Michael Walden

Phase stability of tetragonal BiFeO3 and Bi(Fe, Cr)O3 heterostructures by density-functional theory

Bio:

Michael Walden is a third-year Ph.D. student in the Functional Ceramics Group and is advised by Geoff Brennecka. Walden began his studies at Mines in 2016 after finishing his B.S. in Ceramic Engineering at Missouri S&T. He has been active in the American Ceramics Society since 2015 and received several recognitions therein, including the Du-Co Ceramics Scholarship. Walden has served on the ACS President’s Council of Student Advisors (PCSA) since 2017 and currently serves as the chair of the PCSA Programming Committee. Michael maintains a strong commitment to working with undergraduates with interest in ceramics, assisting with the Mines Keramos chapter, managing the Furnace Lab, and teaching slip-casting to sophomore students each year. Following his academic studies, Walden hopes to work in the industry for several years researching quantum computing technologies before ultimately returning to academia in a ceramics engineering program.

Michael Walden-Abstract

Phase stability of tetragonal BiFeO3 and Bi(Fe, Cr)O3 heterostructures by density-functional theory

We use the density-functional method to model the structure and electronic properties of the Bi(Fe, Cr)O3 system. Both BiFeO3 (BFO) and BiCrO3 (BCO) are perovskite ferroelectrics which exhibit antiferromagnetic order of the G-type configuration. The presence of multiple ferroic properties in BFCO makes it a multiferroic. The magnetoelectric coupling of BFCO enables the control of magnetic response via the external electric field, and vice versa, which may allow the use of BFCO in applications such as data storage. By incorporating BCO into the BFO structure, issues with phase instability present of rhombohedral BFO may be avoided, while simultaneously adding mixed-species magnetic coupling (Fe-Cr) not present in BFO or BCO. 

Our calculations use the local spin-density approximation for all structural and electronic calculations, accounting for the magnetic coupling of Fe/Cr cations a Hubbard (+U) correction. The BFCO supercells modeled are layered heterostructures containing integer numbers of BiFeO3 (BFO) and BiCrO3 (BCO) tetragonal monolayers stacked in the (001) direction. A tetragonal BFCO phase enhances ferroelectric polarizability relative to the rhombohedral stage and accommodates control of electric or magnetic response with an external field. We investigate the inverse relationship between c/a ratio and in-plane lattice strain in BFO, confirming a solid super-tetragonal phase with ~5% compressive strain. Further, we relate the composition of the BFCO heterostructures to the component of relaxed energy contributed by BFO-BCO interfaces, finding a strong linear correlation between higher interface energy and greater Fe-to-Cr ratio. Our calculations to date are a reliable means of assessing the relative stability of BFCO structures to lattice strain, and of relating changes in magnetic order and interfacial tension to composition.