Mechanical Control of Polar Order

Abstract

BiFeO3 is a model multiferroic in which the ferroelectric polarization is coupled to ferroelastic lattice distortions, yet deterministic control of its domain structure remains limited by high switching fields and competing polarization variants. Here, we identify a mechanically assisted polarization switching pathway in epitaxial BiFeO3 thin films that fundamentally alters the switching energetics. Using just out-of-plane electric fields, polarization reversal requires voltages of approximately 4 V and stabilizes coexisting polarization states. In contrast, when mechanical pressure is applied concurrently, the coercive voltage can be significantly reduced (even to 0V), resulting in spontaneous switching. Piezoresponse force microscopy measurements reveal that applied mechanical pressure suppresses ferroelastic domain competition, indicating a decrease in the required electrical energy barrier associated with polarization rotation and domain wall motion. These results demonstrate that stress acts as an active thermodynamic control parameter, enabling access to switching pathways that are inaccessible under only an electric field. By directly coupling lattice distortions to polarization reversal, mechanically assisted switching provides a general framework for controlling coupled order parameters in multiferroic oxides, which can be directly applied in the device-level architecture, where a small mechanical pressure can help in achieving lower switching energy of ferroelectric polarization. This work advances the fundamental understanding of electromechanical coupling in complex ferroics and establishes mechanical energy as a powerful tool for probing and manipulating ferroelastic ferroelectric interactions.

Figures (4)

• Figure 1: Voltage-assisted and mechanically-assisted polarization switching in BiFeO$_3$ thin films. (A) In-plane piezoresponse image of an as-grown BiFeO$_3$/SrRuO$_3$ thin film on a SrTiO$_3$ (001) substrate, showing the canonical 4-variant ferroelectric domains. (B) Out-of-plane piezoresponse image acquired after the application of DC voltage, demonstrating that a bias of approximately 4 V is required to induce polarization switching under electric field. (C) Box-in-box (BiB) piezoresponse image recorded following sequential application of voltage, mechanical force, and voltage. Application of a force of 4 $\mu$N induces clear domain switching, and the mechanically switched domains can be reversibly re-switched using an electric field. (D) Corresponding surface topography of the marked region, confirming the absence of measurable surface damage after force-assisted switching. Scale bars are 2 $\mu$m. (E-f) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images acquired from the as-deposited region and the mechanically switched region. Comparison of these images reveals no observable structural defects or lattice disruption associated with mechanical switching. The insets are the zoomed images and schematics of the unit cell. Scale bars are 1 nm.
• Figure 2: Mechanically-assisted switching of polarization in BiFeO$_3$ thin films. (A) Schematic illustration of the experimental configuration for applying electric bias and mechanical force using a conductive AFM tip. The epitaxial SrRuO$_3$ bottom electrode provides an electrical ground. (B) Example piezoresponse hysteresis loops acquired under varying applied mechanical forces. At low applied force, the hysteresis loop remains symmetric about the vertical axis, indicating predominantly voltage-driven polarization switching. With increasing mechanical force, the hysteresis loop progressively shifts along the voltage axis toward the negative direction. (C) Coercive voltage and bias voltage extracted from the hysteresis loops as a function of applied mechanical force. While the coercive voltage remains approximately constant, the negative bias voltage increases approximately with mechanical force, indicating enhanced switching asymmetry under mechanical loading. (D) Example piezoresponse image recorded after mechanically-assisted switching, where the applied voltage is varied from 1.5 V to 5 V at fixed mechanical forces of 3 $\mu$N (top two rows) and 3.5 $\mu$N (bottom two rows). Increasing the mechanical force enables polarization switching at progressively lower applied voltages. (E) Switching fraction of polarization domains as a function of applied voltage for different mechanical forces. At a force of 4 $\mu$N, complete switching is achieved even in the absence of an external electric bias. At lower mechanical forces, a finite voltage is required to achieve deterministic switching. Measured switching fractions are fit to $tanh(V)$. (F) Switching voltage extracted from the fit in D plotted with the corresponding force. Complete switching is obtained at 4 $\mu$N without applied voltage, whereas reducing the mechanical force necessitates increasing electric bias, with voltages of approximately 4.5 V sufficient to fully switch the domains in the absence of mechanical assistance.
• Figure 3: Voltage- and mechanically induced domain switching in BiFeO$_3$ thin films. (A) Vector mapping of the polarization switching induced by electric bias and mechanical force. The outer region corresponds to switching performed using a $-5$ V bias (red and pink arrows), whereas the region enclosed by the green dotted lines indicates areas switched using a $+5$ V bias and a normal force of 4 $\mu$N, respectively. Switching induced by electric field alone results in predominantly 180$^\circ$ polarization reversal, while mechanically assisted switching produces a ferroelastic reconfiguration in which one polarization variant becomes dominant over the other. (B) Polarization histograms extracted from the three regions. In the as-grown state, both polarization variants are present with comparable intensity. Following electric-field-only switching, the polarization undergoes 180$^\circ$ reversal while maintaining a similar proportion of domain variants. In contrast, mechanically assisted switching suppresses one variant and stabilizes the other, driving the system toward a single domain population.
• Figure 4: Mechanical switching and reconfiguration of in-plane polarization domains in BiFeO$_3$ thin films. (A,C) In-plane PFM phase images recorded after mechanical switching, with the applied voltage varied from 1.5 V to 5 V at fixed normal mechanical forces as indicated on the right. Increasing mechanical force enables polarization switching at progressively lower applied voltages and modifies the resulting domain configuration. (B,D) Corresponding intensity profiles extracted from the marked regions in (A,C). In the as-grown state, both in-plane polarization variants are present with comparable intensity. At low applied mechanical force, the polarization undergoes predominantly 180$^{\circ}$ ferroelectric reversal while preserving a similar population of domain variants. In contrast, increasing mechanical force progressively suppresses one ferroelastic variant and stabilizes the other, driving the system toward a configuration with a single in-plane domain.