Remarkable flexibility in freestanding single-crystalline antiferroelectric PbZrO3 membranes.

The ultrahigh flexibility and elasticity achieved in freestanding single-crystalline ferroelectric oxide membranes have attracted much attention recently. However, for antiferroelectric oxides, the flexibility limit and fundamental mechanism in their freestanding membranes are still not explored clearly. Here, we successfully fabricate freestanding single-crystalline PbZrO3 membranes by a water-soluble sacrificial layer technique. They exhibit good antiferroelectricity and have a commensurate/incommensurate modulated microstructure. Moreover, they also have good shape recoverability when bending with a small radius of curvature (about 2.4 µm for the thickness of 120 nm), corresponding to a bending strain of 2.5%. They could tolerate a maximum bending strain as large as 3.5%, far beyond their bulk counterpart. Our atomistic simulations reveal that this remarkable flexibility originates from the antiferroelectric-ferroelectric phase transition with the aid of polarization rotation. This study not only suggests the mechanism of antiferroelectric oxides to achieve high flexibility but also paves the way for potential applications in flexible electronics.

Strong Electron-Phonon Coupling Mediates Carrier Transport in BiFeO3.

The electron-phonon interaction is known as one of the major mechanisms determining electrical and thermal properties. In particular, it alters the carrier transport behaviors and sets fundamental limits to carrier mobility. Establishing how electrons interact with phonons and the resulting impact on the carrier transport property is significant for the development of high-efficiency electronic devices. Here, carrier transport behavior mediated by the electron-phonon coupling in BiFeO3 epitaxial thin films is directly observed. Acoustic phonons are generated by the inverse piezoelectric effect and coupled with photocarriers. Via the electron-phonon coupling, doughnut shape carrier distribution has been observed due to the coupling between hot carriers and phonons. The hot carrier quasi-ballistic transport length can reach 340 nm within 1 ps. The results suggest an effective approach to investigating the effects of electron-phonon interactions with temporal and spatial resolutions, which is of great importance for designing and improving electronic devices.

Large in-plane piezo-strain enhanced voltage control of magnetic anisotropy in Si-compatible multiferroic thin films.

Voltage control of magnetic anisotropy (VCMA) in Si-compatible ferroelectric/ferromagnetic multiferroic thin films is promising to enable power-efficient and integrated magnetic memories. However, their VCMA effect is weak and is always smaller than that of the bulk counterparts. Here, we achieve a more substantial VCMA effect in thin films than in the bulk, benefiting from the large in-plane piezo-strain mediated magnetoelectric coupling under strong fields. Si-compatible ferroelectric Pb(Zr,Ti)O3 (PZT) thin films with large breakdown strength of up to 3.2 MV cm-1 are fabricated to further construct multiferroic thin films. Since conventional methods fail to measure the VCMA effect under strong fields, we establish a micro-ferromagnetic resonance method based on micro-fabrication. An enhanced VCMA effect is demonstrated in PZT/CoFeB thin films, whose voltage-induced effective magnetic field (Heff) could experimentally reach 26.1 Oe, which is much stronger than that in bulk control samples "PZT ceramic/CoFeB" (2.6 Oe) and "PMN-PT single crystal/CoFeB" (18.5 Oe) as well as previous reports. Theoretically, the Heff in thin films could be > 60 Oe near the breakdown strength, resulting from a giant in-plane piezo-strain S31 < -0.3%, which is comparable to that of the best ferroelectric single crystals. Si-compatible multiferroic thin films with enhanced VCMA will be a useful platform for developing integrated magnetic and spintronic devices.

Voltage Control of Perpendicular Magnetic Anisotropy in Multiferroic Composite Thin Films under Strong Electric Fields.

"Ferroelectric/ferromagnetic" multiferroic composites with perpendicular magnetic anisotropy (PMA) are useful for developing power-efficient magnetic memories. Voltage control of PMA has been demonstrated in bulk multiferroic composites based on ferroelectric single crystals, but they are not compatible for integration. Multiferroic composite thin films are useful for developing integrated devices; however, voltage control of PMA in them has not been achieved yet at room temperature due to their low magnetoelectric (ME) coupling coefficient. Here, we demonstrate such functionality and propose to enhance their ME coupling effect under a strong electric field by taking full advantage of the large dielectric strength of ferroelectric thin films. First, the thickness-dependent breakdown of Pb(Zr0.384Ti0.576Nb0.04)O3 (PNZT) thin films was studied, and the two-layer (∼200 nm) samples exhibited the highest breakdown strength (3.68 MV/cm) and small surface roughness (<1 nm). Second, we fabricated "PNZT/(Co/Pt)5" thin films with strong PMA whose breakdown strength is nearly independent of the top electrode materials. Finally, voltage-induced effective magnetic field (Heff) in "PNZT/(Co/Pt)5" was studied. It is comparable to that achieved in bulk composites and will induce magnetization switching under strong electric fields. Multiferroic composite thin films with large breakdown strength will provide a useful platform for enabling integrated multiferroic devices.

Phase transition enhanced superior elasticity in freestanding single-crystalline multiferroic BiFeO3 membranes.

The integration of ferroic oxide thin films into advanced flexible electronics will bring multifunctionality beyond organic and metallic materials. However, it is challenging to achieve high flexibility in single-crystalline ferroic oxides that is considerable to organic or metallic materials. Here, we demonstrate the superior flexibility of freestanding single-crystalline BiFeO3 membranes, which are typical multiferroic materials with multifunctionality. They can endure cyclic 180° folding and have good recoverability, with the maximum bending strain up to 5.42% during in situ bending under scanning electron microscopy, far beyond their bulk counterparts. Such superior elasticity mainly originates from reversible rhombohedral-tetragonal phase transition, as revealed by phase-field simulations. This study suggests a general fundamental mechanism for a variety of ferroic oxides to achieve high flexibility and to work as smart materials in flexible electronics.