Ferroelectricity and Self-Polarization in Ultrathin Relaxor Ferroelectric Films

Large Piezoelectricity Induced by Internal Stress in (K,Na)NbO3 Single Crystals

Achieving a high piezoelectric response and excellent stability is essential for practical applications of ferroelectric materials. Herein, large piezoelectricity of d33 = 167 pC/N and kt = 0.52 is found in a K0.7Na0.3NbO3 lead-free ferroelectric single crystal without poling, which is comparable to the artificially poled KNN crystals. The large piezoelectricity is maintained up to 196 °C, showing excellent thermal stability. It was demonstrated that the high piezoelectricity is associated with strong self-polarization in the crystals. The strong internal stress formed during crystal growth gives a preferred spontaneous polarization orientation, resulting in a net macro total polarization. In addition, the internal stress also pins domain wall motions and provides a "restoring force" for the domain switching. This work provides a strategy for designing and optimizing the piezoelectric performance of ferroelectric materials.

Experimental Band Structure of Pb(Zr,Ti)O3 : Mechanism of Ferroelectric Stabilization

Pb(Zr,Ti)O₃ (PZT) is the most common ferroelectric (FE) material widely used in solid-state technology. Despite intense studies of PZT over decades, its intrinsic band structure, electron energy depending on 3D momentum k, is still unknown. Here, Pb(Zr_0.2 Ti_0.8 )O₃ using soft-X-ray angle-resolved photoelectron spectroscopy (ARPES) is explored. The enhanced photoelectron escape depth in this photon energy range allows sharp intrinsic definition of the out-of-plane momentum k and thereby of the full 3D band structure. Furthermore, the problem of sample charging due to the inherently insulating nature of PZT is solved by using thin-film PZT samples, where a thickness-induced self-doping results in their heavy doping. For the first time, the soft-X-ray ARPES experiments deliver the intrinsic 3D band structure of PZT as well as the FE-polarization dependent electrostatic potential profile across the PZT film deposited on SrTiO₃ and La_x SrMn_{1- x} O₃ substrates. The negative charges near the surface, required to stabilize the FE state pointing away from the sample (P+), are identified as oxygen vacancies creating localized in-gap states below the Fermi energy. For the opposite polarization state (P-), the positive charges near the surface are identified as cation vacancies resulting from non-ideal stoichiometry of the PZT film as deduced from quantitative XPS measurements.

Novel Study of Strain-Induced Piezoelectricity in VO2

VO₂ is well known for its dual-phase transitions, electrical and structural, at a single temperature of 340 K. The low-temperature structural phases of VO₂ are different from their high-temperature counterpart in terms of structural symmetry. The strain-induced modification of the structural distortion in VO₂ is studied in detail. A ferroelectric-type distortion is observed, and therefore, the piezoelectric effect in the low-temperature phases of VO₂ is investigated, for the first time, by piezoresponse force microscopy. Strain is one of the factors that can modify the electronic behavior of piezoelectric materials. At the same time, the two low-temperature phases of VO₂ (M1 and M2) can only be separated by the application of strain. The piezoelectric coefficient in the strained phase of VO₂ was found to be 11-12 pm/V, making it eligible for piezotronic applications.

Growth mode and strain effect on relaxor ferroelectric domains in epitaxial 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3/SrRuO3 heterostructures

Controlling the growth of complex relaxor ferroelectric thin films and understanding the relationship between biaxial strain–structural domain characteristics are desirable for designing materials with a high electromechanical response. For this purpose, epitaxial thin films free of extended defects and secondary phases are urgently needed. Here, we used optimized growth parameters and target compositions to obtain epitaxial (40–45 nm) 0.67Pb(Mg_1/3Nb_2/3)O₃–0.33PbTiO₃/(20 nm) SrRuO₃ (PMN–33PT/SRO) heterostructures using pulsed-laser deposition (PLD) on singly terminated SrTiO₃ (STO) and ReScO₃ (RSO) substrates with Re = Dy, Tb, Gd, Sm, and Nd. In situ reflection high-energy electron diffraction (RHEED) and high-resolution X-ray diffraction (HR-XRD) analysis confirmed high-quality and single-phase thin films with smooth 2D surfaces. High-resolution scanning transmission electron microscopy (HR-STEM) revealed sharp interfaces and homogeneous strain further confirming the epitaxial cube-on-cube growth mode of the PMN–33PT/SRO heterostructures. The combined XRD reciprocal space maps (RSMs) and piezoresponse force microscopy (PFM) analysis revealed that the domain structure of the PMN–33PT heterostructures is sensitive to the applied compressive strain. From the RSM patterns, an evolution from a butterfly-shaped diffraction pattern for mildly strained PMN–33PT layers, which is evidence of stabilization of relaxor domains, to disc-shaped diffraction patterns for high compressive strains with a highly distorted tetragonal structure, is observed. The PFM amplitude and phase of the PMN–33PT thin films confirmed the relaxor-like for a strain state below ∼1.13%, while for higher compressive strain (∼1.9%) the irregularly shaped and poled ferroelectric domains were observed. Interestingly, the PFM phase hysteresis loops of the PMN–33PT heterostructures grown on the SSO substrates (strain state of ∼0.8%) exhibited an enhanced coercive field which is about two times larger than that of the thin films grown on GSO and NSO substrates. The obtained results show that epitaxial strain engineering could serve as an effective approach for tailoring and enhancing the functional properties in relaxor ferroelectrics.

