Ferroelectric domain wall memory with embedded selector realized in LiNbO3 single crystals integrated on Si wafers.

Interfacial 'dead' layers between metals and ferroelectric thin films generally induce detrimental effects in nanocapacitors, yet their peculiar properties can prove advantageous in other electronic devices. Here, we show that dead layers with low Li concentration located at the surface of LiNbO3 ferroelectric materials can function as unipolar selectors. LiNbO3 mesa cells were etched from a single-crystal LiNbO3 substrate, and Pt metal contacts were deposited on their sides. Poling induced non-volatile switching of ferroelectric domains in the cell, and volatile switching in the domains in the interfacial (dead) layers, with the domain walls created within the substrate being electrically conductive. These features were also confirmed using single-crystal LiNbO3 thin films bonded to SiO2/Si wafers. The fabricated nanoscale mesa-structured memory cell with an embedded interfacial-layer selector shows a high on-to-off ratio (>106) and high switching endurance (~1010 cycles), showing potential for the fabrication of crossbar arrays of ferroelectric domain wall memories.

Temporary formation of highly conducting domain walls for non-destructive read-out of ferroelectric domain-wall resistance switching memories.

Erasable conductive domain walls in insulating ferroelectric thin films can be used for non-destructive electrical read-out of the polarization states in ferroelectric memories. Still, the domain-wall currents extracted by these devices have not yet reached the intensity and stability required to drive read-out circuits operating at high speeds. This study demonstrated non-destructive read-out of digital data stored using specific domain-wall configurations in epitaxial BiFeO3 thin films formed in mesa-geometry structures. Partially switched domains, which enable the formation of conductive walls during the read operation, spontaneously retract when the read voltage is removed, reducing the accumulation of mobile defects at the domain walls and potentially improving the device stability. Three-terminal memory devices produced 14 nA read currents at an operating voltage of 5 V, and operated up to T = 85 °C. The gap length can also be smaller than the film thickness, allowing the realization of ferroelectric memories with device dimensions far below 100 nm.

Stability of Polar Vortex Lattice in Ferroelectric Superlattices.

A novel mesoscale state comprising of an ordered polar vortex lattice has been demonstrated in ferroelectric superlattices of PbTiO3/SrTiO3. Here, we employ phase-field simulations, analytical theory, and experimental observations to evaluate thermodynamic conditions and geometric length scales that are critical for the formation of such exotic vortex states. We show that the stability of these vortex lattices involves an intimate competition between long-range electrostatic, long-range elastic, and short-range polarization gradient-related interactions leading to both an upper and a lower bound to the length scale at which these states can be observed. We found that the critical length is related to the intrinsic domain wall width, which could serve as a simple intuitive design rule for the discovery of novel ultrafine topological structures in ferroic systems.

Enhanced resistive switching in forming-free graphene oxide films embedded with gold nanoparticles deposited by electrophoresis.

Forming-free resistive random access memory (ReRAM) devices having low switching voltages are a prerequisite for their commercial applications. In this study, the forming-free resistive switching characteristics of graphene oxide (GO) films embedded with gold nanoparticles (Au Nps), having an enhanced on/off ratio at very low switching voltages, were investigated for non-volatile memories. The GOAu films were deposited by the electrophoresis method and as-grown films were found to be in the low resistance state; therefore no forming voltage was required to activate the devices for switching. The devices having an enlarged on/off ratio window of ∼10(6) between two resistance states at low voltages (<1 V) for repetitive dc voltage sweeps showed excellent properties of endurance and retention. In these films Au Nps were uniformly dispersed over a large area that provided charge traps, which resulted in improved switching characteristics. Capacitance was also found to increase by a factor of ∼10, when comparing high and low resistance states in GOAu and pristine GO devices. Charge trapping and de-trapping by Au Nps was the mechanism responsible for the improved switching characteristics in the films.

Some current problems in perovskite nano-ferroelectrics and multiferroics: kinetically-limited systems of finite lateral size.

We describe some unsolved problems of current interest; these involve quantum critical points in ferroelectrics and problems which are not amenable to the usual density functional theory, nor to classical Landau free energy approaches (they are kinetically limited), nor even to the Landau-Kittel relationship for domain size (they do not satisfy the assumption of infinite lateral diameter) because they are dominated by finite aperiodic boundary conditions.

Exploring vertex interactions in ferroelectric flux-closure domains.

