Electric Field Writing of Ferroelectric Nano‐Domains Near 71° Domain Walls with Switchable Interfacial Conductivity

Conducting ferroelectric domain walls attract a wide range of research interest due to their promising applications in nanoelectronics. In this study, we reveal an unexpected enhanced conductivity near the well‐aligned 71° nonpolar domain walls in BiFeO₃. Such an interfacial conductivity is induced by the creation of up‐polarized nano‐domains near the 71° domain walls, as revealed by the combination of the piezo‐response force microscopy (PFM) and conducting atomic force microscopy (c‐AFM) imaging techniques, as well as phase‐field simulations. The upward polarized domains are suggested to lower the Schottky barrier at the interface between the tip and sample surface, and then give rise to the enhanced interfacial conductivity. The result provides a new strategy to tune the local conductance in ferroelectric materials and opens up new opportunities to design novel nanoelectronic devices.     

Magnetic domain configurations in exchange-coupled NiO/Co bilayer films

The magnetic domain configurations of exchange-coupled NiO/Co bilayers were investigated by magnetic force microscopy. These bilayers exhibit a well-defined uniaxial anisotropy resulting from the deposition at oblique incidence of the NiO layer. Two types of magnetic contrast are identified: (i) bipolar contrast due to 180° Néel walls in the parts of the walls which are parallel to the easy axis of magnetization, and (ii) monopolar contrast in the parts of the walls separating domains with meeting head-on magnetizations. These latter domain walls have a zigzag shape which represents a compromise between a decrease in the local density of magnetostatic energy and an increase in the wall length. The effect of the Co thickness of the shape on the domains is also discussed.     

Shape instability in out of equilibrium magnetic domains observed in ultrathin magnetic films with perpendicular anisotropy

The magnetic configurations induced by the growth process in a thin film with perpendicular magnetisation have been observed by magnetic force microscopy (MFM). The FePd thin film has been grown by molecular beam epitaxy. A high uniaxial chemical ordering of the alloy into the tetragonal L1₀ structure induces the development of a large perpendicular anisotropy. As the growth process is performed below the Curie temperature of the FePd alloy, domain nucleation occurs during the growth process. The magnetic configuration has been imaged in the as grown state. As the equilibrium size of the magnetic domains decreases when the thickness of the layer increases, the domains obtained from spontaneous nucleation at the beginning of the growth of the thin film are submitted to very large strains as the layer thickness increases. At low thicknesses (low strains), the domain wall instability gives rise to an undulation of the domain walls. Thereafter, it leads to the formation of well-defined magnetic fingers, thus giving birth to the coexistence of two length scale in the domain structure. A quantitative estimation of the strain leading to the fingering instability is obtained. Last, the implications of these observations on the kinetic of domain wall distortion in ultrathin layers are discussed.     

Domain wall propagation in antiferroelectric donor chains as a possible explanation for the 1-D transport properties of complex TCNQ salts

The structures are explored of viewing the structure of the permanent electric dipoles on the donor molecules in complex TCNQ salts as a set of independent, Ising-type, antiferro-electric chains. The chain character will entail the thermal excitation of 1-d domains. The walls in between these will produce extra local polarized charges on the TCNQ chainsm, so that wall propagation should contribute to the conductivity. The observed temperature and frequency dependences of conductivity and permeability can likewise be explained.     

Simultaneous observation of atomic step and domain wall structure of ultrathin Co films by magnetic force microscopy in ultrahigh vacuum

We have applied magnetic force microscopy in ultrahigh vacuum to study the correlation between the atomic step and magnetic domain wall structure of ultrathin Co films prepared in situ on Au(111) substrates. For the first time we were able to achieve high-resolution images showing simultaneously a clear domain wall contrast and the underlying atomic step structure. Although for in-plane magnetized Co films the domain walls were found to run preferentially in a direction perpendicular to the steps, no such correlation could be observed for out-of-plane magnetized Co films. Received: 3 June 1999 / Accepted: 4 June 1999 / Published online: 29 July 1999     

