A novel concept to obtain a ferroelectric material with enhanced piezoelectric properties is proposed. This approach is based on the combination of two pathways: (i) the evolution of a ferroelectric monoclinic phase and, (ii) the coexistence of different types of ferroelectric domains leading to polarization discontinuities at the domain walls. Each of these pathways enables polarization rotation in the material which is responsible for giant piezoelectricity. Targeted incorporation of anisotropic epitaxial lattice strain is used to implement this approach. The feasibility of our concept is demonstrated for K _0.9 Na _0.1 NbO ₃ epitaxial layers grown on NdScO ₃ substrates where the coexistence of (100) _pc and (001) _pc pseudocubic oriented monoclinic domains is experimentally verified. This coexistence results in a complex periodic domain pattern with alternating emergence of ferroelectric in-plane a ₁ a ₂ and inclined M _C monoclinic phases, which differ in the direction of the electrical polarization vector. Our approach opens the possibility to exploit ferroelectric properties in both vertical and lateral directions and to achieve enhanced piezoelectric properties in lead-free material caused by singularities at the domains walls.

Synapses play a vital role in information processing, learning, and memory formation in the brain. By emulating the behavior of biological synapses, electronic synaptic devices hold the promise of enabling high‐performance, energy‐efficient, and scalable neuromorphic computing. Ferroelectric memristive devices integrate the characteristics of both ferroelectric and memristive materials and present a far‐reaching potential as artificial synapses. Here, it is reported on a new ferroelectric device on silicon, a field‐effect memristor, consisting of an epitaxial ultrathin ferroelectric Hf_0.5Zr_0.5O₂ film sandwiched between an epitaxial highly doped oxide semiconductor SrTiO_3‐δ and a top metal. Upon a low voltage of less than 2 V, the field‐effect modulation in the semiconductor enables to access multiple states. The device works in a large time domain ranging from milliseconds down to tens of nanoseconds. By gradually switching the polarization by identical pulses, the ferroelectric diode devices can dynamically adjust the synaptic strength to mimic short‐ and long‐term memory plasticity. Ionic contributions due to redox processes in the oxide semiconductor beneficially influence the device operation and retention.

We report on time- and angle-resolved transverse magneto-optical Kerr effect spectroscopy in the soft x-ray range that, by analysis via polarization-dependent magnetic scattering simulations, allows us to determine the spatiotemporal and element-specific evolution of femtosecond laser-induced spin dynamics in nanostructured magnetic materials. In a ferrimagnetic GdFe thin-film system, we correlate a reshaping spectrum of the magneto-optical Kerr signal to depth-dependent magnetization dynamics and disentangle contributions due to nonequilibrium electron transport and nanoscale heat diffusion on their intrinsic timescales. Our Letter provides a quantitative insight into light-driven spin dynamics occurring at buried interfaces of complex magnetic heterostructures, which can be tailored and functionalized for future optospintronic devices.

NbO₂ is a promising candidate for resistive switching devices due to an insulator-metal transition above room temperature, which is related to a phase transition from a distorted rutile structure to an undistorted one. However, the electrical resistivity of the NbO₂ thin films produced so far has been too low to achieve high on-off switching ratios. Here, we report on the structural, electrical, and optical characterization of single-crystalline NbO₂ (001) thin films grown by pulsed laser deposition on MgF₂ (001) substrates. An annealing step reduced the full width at half maximum of the NbO₂ (004) x-ray Bragg reflection by one order of magnitude, while the electrical resistivity of the films increased by two orders of magnitude to about 1 kΩcm at room temperature. Temperature-dependent resistivity measurements of an annealed sample revealed that below 650 K, two deep-level defects with activation energies of 0.25 eV and 0.37 eV dominate the conduction, while above 650 K, intrinsic conduction prevails. Optical characterization by spectroscopic ellipsometry and by absorption measurements with the electric field vector of the incident light perpendicular to the c-axis of the distorted rutile structure indicates the onset of fundamental absorption at about 0.76 eV at room temperature, while at 4 K, the onset shifts to 0.85 eV. These optical transitions are interpreted to take place across the theoretically predicted indirect bandgap of distorted rutile NbO₂.

We have investigated high temperature phase transitions in NaNbO₃ thin films epitaxially grown under tensile lattice strain on (110) DyScO₃ substrates using metal-organic vapor phase epitaxy. At room temperature, a very regular stripe domain pattern consisting of the monoclinic a₁a₂ ferroelectric phase was observed. Temperature-dependent studies of the refractive index and the optical bandgap as well as in situ high-resolution x-ray diffraction measurements prove a ferroelectric–ferroelectric phase transition in the range between 250 and 300 °C. The experimental results strongly suggest that the high-temperature phase exhibits a distorted orthorhombic a₁/a₂ crystal symmetry, with the electric polarization vector lying exclusively in the plane. A second phase transition was observed at about 500 °C, which presumably signifies the transition to the paraelectric phase. Both phase transitions show a pronounced temperature-dependent hysteresis, indicating first-order phase transitions.