Using piezoresponse force microscopy, we have observed the progressive development of ferroelectric flux-closure domain structures and Landau-Kittel-type domain patterns, in 300 nm thick single-crystal BaTiO3 platelets. As the microstructural development proceeds, the rate of change of the domain configuration is seen to decrease exponentially. Nevertheless, domain wall velocities throughout are commensurate with creep processes in oxide ferroelectrics. Progressive screening of macroscopic destabilizing fields, primarily the surface-related depolarizing field, successfully describes the main features of the observed kinetics. Changes in the separation of domain-wall vertex junctions prompt a consideration that vertex-vertex interactions could be influencing the measured kinetics. However, the expected dynamic signatures associated with direct vertex-vertex interactions are not resolved. If present, our measurements confine the length scale for interaction between vertices to the order of a few hundred nanometers.

Magnetoelectrics: is CdCr2S4 a multiferroic relaxor?

Materials showing simultaneous ferroelectric and magnetic ordering are attracting a great deal of interest because of their unusual physics and potential applications. Hemberger et al. have reported relaxor-like dielectric properties and colossal magnetocapacitance (in excess of 500%) for the cubic spinel compound CdCr2S4 and related isomorphs, concluding that CdCr2S4 is a multiferroic relaxor. We argue here, however, that their results might also be explained by a conductive artefact.

Structurally tailored hexagonal ferroelectricity and multiferroism in epitaxial YbFeO3 thin-film heterostructures.

Multiferroics have received a great deal of attention because of their fascinating physics of order-parameter cross-couplings and their potential for enabling new device paradigms. Considering the rareness of multiferroic materials, we have been exploring the possibility of artificially imposing ferroelectricity by structurally tailoring antiferromagnets in thin-film forms. YbFeO(3) (YbFO hereafter), a family of centrosymmetric rare-earth orthoferrites, is known to be nonferroelectric (space group Pnma). Here we report that a YbFO thin-film heterostructure fabricated by adopting a hexagonal template surprisingly exhibits nonferroelastic ferroelectricity with the Curie temperature of 470 K. The observed ferroelectricity is further characterized by an extraordinary two-step polarization decay, accompanied by a pronounced magnetocapacitance effect near the lower decay temperature, ~225 K. According to first-principles calculations, the hexagonal P6(3)/mmc-P6(3)mc-P6(3)cm consecutive transitions are primarily responsible for the observed two-step polarization decay, and the ferroelectricity originates from the c-axis-oriented asymmetric Yb 5d(z(2))-O 2p(z) orbital hybridization. Temperature-dependent magnetization curves further reveal an interesting phenomenon of spontaneous magnetization reversal at 83 K, which is attributed to the competition between two distinct magnetocrystalline anisotropy terms, Fe 3d and Yb 4f moments.

Ferroelectric-carbon nanotube memory devices.

One-dimensional ferroelectric nanostructures, carbon nanotubes (CNT) and CNT-inorganic oxides have recently been studied due to their potential applications for microelectronics. Here, we report coating of a registered array of aligned multi-wall carbon nanotubes (MWCNT) grown on silicon substrates by functional ferroelectric Pb(Zr,Ti)O3 (PZT) which produces structures suitable for commercial prototype memories. Microstructural analysis reveals the crystalline nature of PZT with small nanocrystals aligned in different directions. First-order Raman modes of MWCNT and PZT/MWCNT/n-Si show the high structural quality of CNT before and after PZT deposition at elevated temperature. PZT exists mostly in the monoclinic Cc/Cm phase, which is the origin of the high piezoelectric response in the system. Low-loss square piezoelectric hysteresis obtained for the 3D bottom-up structure confirms the switchability of the device. Current-voltage mapping of the device by conducting atomic force microscopy (c-AFM) indicates very low transient current. Fabrication and functional properties of these hybrid ferroelectric-carbon nanotubes is the first step towards miniaturization for future nanotechnology sensors, actuators, transducers and memory devices.

Unexpected controllable pair-structure in ferroelectric nanodomains.

The imminent inability of silicon-based memory devices to satisfy Moore's Law is approaching rapidly. Controllable nanodomains of ferroic systems are anticipated to enable future high-density nonvolatile memory and novel electronic devices. We find via piezoresponse force microscopy (PFM) studies on lead zirconate titanate (PZT) films an unexpected nanostructuring of ferroelectric-ferroelastic domains. These consist of c-nanodomains within a-nanodomains in proximity to a-nanodomains within c-domains. These structures are created and annihilated as pairs, controllably. We treat these as a new kind of vertex-antivertex pair and consider them in terms of the Srolovitz-Scott 4-state Potts model, which results in pairwise domain vertex instabilities that resemble the vortex-antivortex mechanism in ferromagnetism, as well as dislocation pairs (or disclination pairs) that are well-known in nematic liquid crystals. Finally, we show that these nanopairs can be scaled up to form arrays that are engineered at will, paving the way toward facilitating them to real technologies.