Conduction of topologically protected charged ferroelectric domain walls

We report on the observation of nanoscale conduction at ferroelectric domain walls in hexagonal HoMnO(3) protected by the topology of multiferroic vortices using in situ conductive atomic force microscopy, piezoresponse force microscopy, and Kelvin-probe force microscopy at low temperatures. In addition to previously observed Schottky-like rectification at low bias [Phys. Rev. Lett. 104, 217601 (2010)], conductance spectra reveal that negatively charged tail-to-tail walls exhibit enhanced conduction at high forward bias, while positively charged head-to-head walls exhibit suppressed conduction at high reverse bias. Our results pave the way for understanding the semiconducting properties of the domains and domain walls in small-gap ferroelectrics.     

Coercivity of precipitation hardened cobalt rare earth 17:2 permanent magnets

The influence of the microstructure on the coercivity has been investigated by means of transmission electron microscopy. It is shown that a thin coherent (CoCu)₅Sm-cell boumdary phase, separating cells of 17:2-crystal structure, acts as a pinning centre for magnetic domain walls. The attractive interaction force is interpreted in terms of a micromagnetic theory for domain wall pinning. The coercive force is determined by the domain wall energy gradient and by the magnetoelastic coupling energy between domain wall stresses and lattice deformation strains. The calculated coercive force due to the lattice mismatch, originated by the cellular coherent precipitation structure, is comparable to the experimentally obtained values.     

Thermal motion of one-dimensional domain walls in monolayers of a polar polymer observed by Video-STM

Scanning tunneling microscopy (STM) has been used to investigate monolayers of the ferroelectric copolymer polyvinylidenefluoride/trifluoroethylene P(VDF/TrFE) showing images of ordered polymer monolayers. By scanning with video frame rate, direct observation of the motion of onedimensional domain walls was also possible for the first time. The images clearly show domain walls normal to the polymer chains. From measurements of the temperature dependence of the domain wall velocities the activation energy for the thermally generated kink motion was estimated. These results are compared with theoretical models describing domain wall motion in ferroelectric PVDF.     

Domain walls in gapped graphene

The electronic properties of a particular class of domain walls in gapped graphene are investigated. We show that they can support midgap states which are localized in the vicinity of the domain wall and propagate along its length. With a finite density of domain walls, these states can alter the electronic properties of gapped graphene significantly. If the midgap band is partially filled, the domain wall can behave like a one-dimensional metal embedded in a semiconductor and could potentially be used as a single-channel quantum wire.     

Atomic-scale magnetic domain walls in quasi-one-dimensional Fe nanostripes

Fe nanostripes on W(110) are investigated by Kerr magnetometry and spin-polarized scanning tunneling microscopy (SP-STM). An Arrhenius law is observed for the temperature dependent magnetic susceptibility indicating a one-dimensional magnetic behavior. The activation energy for creating antiparallel spin blocks indicates extremely narrow domain walls with a width on a length scale of the lattice constant. This is confirmed by imaging the domain wall by SP-STM. This information allows the quantification of the exchange stiffness and the anisotropy constant.     

DYNAMICS OF PLANAR RELATIVISTIC DOMAIN WALLS

The dynamics of planar relativistic domain walls is investigated with the help of a new method proposed by Arodz and Larsen. Two solutions are found: one is a steady state domain wall, the other is a nonstationary domain wall. They move with the same constant velocity in the laboratory frame coordinates. When the velocity approaches light velocity c, the "width" of the nonstationary wall inflates very slowly. These domain walls can disappear again soon after their creation in the early universe by moving away from our visible universe. So they would not dominate the universe completely, but keep the observed universe approximately isotropic and homogeneous.     

