Xiaoxing Cheng

Fast Magnetic Domain-Wall Motion in a Ring-Shaped Nanowire Driven by a Voltage

Magnetic domain-wall motion driven by a voltage dissipates much less heat than by a current, but none of the existing reports have achieved speeds exceeding 100 m/s. Here phase-field and finite-element simulations were combined to study the dynamics of strain-mediated voltage-driven magnetic domain-wall motion in curved nanowires. Using a ring-shaped, rough-edged magnetic nanowire on top of a piezoelectric disk, we demonstrate a fast voltage-driven magnetic domain-wall motion with average velocity up to 550 m/s, which is comparable to current-driven wall velocity. An analytical theory is derived to describe the strain dependence of average magnetic domain-wall velocity. Moreover, one 180° domain-wall cycle around the ring dissipates an ultrasmall amount of heat, as small as 0.2 fJ, approximately 3 orders of magnitude smaller than those in current-driven cases. These findings suggest a new route toward developing high-speed, low-power-dissipation domain-wall spintronics.

Phase-Field Based Multiscale Modeling of Heterogeneous Solid Electrolytes: Applications to Nanoporous Li3PS4

Modeling the effective ion conductivities of heterogeneous solid electrolytes typically involves the use of a computer-generated microstructure consisting of randomly or uniformly oriented fillers in a matrix. However, the structural features of the filler/matrix interface, which critically determine the interface ion conductivity and the microstructure morphology, have not been considered during the microstructure generation. Using nanoporous β-Li3PS4 electrolyte as an example, we develop a phase-field model that enables generating nanoporous microstructures of different porosities and connectivity patterns based on the depth and the energy of the surface (pore/electrolyte interface), both of which are predicted through density functional theory (DFT) calculations. Room-temperature effective ion conductivities of the generated microstructures are then calculated numerically, using DFT-estimated surface Li-ion conductivity (3.14 × 10-3 S/cm) and experimentally measured bulk Li-ion conductivity (8.93 × 10-7 S/cm) of β-Li3PS4 as the inputs. We also use the generated microstructures to inform effective medium theories to rapidly predict the effective ion conductivity via analytical calculations. When porosity approaches the percolation threshold, both the numerical and analytical methods predict a significantly enhanced Li-ion conductivity (1.74 × 10-4 S/cm) that is in good agreement with experimental data (1.64 × 10-4 S/cm). The present phase-field based multiscale model is generally applicable to predict both the microstructure patterns and the effective properties of heterogeneous solid electrolytes.

Anisotropic polarization-induced conductance at a ferroelectric–insulator interface

Coupling between different degrees of freedom, that is, charge, spin, orbital and lattice, is responsible for emergent phenomena in complex oxide heterostrutures1,2. One example is the formation of a two-dimensional electron gas (2DEG) at the polar/non-polar LaAlO3/SrTiO3 (LAO/STO)3–7 interface. This is caused by the polar discontinuity and counteracts the electrostatic potential build-up across the LAO film3. The ferroelectric polarization at a ferroelectric/insulator interface can also give rise to a polar discontinuity8–10. Depending on the polarization orientation, either electrons or holes are transferred to the interface, to form either a 2DEG or two-dimensional hole gas (2DHG)11–13. While recent first-principles modelling predicts the formation of 2DEGs at the ferroelectric/insulator interfaces9,10,12–14, experimental evidence of a ferroelectrically induced interfacial 2DEG remains elusive. Here, we report the emergence of strongly anisotropic polarization-induced conductivity at a ferroelectric/insulator interface, which shows a strong dependence on the polarization orientation. By probing the local conductance and ferroelectric polarization over a cross-section of a BiFeO3–TbScO3 (BFO/TSO) (001) heterostructure, we demonstrate that this interface is conducting along the 109° domain stripes in BFO, whereas it is insulating in the direction perpendicular to these domain stripes. Electron energy-loss spectroscopy and theoretical modelling suggest that the anisotropy of the interfacial conduction is caused by an alternating polarization associated with the ferroelectric domains, producing either electron or hole doping of the BFO/TSO interface.The striped polarization domains in a BiFeO3/TbScO3 heterostructure induce alternating p- and n-type doping at the interface, giving rise to strongly anisotropic in-plane conductance.

Defect-assisted Reorganization of Ferroelectric Domain Walls Revealed by Aberration-corrected Electron Microscopy

Domain walls (DWs) in ferroelectrics are quasi-2D functional units possessing peculiar properties that are absent in the bulk materials. The ability to produce ordered patterns of DWs is a prerequisite both for the fundamental study of DW properties and the design of novel nanodevices based on DW functionalites. In recent years, extensive efforts have been made towards fabricating periodic domain and DW structures in ferroelectric thin films, mainly through modifying elastic and electrostatic boundary conditions at the film interfaces. One of the major limitations of such a method, however, has been that once the choice of substrate is set further modifications to control or alter domain patterns during material synthesis becomes difficult, reducing the parameter space for creating more complex structures with ordered DW patterns and thus imposing resitrictions on the functionalities of the system. Here, with a combination of aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS), we show that atomically-thin charged defects that are deliberately introduced during the film growth can be used as nano-building-blocks for tailoring and reorganizing DW patterns. Such defect engineering can produce novel periodic mixed-type DW structures that are inaccessible by conventional boundary-condition-tuning methods

Correction to Fast Magnetic Domain-Wall Motion in a Ring-Shaped Nanowire Driven by a Voltage

T original article [J.-M. Hu et al., Nano Letters 2016, 16 (4), 2341−2348] contains a typographical error in a grant number in the Acknowledgments section. “The work is supported by National Science Foundation (NSF) with Grant Nos. of DMR-1235092 (J.-M.H.), ...” in the original article should read: “The work is supported by National Science Foundation (NSF) with Grant Nos. of DMR-1234096 (J.-M.H.), ...”. All of the other grant numbers in the original article are correctly listed. Addition/Correction

Interaction between Ferroelectric Polarization and Defects in BiFeO3 Thin Films

Nanoscale impurity defects, with structures different from host materials, are known to commonly exist in functional complex oxides as a result of slight stoichiometry fluctuations that occur during material growth. Local perturbations induced by these defects, such as charge, strain, and atomic interaction, could have a profound effect on the physical properties of oxide nanomaterials. A direct correlation of the defects to the material functionalities, however, are often hampered by the lack of a fundamental understanding of the microscopic mechanisms underlying the coupling between the defects and the host lattice. Here, with a combination of atomic-scale STEM and in situ TEM, we perform a systematic study of atomic-scale polarization structures and microscopic domain-switching processes in the prototypical multiferroic BiFeO3 thin films to explore the interaction between ferroelectric polarization and defects.