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Pure and other concentrations of cobalt (Co2+) ions incorporated into cuprous oxide Cu2−xCoxO (x = 0–14 mM) thin films were successfully deposited in fluorine-doped tin oxide (FTO) glass substrate by employing electro deposition technique. The crystallite size of pure and Co-doped thin films was investigated from 29.03 to 43.38 nm using X-ray diffraction (XRD) patterns. Scanning electron microscope (SEM) images display three-sided pyramid shape morphology of pure Cu2O thin films that get significantly changed, as and when Co concentrations get increased. The optical bandgap value gets continuously increased from 2.128 to 2.297 eV for a 0–14 mM change in Co doping concentration. Vibrating sample magnetometer (VSM) pictures the film that exhibits a better ferromagnetic property with a saturation magnetism of 159.96 E−6 emu for 14 mM Co dopant. X-ray photoelectron spectroscopy (XPS) confirm the presence of cobalt (Co2+) as a dopant in the host Cu2O thin films. The Cu2−86Co14O (x = 14 mM) thin film has a higher specific capacitance of 164.90 F/g with 3.66 Wh/kg energy density at the current density of 2 A/g. It has a good energy and a power density in the higher concentration Co-doped Cu2O thin films applicable for energy storage devices.

A semi-flexible binder-free graphitic carbon nitride (g-C3N4) intercalated polypyrrole (PPy) nanocomposite thin film was prepared via hybrid electrospray technique. A homogeneous thin film was achieved by the controlled spray process. A deposited thin film illustrated the amorphous nature of composite with high surface purity and good chemical composition. The surface analysis confirmed the smoother surface with well-defined distinct phases of g-C3N4 and PPy matrix in the composite thin film. The electrochemical studies exhibited that a maximum areal capacity of the thin film was achieved to be 289.6 mF/cm2 at a current density of 0.4 mA/cm2, which is higher than that of pristine PPy (194.8 mF/cm2). The heterostructure electrode illustrated that the life cycles of 99% could be up to 10,000 cycles with low resistance due to the synergetic effect of g-C3N4 and PPy matrix with strong material adhesion (hybrid spray process) on the surface. A binder-free heterostructure thin film will play a significant role in the energy storage application.

Magnetostrictive materials transduce magnetic and mechanical energies and when combined with piezoelectric elements, evoke magnetoelectric transduction for high-sensitivity magnetic field sensors and energy-efficient beyond-CMOS technologies. The dearth of ductile, rare-earth-free materials with high magnetostrictive coefficients motivates the discovery of superior materials. Fe1−xGax alloys are amongst the highest performing rare-earth-free magnetostrictive materials; however, magnetostriction becomes sharply suppressed beyond x = 19% due to the formation of a parasitic ordered intermetallic phase. Here, we harness epitaxy to extend the stability of the BCC Fe1−xGax alloy to gallium compositions as high as x = 30% and in so doing dramatically boost the magnetostriction by as much as 10x relative to the bulk and 2x larger than canonical rare-earth based magnetostrictors. A Fe1−xGax − [Pb(Mg1/3Nb2/3)O3]0.7−[PbTiO3]0.3 (PMN-PT) composite magnetoelectric shows robust 90° electrical switching of magnetic anisotropy and a converse magnetoelectric coefficient of 2.0 × 10−5 s m−1. When optimally scaled, this high coefficient implies stable switching at ~80 aJ per bit.

The ultimate challenge for the development of new multiferroics with enhanced properties lies in achieving nanoscale control of the coupling between different ordering parameters. In oxide-based multiferroics, substitutional cation dopants offer the unparalleled possibility to modify both the electric and magnetic properties at a local scale. Herein it is demonstrated the formation of a dopant-controlled polar pattern in BiFeO3 leading to the spontaneous instauration of periodic polarization waves. In particular, nonpolar Ca-doped rich regions act as spacers between consecutive dopant-depleted regions displaying coupled ferroelectric states. This alternation of layers with different ferroelectric state creates a novel vertical polar structure exhibiting giant polarization gradients as large as 70 μC cm–2 across 30 A thick domains. The drastic change in the polar state of the film is visualized using high-resolution differential phase-contrast imaging able to map changes in ferroelectric polarization at at...

