Seminar Speaker: John Heron, University of Michigan

Entropy-Stabilized Oxides: Manipulating Local Structure and Charge for Macroscopic Properties

Abstract

Entropy-stabilized oxides demonstrate a new and unprecedented degree of chemical control in materials. This broadens the compositional space of crystalline oxides and presents opportunities to explore the interplay between local chemical and structural disorders and physical properties. Here, I will discuss the control of stereochemically-driven structural disorder in single crystalline, rocksalt, (MgCoNiCuZn)O-type entropy-stabilized oxides by incorporating Co, Cu, and Mg cations. Point defect formation energies from first principles calculations predict the local strain is the driver for defect formation and is largely dictated by Cu incorporation and rare local configurations. Electron transport measurements reveal a transition from Schottky to hopping conduction. The hopping conductivity is tunable via the Mg concentration (inversely correlated to Cu). Pulse measurements reveal a low resistance state with a short retention time that can be harnessed for memristor function. As theory predicts filled defect levels deep within the bandgap, we interpret the high resistance state as Schottky emission via these deep-level states and the low resistance state at high applied voltage as electron hopping conduction through empty deep-level states via the excitation of electrons from the defect levels. Our findings demonstrate a novel interplay between local structure and macroscopic charge transport in entropy-stabilized oxides for tunable electronic responses and transformative computing applications.

Biography

John Heron is an Associate Professor of Materials Science and Engineering at the University of Michigan, leading the ferroelectronics group. The group focuses on understanding and designing ferroic properties (such as ferroelectricity and ferromagnetism) typically with an application focus toward next-generation energy-efficient microelectronic devices. He is best known for his work on the pulsed laser deposition thin film ferroic and multiferroic oxides and the characterization of magnetoelectric and multiferroic materials. Particular interest resides in interface, spin, structure, and charge effects in layered structures with ferroic materials, such as (anti)ferromagnets, (anti)ferroelectrics, and multiferroics. Recent interest has also emerged in novel high entropy oxide materials where composition leads to local disorder and highly tunable properties.