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Abstract Reducing the switching energy of ferroelectric thin films remains an important goal in the pursuit of ultralow-power ferroelectric memory and logic devices. Here, we elucidate the fundamental role of lattice dynamics in ferroelectric switching by studying both freestanding bismuth ferrite (BiFeO 3 ) membranes and films clamped to a substrate. We observe a distinct evolution of the ferroelectric domain pattern, from striped, 71° ferroelastic domains (spacing of ~100 nm) in clamped BiFeO 3 films, to large (10’s of micrometers) 180° domains in freestanding films. By removing the constraints imposed by mechanical clamping from the substrate, we can realize a ~40% reduction of the switching voltage and a consequent ~60% improvement in the switching speed. Our findings highlight the importance of a dynamic clamping process occurring during switching, which impacts strain, ferroelectric, and ferrodistortive order parameters and plays a critical role in setting the energetics and dynamics of ferroelectric switching. 

Over the last 30 years, the study of ferroelectric oxides has been revolutionized by the implementation of epitaxial‐thin‐film‐based studies, which have driven many advances in the understanding of ferroelectric physics and the realization of novel polar structures and functionalities. New questions have motivated the development of advanced synthesis, characterization, and simulations of epitaxial thin films and, in turn, have provided new insights and applications across the micro‐, meso‐, and macroscopic length scales. This review traces the evolution of ferroelectric thin‐film research through the early days developing understanding of the roles of size and strain on ferroelectrics to the present day, where such understanding is used to create complex hierarchical domain structures, novel polar topologies, and controlled chemical and defect profiles. The extension of epitaxial techniques, coupled with advances in high‐throughput simulations, now stands to accelerate the discovery and study of new ferroelectric materials. Coming hand‐in‐hand with these new materials is new understanding and control of ferroelectric functionalities. Today, researchers are actively working to apply these lessons in a number of applications, including novel memory and logic architectures, as well as a host of energy conversion devices.

 

Epitaxial strain has been shown to produce dramatic changes to the orbital structure in transition metal perovskite oxides and, in turn, the rate of oxygen electrocatalysis therein. Here, epitaxial strain is used to investigate the relationship between surface electronic structure and oxygen electrocatalysis in prototypical fuel cell cathode systems. Combining high‐temperature electrical‐conductivity‐relaxation studies and synchrotron‐based X‐ray absorption spectroscopy studies of La_0.5Sr_0.5CoO₃and La_0.8Sr_0.2Co_0.2Fe_0.8O₃thin films under varying degrees of epitaxial strain reveals a strong correlation between orbital structure and catalysis rates. In both systems, films under biaxial tensile strain simultaneously exhibit the fastest reaction kinetics and lowest electron occupation in thed_z²orbitals. These results are discussed in the context of broader chemical trends and electronic descriptors are proposed for oxygen electrocatalysis in transition metal perovskite oxides.

 

The hafnate perovskites PbHfO₃(antiferroelectric) and SrHfO₃(“potential” ferroelectric) are studied as epitaxial thin films on SrTiO₃(001) substrates with the added opportunity of observing a morphotropic phase boundary (MPB) in the Pb_1−xSr_xHfO₃system. The resulting (240)‐oriented PbHfO₃(Pba2) films exhibited antiferroelectric switching with a saturation polarization ≈53 µC cm⁻²at 1.6 MV cm⁻¹, weak‐field dielectric constant ≈186 at 298 K, and an antiferroelectric‐to‐paraelectric phase transition at ≈518 K. (002)‐oriented SrHfO₃films exhibited neither ferroelectric behavior nor evidence of a polarP4mmphase . Instead, the SrHfO₃films exhibited a weak‐field dielectric constant ≈25 at 298 K and no signs of a structural transition to a polar phase as a function of temperature (77–623 K) and electric field (–3 to 3 MV cm⁻¹). While the lack of ferroelectric order in SrHfO₃removes the potential for MPB, structural and property evolution of the Pb_1−xSr_xHfO₃(0 ≤x < 1) system is explored. Strontium alloying increased the electric‐breakdown strength (E_B) and decreased hysteresis loss, thus enhancing the capacitive energy storage density (U_r) and efficiency (η). The composition, Pb_0.5Sr_0.5HfO₃produced the best combination ofE_B = 5.12 ± 0.5 MV cm⁻¹,U_r = 77 ± 5 J cm⁻³, and η = 97 ± 2%, well out‐performing PbHfO₃and other antiferroelectric oxides.