Applied Physics Seminar:Tailoring magnetic and dielectric properties through entropic design

Well-ordered materials form the foundation of solid state physics. Much of modern
research focuses on understanding and designing novel materials through the atomic precision of advanced synthesis, computational techniques, and electron microscopy. However, less discussed is the role of disorder in materials despite its ability to enhance or completely alter the physical properties (an example being the giant piezoelectricity in relaxor ferroelectrics that emerges from the chemical disorder that creates competing local antiferroelectric and ferroelectric interactions). Here, I will present our work that investigates the correlative disorder (structural and chemical) and unique chemistry in materials stabilized by entropy. Entropy-stabilized materials are a new and exciting class of materials as they are stabilized by the configurational entropy of the constituents, rather than the enthalpy of formation of the compound. Particularly for oxides, pioneering work has demonstrated the
emergence of a homogenous single phase (MgCoNiCuZn)O from a mix of five binary oxide constituents (MgO, CoO, NiO, CuO, and ZnO) at a critical entropy. These, so-called, entropy-stabilized oxides (ESOs) demonstrate a new and unprecedented degree of chemical control in materials. The technique can be used to incorporate typically immiscible concentrations of cationic species in an atypical coordination, such as Cu 2+ and Zn 2+ into octahedral coordination and increase the solubility of elements, which opens a broad compositional space with subsequent local chemical and structural disorder resulting from different atomic sizes and the
preferred coordination of the constituents. Particularly in oxides, where the physical behavior is strongly correlated to stereochemistry and electronic structure, entropic stabilization creates a unique platform to tailor the interplay between structural and chemical disorder to realize unprecedented functionalities. We find that the canonical entropy-stabilized oxide (MgCoNiCuZn)O has long-range antiferromagnetic order that can be rather independently tuned with appropriate incorporation or reduction of a cation to either induce or reclaim a large degree of frustration in the magnetic lattice of the material. This effect can then be engineered to enhance the strength of the magnetic exchange field by a factor of 10x in ferromagnetic/antiferromagnetic heterostructures, when compared to a rocksalt antiferromagnetic oxide, such as CoO. Further, we find that the local disorder gives rise to randomly oriented electric dipoles which gives rise to a large dielectric constant and suggests a pathway to tune correlative structural disorder with applied electric field.I will conclude with an outlook and prospects for this nascent field.