4d-5p orbital mixing and asymmetric In 4d-O 2p hybridization in InMnO3: a new bonding mechanism for hexagonal ferroelectricity.

Recent studies on the ferroelectricity origin of YMnO(3), a prototype of hexagonal manganites (h-RMnO(3), where R is a rare-earth-metal element), reveal that the d(0)-ness of a Y(3+) ion with an anisotropic Y 4d-O 2p hybridization is the main driving force of ferroelectricity. InMnO(3) (IMO) also belongs to the h-RMnO(3) family. However, the d(0)-ness-driven ferroelectricity cannot be expected because the trivalent In ion is characterized by a fully filled 4d orbital. Here we propose a new bonding mechanism of the hexagonal ferroelectricity in IMO: intra-atomic 4d(z(2))-5p(z) orbital mixing of In followed by asymmetric 4d(z(2))(In)-2p(z)(O) covalent bonding along the c axis.

Spin-canting-induced improper ferroelectricity and spontaneous magnetization reversal in SmFeO3.

SmFeO3, a family of centrosymmetric rare-earth orthoferrites, is known to be nonferroelectric. However, we have found that SmFeO3 is surprisingly ferroelectric at room temperature with a small polarization along the b axis of Pbnm. First-principles calculations indicate that the canted antiferromagnetic ordering with two nonequivalent spin pairs is responsible for this extraordinary polarization and that the reverse Dzyaloshinskii-Moriya interaction dominates over the exchange-striction mechanism in the manifestation of the improper ferroelectricity. SmFeO3 further exhibits an interesting phenomenon of spontaneous magnetization reversal at cryogenic temperatures. This reversal is attributed to the activation of the Sm-spin moment which is antiparallel to the Fe-spin moment below ∼5 K.

Fabrication and field emission study of novel rod-shaped diamond-like carbon nanostructures.

Novel sp(3) rich diamond-like carbon nanorod films were fabricated by a hot filament chemical vapour deposition technique. The results are indicative of a bottom-up synthesis process, which results in a hierarchical structure that consists of microscale papillae comprising numerous nanorods. The papillae have diameters ranging from 2 to 4 microm and the nanorods have diameters in the 35-45 nm range. A growth mechanism based on the vapour-liquid-solid mechanism is proposed that accounts for the morphological aspects at the microscale and nanoscale. Investigation of field emission properties of fabricated nanorods reveals a low turn-on field of about 4.9 V microm( - 1) at 1 nA and a high field-enhancement factor.

Settling the "dead layer" debate in nanoscale capacitors.

Permittivity peaks in single crystal thin film capacitors are strongly suppressed compared to bulk in the case of Pt/SrTiO3 /Pt, but are relatively unaffected in Pt/BaTiO3 /Pt structures. This is consistent with the recent suggestion that subtle variations in interfacial bonding between the dielectric and electrode are critical in determining the presence or absence of inherent dielectric "dead layers".

Non-Polar and Complementary Resistive Switching Characteristics in Graphene Oxide devices with Gold Nanoparticles: Diverse Approach for Device Fabrication.

Downscaling limitations and limited write/erase cycles in conventional charge-storage based non-volatile memories stimulate the development of emerging memory devices having enhanced performance. Resistive random-access memory (RRAM) devices are recognized as the next-generation memory devices for employment in artificial intelligence and neuromorphic computing, due to their smallest cell size, high write/erase speed and endurance. Unipolar and bipolar resistive switching characteristics in graphene oxide (GO) have been extensively studied in recent years, whereas the study of non-polar and complementary switching is scarce. Here we fabricated GO-based RRAM devices with gold nanoparticles (Au Nps). Diverse types of switching behavior are observed by changing the processing methods and device geometry. Tri-layer GO-based devices illustrated non-polar resistive switching, which is a combination of unipolar and bipolar switching. Five-layer GO-based devices depicted complementary resistive switching having the lowest current values ~12 µA; and this structure is capable of resolving the sneak path issue. Both devices show good retention and endurance performance. Au Nps in tri-layer devices assisted the conducting path, whereas in five-layer devices, Au Nps layer worked as common electrodes between co-joined cells. These GO-based devices with Au Nps comprising different configuration are vital for practical applications of emerging non-volatile resistive memories.

Giant polarization in super-tetragonal thin films through interphase strain.