Author(s): Zhao, W; Kim, J; Huang, X; Zhang, L; Pesquera, D; Velarde, GAP; Gosavi, T; Lin, CC; Nikonov, DE; Li, H; Young, IA; Ramesh, R; Martin, LW | Abstract: The rapid development of computing applications demands novel low-energy consumption devices for information processing. Among various candidates, magnetoelectric heterostructures hold promise for meeting the required voltage and power goals. Here, a route to low-voltage control of magnetism in 30nnm Fe0.5Rh0.5/100nnm 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 (PMN-PT) heterostructures is demonstrated wherein the magnetoelectric coupling is achieved via strain-induced changes in the Fe0.5Rh0.5 mediated by voltages applied to the PMN-PT. We describe approaches to achieve high-quality, epitaxial growth of Fe0.5Rh0.5 on the PMN-PT films and, a methodology to probe and quantify magnetoelectric coupling in small thin-film devices via studies of the anomalous Hall effect. By comparing the spin-flop field change induced by temperature and external voltage, the magnetoelectric coupling coefficient is estimated to reach ≈7 × 10−8ns m−1 at 325 K while applying a −0.75nV bias.

Using epitaxial stabilization, we synthesized single-phase (001)-oriented thin films of DyFe2O4+x on (111) MgAl2O4 substrates by molecular-beam epitaxy. The metastable DyFe2O4 polymorph formed is isostructural to known trigonal ferrimagnetic RFe2O4 phases with space group R3̄m, where R = Ho to Lu. The epitaxial DyFe2O4 thin films have two in-plane orientation relationships: [100] DyFe2O4 || 211̄ MgAl2O4 plus a twin variant related by a 60° in-plane rotation. DyFe2O4 is not bulk stable and has never been synthesized before. Indeed, it has been predicted to be on the edge energetically of what may be possible to stabilize. The fact that the RFe2O4 phase is stable for all elements leading up to dysprosium (Ho–Lu) leads us to believe that DyFe2O4 could be a “remnant metastable phase,” one which, given the right thermodynamic conditions, could become the lowest free energy phase. We find that although we are able to get structurally very close to R3̄m DyFe2O4, the films are not stoichiometric as they have an increased c lattice parameter, indicative of extra oxygen as is sometimes seen in other RFe2O4 phases. The unintended surplus oxygen opens questions regarding what may be achievable using such tricks as epitaxial stabilization to access metastable phases and whether this indeed constitutes “remnant metastability.”

Electric-field (E-field) control of magnetism enabled by multiferroic materials has the potential to revolutionize the landscape of present memory devices plagued with high energy dissipation. To date, this E-field controlled multiferroic scheme has only been demonstrated at room temperature using BiFeO3 films grown on DyScO3, a unique and expensive substrate, which gives rise to a particular ferroelectric domain pattern in BiFeO3. Here, we demonstrate reversible electric-field-induced switching of the magnetic state of the Co layer in Co/BiFeO3 (BFO) (001) thin film heterostructures fabricated on (001) SrTiO3 (STO) substrates. The angular dependence of the coercivity and the remanent magnetization of the Co layer indicates that its easy axis reversibly switches back and forth 45° between the (100) and the (110) crystallographic directions of STO as a result of alternating application of positive and negative voltage pulses between the patterned top Co electrode layer and the (001) SrRuO3 (SRO) layer on which the ferroelectric BFO is epitaxially grown. The coercivity (HC) of the Co layer exhibits a hysteretic behavior between two states as a function of voltage. A mechanism based on the intrinsic magnetoelectric coupling in multiferroic BFO involving projection of antiferromagnetic G-type domains is used to explain the observation. We have also measured the exact canting angle of the G-type domain in strained BFO films for the first time using neutron diffraction. These results suggest a pathway to integrating BFO-based devices on Si wafers for implementing low power consumption and nonvolatile magnetoelectronic devices.

Ultrathin ferroelectric materials could potentially enable low-power logic and nonvolatile memories1,2. As ferroelectric materials are made thinner, however, the ferroelectricity is usually suppressed. Size effects in ferroelectrics have been thoroughly investigated in perovskite oxides—the archetypal ferroelectric system3. Perovskites, however, have so far proved unsuitable for thickness scaling and integration with modern semiconductor processes4. Here we report ferroelectricity in ultrathin doped hafnium oxide (HfO2), a fluorite-structure oxide grown by atomic layer deposition on silicon. We demonstrate the persistence of inversion symmetry breaking and spontaneous, switchable polarization down to a thickness of one nanometre. Our results indicate not only the absence of a ferroelectric critical thickness but also enhanced polar distortions as film thickness is reduced, unlike in perovskite ferroelectrics. This approach to enhancing ferroelectricity in ultrathin layers could provide a route towards polarization-driven memories and ferroelectric-based advanced transistors. This work shifts the search for the fundamental limits of ferroelectricity to simpler transition-metal oxide systems—that is, from perovskite-derived complex oxides to fluorite-structure binary oxides—in which ‘reverse’ size effects counterintuitively stabilize polar symmetry in the ultrathin regime. Enhanced switchable ferroelectric polarization is achieved in doped hafnium oxide films grown directly onto silicon using low-temperature atomic layer deposition, even at thicknesses of just one nanometre.

