Ferroelectricity, High Permittivity, and Tunability in Millimeter-Size Crack-Free Ba_1–<i>x</i>Sr_<i>x</i>TiO₃ Flexible Epitaxial Sheets

Flexible oxide sheets exhibiting ferroelectricity and high permittivity are crucial for the advancement of various emerging technologies. However, achieving large-area crack-free flexible oxide sheets remains difficult because oxides easily crack when their thicknesses are significantly reduced. In this study, we focused on Ba1–xSrxTiO3 (BST), which is an important material owing to its high permittivity and electric-field-induced tunability. By employing an amorphous AlOx protective layer with a thickness greater than 10 nm, we successfully fabricated millimeter-sized crack-free BST epitaxial sheets. In contrast, the sheets fabricated without protective layers exhibited breakage. In addition, we observed that a polycrystalline indium tin oxide layer acted as a suitable bottom electrode. The BST sheet with a composition of x = 0.25 exhibited excellent ferroelectric switching behavior and minimal current leakage, even when used with electrodes with a diameter of 100 μm. Furthermore, the BST sheet with a composition of x = 0.5 simultaneously exhibited high permittivity (εr ∼ 3500 at 10 kHz) and tunability (56%), combining the desirable characteristics of both bulk and thin-film materials. These improved dielectric properties are attributed to the absence of substrate-induced strain, which is a characteristic not observed in thin-film materials

Significant Suppression of Cracks in Freestanding Perovskite Oxide Flexible Sheets Using a Capping Oxide Layer

Flexible and functional perovskite oxide sheets with high orientation and crystallization are the next step in the development of next-generation devices. One promising synthesis method is the lift-off and transfer method using a water-soluble sacrificial layer. However, the suppression of cracks during lift-off is a crucial problem that remains unsolved. In this study, we demonstrated that this problem can be solved by depositing amorphous Al2O3 capping layers on oxide sheets. Using this simple method, over 20 mm2 of crack-free, deep-ultraviolet transparent electrode La:SrSnO3 and ferroelectric Ba0.75Sr0.25TiO3 flexible sheets were obtained. By contrast, the sheets without any capping layers broke. The obtained sheets showed considerable flexibility and high functionality. The La:SrSnO3 sheet simultaneously exhibited a wide bandgap (4.4 eV) and high electrical conductivity (>103 S/cm). The Ba0.75Sr0.25TiO3 sheet exhibited clear room-temperature ferroelectricity with a remnant polarization of 17 μC/cm2. Our findings provide a simple transfer method for obtaining large, crack-free, high-quality, single-crystalline sheets

Magnetic Phase Transition-Induced Modulation of Ferroelectric Properties in Hexagonal <i>R</i>FeO₃ (<i>R</i> = Tb and Ho)

Hexagonal rare-earth iron oxides (h-RFeO3) exhibit spontaneous magnetization and room-temperature ferroelectricity simultaneously. However, achieving a large magnetoelectric coupling necessitates further exploration. Herein, we report the impact of the magnetic phase transition on the ferroelectric properties of epitaxial h-RFeO3 (R = Tb and Ho) films prepared by pulsed laser deposition. The metastable h-RFeO3 phase is successfully stabilized with high crystallinity and low leakage current due to the ITO buffer layer, making it possible to investigate the ferroelectric properties. The h-TbFeO3 film exhibits a magnetic-field-induced transition from antiferromagnetic (AFM) to weak ferromagnetic (wFM) phases below 30 K, while also exhibiting ferroelectricity at 300 K. The dielectric constants change with the magnetic phase transition, demonstrating hysteresis in the magnetocapacitance. In contrast, the h-HoFeO3 film exhibits antiferroelectric-like behavior and an AFM–wFM phase transition. Notably, the h-HoFeO3 film shows a rapid increase in the remnant polarization during the AFM–wFM phase transition accompanied by an increase in the ferroelectric component. Considering the strong connection between the antiferroelectric behavior in the h-RFeO3 system and the ferroelectric domain wall motion, this considerable modification of ferroelectric properties during the magnetic phase transition is probably due to the faster movement of the ferroelectric domain walls in the wFM phase induced by the clamping effect. Our findings indicate the effectiveness of magnetic phase transitions in enhancing the magnetoelectric coupling, particularly when utilizing domain wall clamping properties