Strain engineering has emerged as a powerful tool to enhance the performance of known functional materials. Here we demonstrate a general and practical method to obtain super-tetragonality and giant polarization using interphase strain. We use this method to create an out-of-plane-to-in-plane lattice parameter ratio of 1.238 in epitaxial composite thin films of tetragonal lead titanate (PbTiO3), compared to 1.065 in bulk. These thin films with super-tetragonal structure possess a giant remanent polarization, 236.3 microcoulombs per square centimeter, which is almost twice the value of known ferroelectrics. The super-tetragonal phase is stable up to 725°C, compared to the bulk transition temperature of 490°C. The interphase-strain approach could enhance the physical properties of other functional materials.

Nonequilibrium ferroelectric-ferroelastic 10 nm nanodomains: wrinkles, period-doubling, and power-law relaxation.

Since the 1935 work of Landau-Lifshitz and of Kittel in 1946 all ferromagnetic, ferroelectric, and ferroelastic domains have been thought to be straight-sided with domain widths proportional to the square root of the sample thickness. We show in the present work that this is not true. We also discover period doubling domains predicted by Metaxas et al (2008 Phys. Rev. Lett. 99 217208) and modeled by Wang and Zhao (2015 Sci. Rep. 5 8887). We examine non-equilibrium ferroic domain structures in perovskite oxides with respect to folding, wrinkling, and relaxation and suggest that structures are kinetically limited and in the viscous flow regime predicted by Metaxas et al in 2008 but never observed experimentally. Comparisons are made with liquid crystals and hydrodynamic instabilities, including chevrons, and fractional power-law relaxation. As Shin et al (2016 Soft Matter 12 3502) recently emphasized: 'An understanding of how these folds initiate, propagate, and interact with each other is still lacking'. Inside each ferroelastic domain are ferroelectric 90° nano-domains with 10 nm widths and periodicity in agreement with the 10 nm theoretical minima predicted by Feigl et al (2014 Nat. Commun. 5 4677). Evidence is presented for domain-width period doubling, which is common in polymer films but unknown in ferroic domains. A discussion of the folding-to-period doubling phase transition model of Wang and Zhao is included.

Investigating the effects of reduced size on the properties of ferroelectrics.

A series of experiments has been undertaken to understand more about the fundamental origin of the thickness-induced permittivity collapse often observed in conventional thin film ferroelectric heterostructures. The various experiments are discussed, highlighting the eventual need to examine permittivity collapse in thin film single crystal material. It has been seen that dielectric collapse is not a direct consequence of reduced size, and neither is it a consequence of unavoidable physics associated with the ferroelectric-electrode boundary. Research on three-dimensional shape-constrained ferroelectrics, emphasizing self-assembled structures based on nanoporous alumina templates and on FIB-milled single crystals, is also presented, and appears to represent an exciting area for ongoing research.

Spin phonon interactions and magnetodielectric effects in multiferroic BiFeO3-PbTiO3.

Here, we report the observation of magneto-dielectric and magneto-structural coupling in (1 - x)BiFeO3-xPbTiO3 i.e.(1 - x)BF-xPT) solid solutions with compositions in the vicinity of morphotropic phase boundary, as manifested by a combination of temperature dependent magnetic, Raman and dielectric measurements. Whilst x-ray diffraction and Raman spectroscopy suggest absence of any structural phase transition between 90-300 K, temperature dependent magnetic studies reveal magnetic anomalies in the solid solutions. These results are complemented by identical observations in the dielectric measurements at similar temperatures indicating a coupling between magnetic and electric order parameters. Further, Raman studies on rhombohedral i.e. x = 0.20 samples reveal a coupling between the magnetic structure and the lattice, causing spin-phonon interactions that are possibly responsible for observed magneto-dielectric effects. Our results illustrate that the phase transitions in BiFeO3-PbTiO3 system are fewer than expected and are attributed to a spatial averaging in an inhomogeneous albeit single-phase material due to clustering of Fe- or Ti-ions on different length scales.

Implementing Room-Temperature Multiferroism by Exploiting Hexagonal-Orthorhombic Morphotropic Phase Coexistence in LuFeO3 Thin Films.

Room-temperature multiferroism in LuFeO3 (LFO) films is demonstrated by exploiting the orthorhombic-hexagonal (o-h) morphotrophic phase coexistence. The LFO film further reveals a magnetoelectric coupling effect that is not shown in single-phase (h- or o-) LFO. The observed multiferroism is attributed to the combination of sufficient polarization from h-LFO and net magnetization from o-LFO.