Anisotropic epitaxial stabilization of a low-symmetry ferroelectric with enhanced electromechanical response

Reversible Mechanical Switching of Ferroelastic Stripe Domains in Multiferroic Thin Films

The application potential of ferroelectric thin films largely relies on the controllability of their domain structure. Among the various proposed strategies, mechanical switching is being considered as a potential alternative to replace electrical switching for control of the domain structure of ferroelectric thin films via, e.g., the flexoelectric effect. So far, studies on mechanical switching are confined to out-of-plane polarization switching in ferroelectric thin films, which are in pristine or prepoled single-domain states. In this work, we report reversible in-plane mechanical switching of the monoclinic phase (MC phase) stripe domains in BiFeO3 thin films can be realized by scanning tip force. Via controlling the fast scan direction of the scanning probe microscopy tip and the magnitude of the tip force, the effective trailing field induced by the local tip force can be rotated to consequently switch the net in-plane polarization of the two-variant stripe domain patterns by either 90° or 180°. Moreover, the monoclinic to rhombohedral (MC-R) phase transition occurs during mechanical switching with the distribution of R-phase domains dependent on the switching paths. These results extend our current understanding of the mechanical switching behavior in ferroelectric thin films and should be instructive for their future applications.

Colossal electrocaloric effect in an interface-augmented ferroelectric polymer

The electrocaloric effect demands the maximized degree of freedom (DOF) of polar domains and the lowest energy barrier to facilitate the transition of polarization. However, optimization of the DOF and energy barrier-including domain size, crystallinity, multiconformation coexistence, polar correlation, and other factors in bulk ferroelectrics-has reached a limit. We used organic crystal dimethylhexynediol (DMHD) as a three-dimensional sacrificial master to assemble polar conformations at the heterogeneous interface in poly(vinylidene fluoride)-based terpolymer. DMHD was evaporated, and the epitaxy-like process induced an ultrafinely distributed, multiconformation-coexisting polar interface exhibiting a giant conformational entropy. Under a low electric field, the interface-augmented terpolymer had a high entropy change of 100 J/(kg·K). This interface polarization strategy is generally applicable to dielectric capacitors, supercapacitors, and other related applications.

Guided acoustic waves in thin epitaxial films: Experiment and inverse problem solution for NiTi

Despite the fundamental and technological importance of the elastic constants, a suitable method for their full characterization in epitaxial films is missing. Here we show that transient grating spectroscopy (TGS) with highly k-vector-selective generation and detection of acoustic waves is capable of determination of all independent elastic coefficients of an epitaxial thin film grown on a single-crystalline substrate. This experimental setup enables detection of various types of guided acoustic waves and evaluation of the directional dependence of their speeds of propagation. For the studied model system, which is a 3μm thin epitaxial film of the NiTi shape memory alloy on an MgO substrate, the TGS angular maps include Rayleigh-type surface acoustic waves as well as Sezawa-type and Love-type modes, delivering rich information on the elastic response of the film under different straining modes. The resulting inverse problem, which means the calculation of the elastic constants from the TGS maps, is subsequently solved using the Ritz-Rayleigh numerical method. Using this approach, tetragonal elastic constants of the NiTi film and their changes with the austenite→martensite phase transition are analyzed.

Large bi-axial tensile strain effect in epitaxial BiFeO3 film grown on single crystal PrScO3

A BiFeO3 film is grown epitaxially on a PrScO3 single crystal substrate which imparts ~ 1.45% of biaxial tensile strain to BiFeO3 resulting from lattice misfit. The biaxial tensile strain effect on BiFeO3 is investigated in terms of crystal structure, Poisson ratio, and ferroelectric domain structure. Lattice resolution scanning transmission electron microscopy, precession electron diffraction, and X-ray diffraction results clearly show that in-plane interplanar distance of BiFeO3 is the same as that of PrScO3 with no sign of misfit dislocations, indicating that the biaxial tensile strain caused by lattice mismatch between BiFeO3 and PrScO3 are stored as elastic energy within BiFeO3 film. Nano-beam electron diffraction patterns compared with structure factor calculation found that the BiFeO3 maintains rhombohedral symmetry, i.e., space group of R3c. The pattern analysis also revealed two crystallographically distinguishable domains. Their relations with ferroelectric domain structures in terms of size and spontaneous polarization orientations within the domains are further understood using four-dimensional scanning transmission electron microscopy technique.

Tuning Piezoelectricity via Thermal Annealing at a Freestanding Ferroelectric Membrane

Tuning the ferroelectric domain structure by a combination of elastic and electrostatic engineering provides an effective route for enhanced piezoelectricity. However, for epitaxial thin films, the clamping effect imposed by the substrate does not allow aftergrowth tuning and also limits the electromechanical response. In contrast, freestanding membranes, which are free of substrate constraints, enable the tuning of a subtle balance between elastic and electrostatic energies, giving new platforms for enhanced and tunable functionalities. Here, highly tunable piezoelectricity is demonstrated in freestanding PbTiO₃ membranes, by varying the ferroelectric domain structures from c-dominated to c/a and a domains via aftergrowth thermal treatment. Significantly, the piezoelectric coefficient of the c/a domain structure is enhanced by a factor of 2.5 compared with typical c domain PbTiO₃. This work presents a new strategy to manipulate the piezoelectricity in ferroelectric membranes, highlighting their great potential for nano actuators, transducers, sensors and other NEMS device applications.

