Mixed electrochemical–ferroelectric states in nanoscale ferroelectrics

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Abstract

Ferroelectricity on the nanoscale has been the subject of much fascination in condensed-matter physics for over half a century. In recent years, multiple reports claiming ferroelectricity in ultrathin ferroelectric films based on the formation of remnant polarization states, local electromechanical hysteresis loops, and pressure-induced switching were made. However, similar phenomena were reported for traditionally non-ferroelectric materials, creating a significant level of uncertainty in the field. Here we show that in nanoscale systems the ferroelectric state is fundamentally inseparable from the electrochemical state of the surface, leading to the emergence of a mixed electrochemical–ferroelectric state. We explore the nature, thermodynamics, and thickness evolution of such states, and demonstrate the experimental pathway to establish its presence. This analysis reconciles multiple prior studies, provides guidelines for studies of ferroelectric materials on the nanoscale, and establishes the design paradigm for new generations of ferroelectric-based devices.

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Figure 1: Electrochemical effects on ferroelectric phase instability.
Figure 2: Different ferroelectric states in phase diagram.
Figure 3: Effect of polarization and temperature on the MC-FE states at different thicknesses.
Figure 4: PFM measurements for different thickness BTO films.
Figure 5: cKPFM measurements for BTO films of different thickness.

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Acknowledgements

We thank P. Maksymovych (ORNL) and S. Schamm-Chardon (CEMES-CNRS, Toulouse) for their valuable discussion. This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Support for S.M.Y. was provided by a DOE Presidential Early Career for Scientists and Engineers. This research was also supported (S.M.Y.) by the Sookmyung Women’s University Research Grants (1-1603-2049). This research was also sponsored by the Division of Materials Sciences and Engineering, BES, US DOE (R.K.V. and S.V.K.). C.D. and L.M. acknowledge support and access to the Center for Nanophase Materials Sciences through the proposal CNMS2014-323. C.D. and L.M. also acknowledge financial support for this research at INL by the LABEX iMUST (ANR-10-LABX-0064) of Université de Lyon, within the program ‘Investissements d’Avenir’ (ANR-11-IDEX-0007) operated by the French National Research Agency (ANR).

Author information

S.V.K. and C.D. conceived the project and directed the research; S.M.Y. designed and performed the SPM experiments; L.M. performed a part of the PFM measurements with S.M.Y.; S.M.Y. analysed and discussed the data with N.B., S. J., R.K.V., C.D. and S.V.K.; A.N.M. and E.A.E. performed thermodynamic calculations. R.K. and Y.C. contributed theoretical calculations. L.M. performed the growth and diffraction characterization of the thin films and analysed the data with C.D.; S.M.Y., A.N.M., and S.V.K. wrote the paper with contributions and feedback from all the authors, mainly R.K.V. and C.D.

Correspondence to Sergei V. Kalinin.

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Yang, S., Morozovska, A., Kumar, R. et al. Mixed electrochemical–ferroelectric states in nanoscale ferroelectrics. Nature Phys 13, 812–818 (2017) doi:10.1038/nphys4103

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