Remanence due to wall magnetization and counterintuitive magnetometry data in 200-nm films of Ni

200-nm-thick Ni films in an epitaxial Cu/Ni/Cu/Si(001) structure are expected to have an in-plane effective magnetic anisotropy. However, the in-plane remanence is only 42%, and magnetic force microscopy domain images suggest perpendicular magnetization. Quantitative magnetic force microscopy analysis can resolve the inconsistencies and show that (i) the films have perpendicular domains capped by closure domains with magnetization canted at 51 degrees from the film normal, (ii) the magnetization in the Bloch domain walls between the perpendicular domains accounts for the low in-plane remanence, and (iii) the perpendicular magnetization process requires a short-range domain wall motion prior to wall-magnetization rotation and is nonhysteretic, whereas the in-plane magnetization requires long-range motion before domain-magnetization rotation and is hysteretic.     

Effects of Sm substitution on ferroelectric domains and conductivity in bismuth ferrite ceramics

The large piezoelectric coefficient and multiferroicity of bismuth ferrite (BFO) make it an attractive candidate for lead-free ferroelectric devices. However, large leakage currents have limited broader applications. Rare-earth substitutions in BFO have been shown to improve ferroelectric and magnetic properties. In this work, we employed piezoresponse and conductive atomic force microscopy to study ferroelectric domains in Bi_1-xSm_xFeO₃ (x = 0–0.150) grown by the co-precipitation method. The combined piezoresponse and conductivity measurements can directly visualize the local ferroelectric domains under different sample bias. At Sm mol% > 7.5, Sm-substitution effectively lowers defect-generated conductivity. At Sm mol% < 7.5, conductivity increases due to conductive domain walls inside sample grains. The surfaces of these conductive samples exhibit a p-type rectifying behavior while the bulk is n-type. Our work details how the local piezoelectric properties and transport behaviors of BFO ceramics change as a function of Sm-substitution.     

Domain wall roughness in epitaxial ferroelectric PbZr0.2Ti0.8O3 thin films

The static configuration of ferroelectric domain walls was investigated using atomic force microscopy on epitaxial PbZr(0.2)Ti(0.8)O(3) thin films. Measurements of domain wall roughness reveal a power-law growth of the correlation function of relative displacements B(L) alpha L(2zeta) with zeta approximately 0.26 at short length scales L, followed by an apparent saturation at large L. In the same films, the dynamic exponent mu was found to be approximately 0.6 from independent measurements of domain wall creep. These results give an effective domain wall dimensionality of d = 2.5, in good agreement with theoretical calculations for a two-dimensional elastic interface in the presence of random-bond disorder and long-range dipolar interactions.     

The effect of annealing temperature on the morphology and piezoelectric characteristics of BaTiO3 nanofibers and domain switching under different temperatures

Effects of annealing temperature (600–750 °C) on crystalline structure, the morphology and piezoresponse hysteresis loops of BaTiO₃ nanofibers prepared by electrospinning are characterized by X-ray diffraction, scanning electronic microscopy, transmission electron microscope and piezoresponse force microscope. When the annealing temperature is 700 °C, the nanofibers become smoother and have a diameter of 100–300 nm. Meanwhile the typical butterfly-shaped amplitude loop and 180°phase change represents the best ferroelectric and piezoelectric properties at 700 °C. So the 700 °C was found to be optimum for good piezoelectric characteristics at annealing temperature of 600 °C–750 °C. In order to give more clear evolution of domain states at different external fields, the three dimensional topographic and phase images of the nanofiber at different temperatures are observed by piezoresponse force microscope. The 90° domain switching is observed during heating from room temperature to 125 °C and the domain switching tends to be stable when the temperature exceeds a critical value. The thermal stress due to the high temperatures is responsible for switching mechanism from the perspective of equilibrium state free energy. This work suggests that the temperature variation should be considered while designing the ferroelectric devices based on one dimensional material.     

