and computing devices. It has been a challenge to find materials that possess both magnetic and ferroelectric properties at room temperature, but recent work shows a path to this goal by manipulating the composition of a class of oxides that contain iron and a rare earth metal. The change in composition produces a particular type of defect which is the source of the ferroelectricity. This research project investigates ferroelectricity in material compositions that are predicted to show the strongest effects. Broader impacts include student training, activities to promote diversity, public outreach via events and online classes, and technology transfer to industrial groups. Public outreach is offered at The Nano-Observatory, an annual event where attendees visit a nanofabrication and characterization lab at MIT for demonstrations of nanotechnology, and through free online classes that contribute to a micro-minor certification. Outreach to underserved minorities is carried out through a program that brings undergraduates from minority populations to MIT, and through a departmental Diversity, Equity and Inclusion Collaborative.Technical Description: Ferroelectrics are useful and interesting materials, with applications in energy storage, memory and actuators. Furthermore, when combined with ferro- or antiferromagnetic order, the materials are multiferroic and exhibit additional functionality such as voltage-induced changes in magnetization or magnetic field-induced changes in polarization. This research project explores the hypothesis that ferroelectricity can be induced in perovskites with generic formula ABO_3 as a result of antisite defects, i.e. the presence of one cation on a site that should contain a different cation. Recent work has shown that antisite defects in yttrium orthoferrite YFeO_3 with Y:Fe 1 lead to a robust room-temperature ferroelectricity, even though the bulk stoichiometric material is non-ferroelectric, and density functional theory predicts that this mechanism would be even stronger in orthoferrites with smaller rare earth cations such as Lu. This research investigates antisite-defect-mediated ferroelectricity in thin films of rare earth-rich orthoferrites LuFeO_3 and YbFeO_3, and measures the effect of magnetic field on their ferroelectric response. Broader impacts include student training, activities to promote diversity, public outreach via events and online classes, and technology transfer to industrial groups. Public outreach is offered at The Nano-Observatory, an annual event where attendees visit a nanofabrication and characterization lab at MIT for demonstrations of nanotechnology, and through free online classes that contribute to a micro-minor certification. Outreach to underserved minorities is carried out through a program that brings undergraduates from minority populations to MIT, and through a departmental Diversity, Equity and Inclusion Collaborative.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.