Strain engineering in epitaxial PMN–33PT films revealed an evolution from a butterfly-shaped diffraction for mildly strained films, evidencing the stabilization of relaxor domains, to disc-shaped diffraction patterns for high compressive strains.

Defect-Induced (Dis)Order in Relaxor Ferroelectric Thin Films

The effect of intrinsic point defects on relaxor properties of 0.68 PbMg_{1/3}Nb_{2/3}O_{3}-0.32 PbTiO_{3} thin films is studied across nearly 2 orders of magnitude of defect concentration via ex post facto ion bombardment. A weakening of the relaxor character is observed with increasing concentration of bombardment-induced point defects, which is hypothesized to be related to strong interactions between defect dipoles and the polarization. Although more defects and structural disorder are introduced in the system as a result of ion bombardment, the special type of defects that are likely to form in these polar materials (i.e., defect dipoles) can stabilize the direction of polarization against thermal fluctuations, and in turn, weaken relaxor behavior.

Epitaxial Strain Control of Relaxor Ferroelectric Phase Evolution

Understanding and ultimately controlling the large electromechanical effects in relaxor ferroelectrics requires intimate knowledge of how the local-polar order evolves under applied stimuli. Here, the biaxial-strain-induced evolution of and correlations between polar structures and properties in epitaxial films of the prototypical relaxor ferroelectric 0.68PbMg_1/3 Nb_2/3 O₃ -0.32PbTiO₃ are investigated. X-ray diffuse-scattering studies reveal an evolution from a butterfly- to disc-shaped pattern and an increase in the correlation-length from ≈8 to ≈25 nm with increasing compressive strain. Molecular-dynamics simulations reveal the origin of the changes in the diffuse-scattering patterns and that strain induces polarization rotation and the merging of the polar order. As the magnitude of the strain is increased, relaxor behavior is gradually suppressed but is not fully quenched. Analysis of the dynamic evolution of dipole alignment in the simulations reveals that, while, for most unit-cell chemistries and configurations, strain drives a tendency toward more ferroelectric-like order, there are certain unit cells that become more disordered under strain, resulting in stronger competition between ordered and disordered regions and enhanced overall susceptibilities. Ultimately, this implies that deterministic creation of specific local chemical configurations could be an effective way to enhance relaxor performance.

Flexible, Temperature-Resistant, and Fatigue-Free Ferroelectric Memory Based on Bi(Fe0.93Mn0.05Ti0.02)O3 Thin Film

A recent hot-spot topic for flexible and wearable devices involves high-performance nonvolatile ferroelectric memories operating under compressive or tensile mechanical deformations. This work presents the direct fabrication of a flexible (Mn,Ti)-codoped multiferroic BiFeO₃ film capacitor with Pt bottom and Au top electrodes on mica substrate. The fabricated polycrystalline Bi(Fe_0.93Mn_0.05Ti_0.02)O₃ film on mica exhibits superior ferroelectric switching behavior with robust saturated polarization ( P_s ∼ 93 μC/cm²) and remanent polarization ( P_r ∼ 66 μC/cm²) and excellent frequency stability (1-50 kHz) and temperature resistance (25-200 °C), as well as reliable long-lifetime operation. More saliently, it can be safely bent to a small radius of curvature, as low as 2 mm, or go through repeated compressive/tensile mechanical flexing for 10³ bending times at 4 mm radius without any obvious deterioration in polarization, retention time at 10⁵ s, or fatigue resistance after 10⁹ switching cycles. These findings demonstrate a novel route to designing flexible BiFeO₃-based ferroelectric memories for information storage and data processing, with promising applications in next-generation smart electronics.

An Unusual Mechanism for Negative Differential Resistance in Ferroelectric Nanocapacitors: Polarization Switching-Induced Charge Injection Followed by Charge Trapping

Negative differential resistance (NDR) has been extensively investigated for its wide device applications. However, a major barrier ahead is the low reliability. To address the reliability issues, we consider ferroelectrics and propose an alternative mechanism for realizing the NDR with deterministic current peak positions, in which the NDR results from the polarization switching-induced charge injection and subsequent charge trapping at the metal/ferroelectric interface. In this work, ferroelectric Au/BiFe_0.6Ga_0.4O₃ (BFGO)/Ca_0.96Ce_0.04MnO₃ (CCMO) nanocapacitors are prepared, and their ferroelectricity and NDR behaviors are studied concurrently. It is observed that the NDR current peaks are located at the vicinity of coercive voltages (V_c) of the ferroelectric nanocapacitors, thus evidencing the proposed mechanism. In addition, the NDR effect is reproducible and robust with good endurance and long retention time. This study therefore demonstrates a ferroelectric-based NDR device, which may facilitate the development of highly reliable NDR devices.