Direct observation of domain-wall configurations transformed by spin currents

Direct observations of current-induced domain-wall propagation by spin-polarized scanning electron microscopy are reported. Current pulses move head-to-head as well as tail-to-tail walls in submicrometer Fe20Ni80 wires in the direction of the electron flow, and a decay of the wall velocity with the number of injected current pulses is observed. High-resolution images of the domain walls reveal that the wall spin structure is transformed from a vortex to a transverse configuration with subsequent pulse injections. The change in spin structure is directly correlated with the decay of the velocity.     

Hopping domain wall induced by paired adatoms on an atomic wire: si(111)-(5 x 2)-Au

We observed an inhomogeneous fluctuation along one-dimensional atomic wires self-assembled on a Si(111) surface using scanning tunneling microscopy. The fluctuation exhibits dynamic behavior at room temperature and is observed only in a specific geometric condition; the spacing between two neighboring adatom defects is discommensurate with the wire lattice. Upon cooling, the dynamic fluctuation freezes to show the existence of an atomic-scale dislocation or domain wall induced by such "unfavorably" paired adatoms. The microscopic characteristics of the dynamic fluctuation are explained in terms of a hopping solitonic domain wall, and a local potential for this motion imposed by the adatoms is quantified.     

Metal–insulator transition induced by non-stoichiometry of surface layer and molecular reactions on single crystal KTaO3

In the study we present results on topography, morphology, chemical composition, electronic structure and electrical properties of the (100) surface layer of KTaO₃ single crystal caused by sputtering with Ar⁺ ion beam with energy of 1 keV. Several surface sensitive techniques, i.e. X-ray photoelectron spectroscopy (XPS), local conductivity of atomic force microscopy (LC-AFM), and Kelvin Probe Force Microscopy (KPFM) were used. The observed changes in the electronic structure were explained as a result of the chemical decomposition of the surface layer. A correlation between the electronic states which appeared in the energy gap and the changes in charge state of Ta ions was found. The activation energy related to averaged local conductivity temperature dependence was estimated from Arrhenius plot. It was also found, that variations in the local contact potential difference (LCPD) indicated changes in the chemical composition in nano-scale. The chemical reconstruction of the KTaO₃ surface modified by Ar⁺ ion beam was deduced. This non-homogeneity corresponded to 2-D non-homogeneity of the local electric conduction (LC-AFM), which occurred within nano-areas after sputtering. Chemical reactivity of the modified surface with CO₂ and O₂ was observed. The reversibility of the Ar⁺ induced loss of oxygen non-stoichiometry was observed after the sample was exposed to various doses of O₂. The successful reversibility occurred after oxidation process at high temperature, i.e. 300 °C.     

Domain wall motion in nanowires

We investigated the motion of domain walls in ferromagnetic cylindrical nanowires by solving the Landau–Lifshitz–Gilbert equation numerically for a classical spin model in which energy contributions from exchange, crystalline anisotropy, dipole–dipole interactions, and a driving magnetic field are considered. Depending on the diameter, either transverse domain walls or vortex walls are found. A transverse domain wall is observed for diameters smaller than the exchange length of the given system. In this case, the system effectively behaves one dimensionally and the domain wall velocity agrees with the result of Slonczewski for one-dimensional walls. For larger diameters, a crossover to a vortex wall sets in which enhances the domain wall velocity drastically. For a vortex wall the domain wall velocity is described by the Walker formula.     

Conduction through 71° domain walls in BiFeO3 thin films

Local conduction at domains and domain walls is investigated in BiFeO(3) thin films containing mostly 71° domain walls. Measurements at room temperature reveal conduction through 71° domain walls. Conduction through domains could also be observed at high enough temperatures. It is found that, despite the lower conductivity of the domains, both are governed by the same mechanisms: in the low voltage regime, electrons trapped at defect states are temperature activated but the current is limited by the ferroelectric surface charges; in the large voltage regime, Schottky emission takes place and the role of oxygen vacancies is that of selectively increasing the Fermi energy at the walls and locally reducing the Schottky barrier. This understanding provides the key to engineering conduction paths in BiFeO(3).