Strain Tuning of Negative Capacitance in Ferroelectric KNbO3 Thin Films

Ferroelectrics with negative capacitance effects can amplify the gate voltage in field-effect transistors to achieve low power operation beyond the limits of Boltzmann's Tyranny. The reduction of power consumption depends on the capacitance matching between the ferroelectric layer and gate dielectrics, which can be well controlled by adjusting the negative capacitance effect in ferroelectrics. However, it is a great challenge to experimentally tune the negative capacitance effect. Here, the observation of the tunable negative capacitance effect in ferroelectric KNbO₃ through strain engineering is demonstrated. The magnitude of the voltage reduction and negative slope in polarization-electric field (P-E) curves as the symbol of negative capacitance effects can be controlled by imposing various epitaxial strains. The adjustment of the negative curvature region in the polarization-energy landscape under different strain states is responsible for the tunable negative capacitance. Our work paves the way for fabricating low-power devices and further reducing energy consumption in electronics.

Polar Magnetism Above 600 K with High Adaptability in Perovskite Oxides

High magnetic order temperature, sustainable polar insulating state, and tolerance to device integrations are substantial advantages for applications in next-generation spintronics. However, engineering such functionality in a single-phase system remains a challenge owing to the contradicted chemical and electronic requirements for polar nature and magnetism, especially with an ordering state highly above room temperature. Perovskite-related oxides with unique flexibility allow electron-unpaired subsystems to merge into the polar lattice to induce magnetic interactions, combined with their inherent asymmetry, thereby promising polar magnet design. Herein, by atomic-level composition assembly, a family of Ti/Fe co-occupied perovskite oxide films Pb(Ti_1-x,Fe_x)O₃ (PFT(x)) with a Ruddlesden-Popper superstructure are successfully synthesized on several different substrates, demonstrating exceptional adaptability to different integration conditions. Furthermore, second-harmonic generation measurements convince the symmetry-breaking polar character. Notably, a ferromagnetic ground state up to 600 K and a steady insulating state far beyond room temperature were achieved simultaneously in these films. This strategy of constructing layered modular superlattices in perovskite oxides could be extended to other strongly correlated systems for triggering nontrivial quantum physical phenomena.

Ultrahigh Ferroelectric and Piezoelectric Properties in BiFeO3-BaTiO3 Epitaxial Films Near Morphotropic Phase Boundary

Ferroelectric solid solutions with composition near the morphotropic phase boundary (MPB) have gained extensive attention recently due to their excellent ferroelectric and piezoelectric properties. Here, we have demonstrated a strategy to realize the controllable preparation of BiFeO₃-BaTiO₃ (BF-BT) epitaxial films near the MPB. A series of high-quality BF-BT films were fabricated by pulsed laser deposition via adjusting oxygen partial pressure (PO₂) using a BF-BT ceramic target. A continuous transition from rhombohedral to tetragonal phase was observed upon increasing PO₂. Particularly, the film with a pure tetragonal phase exhibited a large remnant polarization of ∼90.6 μC/cm², while excellent piezoelectric performance with an ultrahigh strain (∼0.48%) was obtained in the film with coexisting rhombohedral and tetragonal phases. The excellent ferroelectric and piezoelectric properties endow the BF-BT system near the MPB with great application prospects in lead-free electronic devices.

Designing Monoclinic Heterophase Coexistence for the Enhanced Piezoelectric Performance in Ternary Lead-Based Relaxor Ferroelectrics

Enhanced piezoelectric, dielectric properties and thermal stability in ternary relaxor-PbTiO₃ based ferroelectric crystals are expected to develop the next-generation of electromechanical devices. However, due to their increased disorder compared to other ferroelectrics, designing a controllable phase boundary structure and engineered domain remains a challenging task. Here, we construct a monoclinic heterophase coexisting in a ternary Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ single crystal with optimized composition and an ultrahigh piezoelectric coefficient of 1400 pC N^-1, to quantify the correlation between spontaneous nanopolarity and phase heterogeneity, in an attempt to understand the origin of the exceptional functionalities. By designing an in situ high-resolution spectroscopic-microscopic technique, we have observed M_a and M_c heterophase mixtures spatially separated by the monoclinic heterophase boundary (MHB), which are responsible for the ferroelectric-dominated and relaxor-ferroelectric-dominated nanodomain structure, respectively. Internal energy mapping from optical soft mode dynamics reveals the inhomogeneous polarization and local symmetry on both sides of the MHB. Various molecular polarizabilities and localized octahedral distortions correlate directly with monoclinic regions and electromechanical contribution. This work clarifies the heterogeneity between structure, energy, and polar order and provides a new design freedom for advanced relaxor ferroelectrics.

Strain and orientation engineering in ABO3perovskite oxide thin films

Perovskite oxides with chemical formula ABO₃are widely studied for their properties including ferroelectricity, magnetism, strongly correlated physics, optical effects, and superconductivity. A thriving research direction using such materials is through their integration as epitaxial thin films, allowing many novel and exotic effects to be discovered. The integration of the thin film on a single crystal substrate, however, can produce unique and powerful effects, and can even induce phases in the thin film that are not stable in bulk. The substrate imposed mechanical boundary conditions such as strain, crystallographic orientation, octahedral rotation patterns, and symmetry can also affect the functional properties of perovskite films. Here, the author reviews the current state of the art in epitaxial strain and orientation engineering in perovskite oxide thin films. The paper begins by introducing the effect of uniform conventional biaxial strain, and then moves to describe how the substrate crystallographic orientation can induce symmetry changes in the film materials. Various material case studies, including ferroelectrics, magnetically ordered materials, and nonlinear optical oxides are covered. The connectivity of the oxygen octahedra between film and substrate depending on the strain level as well as the crystallographic orientation is then discussed. The review concludes with open questions and suggestions worthy of the community's focus in the future.