BiFeO3 (BFO) is positioned for success as a magnetoelectric material system, but its optimum usage in faster and more energy-efficient magneto-logic devices require advances. Most importantly, a ferroelastic and antiferromagnetic monodomain state with single-step deterministic switching is desirable for reliable low-power magnetoelectric devices with reproducibility and scaling using BiFeO3. This would allow deterministic and robust control of both the internal magnetoelectric coupling in BiFeO3 and the exchange coupling of its antiferromagnetic order to a ferromagnetic overlayer.

We have fabricated epitaxial (001) and (111) BFO thin films with both ferroelectric and antiferromagnetic monodomain states. Additionally, we have fabricated freestanding membranes of ferroelastic and ferroelectric monodomain BiFeO3 using an Sr2CaAl2O6 (SCAO) sacrificial layer. The membranes exhibit deterministic switching over a hundred thousand electric field cycles with lower voltage and faster switching dynamics than their thin-film counterpart. This progress is promising toward energy-efficient magnetoelectric memory devices. We will discuss additional multiferroic applications of these BFO membranes.

This work has been done in collaboration with Pratap Pal, J. L. Schad, K. J. Lee, Y. Yao, A. M. Vibhakar, R. D. Johnson, P. G. Radaelli M.S. Rzchowski.

CBE acknowledges support for this research through the Gordon and Betty Moore Foundation’s EPiQS Initiative, Grant GBMF9065 and a Vannevar Bush Faculty Fellowship (ONR N00014-20-1-2844). Magnetic and transport measurement at the University of Wisconsin–Madison was supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), under award number DE-FG02-06ER46327.

Epitaxial lift-off techniques have allowed the integration of highly crystalline complex oxide thin films on arbitrary substrates, avoiding the mechanical clamping with the substrate. These free-standing films show improved functional properties compared to the clamped epitaxial films, and also make it possible to widely extend the range of strains that so far was reachable via heteroepitaxy. Here, we report the switching dynamics of antiferroelectric (AFE) Lead Zirconate (PbZrO₃) heteroepitaxial capacitors clamped to the growth substrate, compared to their free-standing counterparts, transferred to silicon. Although the structural characterizations show no significant differences between the two morphologies, we observe a faster switching response and lower residual polarization at zero field in freestanding capacitors as compared to epitaxially-clamped films. Moreover, we observe that the frequency dependence of hysteresis loops is highly dependent on the clamping conditions, with the freestanding films showing higher domain wall and phase boundary mobility. As a result of the improved antiferroelectric switching behavior, freestanding capacitor membranes exhibit better energy storage density and efficiency. Furthermore, we demonstrate the integration of these free-standing capacitors on polymer substrates and study the evolution of switching behavior under different strain conditions. Our results give new insights into design considerations for improving efficiency of AFE capacitors and their integration in flexible microelectronics.

Na0.5Bi0.5TiO3-based compositions attract large interest as good candidates for replacement of nowadays widely-used lead-containing ferroelectrics, production and use of which is being gradually limited due to health and environmental considerations. Particularly (1-x)Na0.5Bi0.5TiO3-xSr1-1.5xBixTiO3 solid solutions are promising for a wide range of applications, including sensors, actuators, energy storage. Although mostly bulk ceramics of these new ferroelectric materials are studied, they do not meet the real requirements for many applications. At the same time, thin films of the same composition seem to be more suitable for miniaturization purposes, but lead to reduced power and sensitivity of the devices where they are implemented. Whereas thick films are a good compromise with respect to bulk ceramics and thin films. They have the advantage in realization of miniaturized devices without the mentioned performance drawbacks of thin films and allow application of much higher electric fields compared to bulk ceramics. In the present research, for the first time, we have produced free-standing 0.9Na0.5Bi0.5TiO3-0.1Sr0.7Bi0.2TiO3 thick films by water-based tape-casting method, using just two organic chemicals, which is an eco-friendly production approach having only several successful attempts in the case of ferroelectric materials before. We conducted a detailed study focusing on development of microstructure at various sintering temperatures and consequences of evaporation of Bi during the production. Our findings show how the choice of the sintering temperature can help to improve density of the thick films, reaching 98.9%, and changing the grain size. It is demonstrated that a secondary phase appears predominantly on the free surface of the films. We propose a method for effective minimization of its formation – by the choice of appropriate embedding powder media during the sintering. Our results show that a stable ferroelectric state is induced by poling the studied thick films, similarly to the bulk ceramics of the same composition. We have also demonstrated that application of higher voltages to 0.9N0.5Bi0.5TiO3-0.1Sr0.7Bi0.2TiO3 thick films, a possibility not available in bulk ceramics, enables the attainment of greater piezoelectric deformations. This affirms the viability and consistency of the proposed methodology for producing competing ferroelectric counterparts, which is especially valuable taking into account its ecological approach.