Abstract
The secondary nature of polarization in improper ferroelectrics promotes functional properties beyond those of conventional ferroelectrics. In technologically relevant ultrathin films, however, the improper ferroelectric behavior remains largely unexplored. Here, we probe the emergence of the coupled improper polarization and primary distortive order parameter in thin films of hexagonal YMnO3. Combining state-of-the-art in situ characterization techniques separately addressing the improper ferroelectric state and its distortive driving force, we reveal a pronounced thickness dependence of the improper polarization, which we show to originate from the strong modification of the primary order at epitaxial interfaces. Nanoscale confinement effects on the primary order parameter reduce the temperature of the phase transition, which we exploit to visualize its order-disorder character with atomic resolution. Our results advance the understanding of the evolution of improper ferroelectricity within the confinement of ultrathin films, which is essential for their successful implementation in nanoscale applications.
Highlights
The secondary nature of polarization in improper ferroelectrics promotes functional properties beyond those of conventional ferroelectrics
To detect the ferroelectric order in the samples, we use in situ second harmonic generation (SHG) (ISHG) implemented in the growth chamber
This light-frequency-doubling process is sensitive to the breaking of inversion symmetry so that the amplitude of the ISHG light wave is proportional to Ps34,35
Summary
The secondary nature of polarization in improper ferroelectrics promotes functional properties beyond those of conventional ferroelectrics. We probe the emergence of the coupled improper polarization and primary distortive order parameter in thin films of hexagonal YMnO3. The leading order can coerce the improper polarization into states that would be avoided in conventional ferroelectrics, allowing the emergence of exotic functional properties to complement those of conventional ferroelectrics, an issue of great interest for the development of next-generation oxide-based electronics[3,4,5]. The elevated ferroelectric transition temperature in bulk hexagonal manganites (TCbulk = 1259 K for bulk YMnO321) imposes major experimental requirements on the probing of the emerging improper polarization, and direct observation and comparison of the two order parameters at the phase transition point has remained elusive. We show that the substrate exerts a clamping of the YMnO3 unit cell at a
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