Author(s): Everhardt, AS; Dc, M; Huang, X; Sayed, S; Gosavi, TA; Tang, Y; Lin, CC; Manipatruni, S; Young, IA; Datta, S; Wang, JP; Ramesh, R | Abstract: Efficient charge to spin conversion is important for low-power spin logic devices. Spin and charge interconversion is commonly performed using heavy metals and topological insulators, while the field of oxides is not yet fully explored. Strontium iridate thin films were grown, where the different crystal structures form a perfect playground to understand the key factors in obtaining high charge to spin conversion efficiency (i.e., large spin Hall angle). It was found that the semiconducting Sr2IrO4 has a spin Hall angle of ∼0.1 (depending on measurement technique), which is promising for a spin-orbit coupled electronic system and comparable to Pt. In contrast, the perovskite SrIrO3, reported to have a Dirac cone near the Fermi level, has a larger spin Hall angle of 0.3-0.4 degrees. The largest difference between the two materials is a large degree of spin-momentum locking in SrIrO3, comparable to known topological insulators. A simple semiclassical relationship is found where the spin Hall angle increases for higher degrees of spin-momentum locking and it also increases for lower Fermi wave vectors. This relationship is then able to explain the decreased spin Hall angle below 10 nm film thickness in SrIrO3, by relating it to the correspondingly higher carrier concentration (related to the higher Fermi wave vector). Breaking the commonly believed anticorrelation between resistivity and carrier concentration paves a pathway to lower power losses due to resistance while keeping large spin Hall angles.

The synthesis of fully epitaxial ferroelectric Hf0.5 Zr0.5 O2 (HZO) thin films through the use of a conducting pyrochlore oxide electrode that acts as a structural and chemical template is reported. Such pyrochlores, exemplified by Pb2 Ir2 O7 (PIO) and Bi2 Ru2 O7 (BRO), exhibit metallic conductivity with room-temperature resistivity of

Multiferroic BiFeO3 (BFO) shows several phonon modes at infrared (IR) to THz energies, which are expected to carry information on any sample property coupled to crystal lattice vibrations. While macroscopic IR studies of BFO are often limited by single-crystal size, scattering-type scanning near-field optical microscopy (s-SNOM) allows for IR thin film spectroscopy of nanoscopic probing volumes with negligible direct substrate contribution to the optical signal. In fact, polaritons such as phonon polaritons of BFO introduce a resonant tip–sample coupling in s-SNOM, leading to both stronger signals and enhanced sensitivity to local material properties. Here, we explore the near-field response of BFO thin films at three consecutive resonances (centered around 5 THz, 13 THz, and 16 THz), by combining s-SNOM with a free-electron laser. We study the dependence of these near-field resonances on both the wavelength and tip–sample distance. Enabled by the broad spectral range of the measurement, we probe phonon modes connected to the predominant motion of either the bismuth or oxygen ions. Therefore, we propose s-SNOM at multiple near-field resonances as a versatile and very sensitive tool for the simultaneous investigation of various sample properties.

Defects in solids can be broadly classified based on their dimensionality (1). Zero-dimensional defects (point defects) arise as a consequence of entropy considerations (configurational entropy) and thus are thermodynamically required in any material. One-dimensional defects such as dislocations are not required by thermodynamics but arise anyway as a consequence of imperfections in the synthetic environment or due to structural constraints imposed by applied stresses from a variety of sources. Dislocations arise as a consequence of stresses imposed through thermal history, processing, or structural mismatch, as in the case of heteroepitaxial thin films, which are so prevalent in modern technology. Extended defects are known to have a profound influence in achieving or impacting the desired material performance. Complex networks of dislocations are designed into alloys to enhance their mechanical properties, such as the tensile strength in steels and other technologically relevant alloys (2). In many cases, defects, such as dislocations, are harmful to the desired performance of the device (3). Volumetric defects (such as second-phase inclusions, some of which are deliberately designed into materials such as the Guinier–Preston zones in Al–Cu alloys) (4, 5) can arise as a consequence of improper processing or through deliberate materials engineering. The characteristic length scale of such defects will depend on the physical phenomenon in question (e.g., pinning of the movement of dislocations would require second phases that are of a certain size and spacing). We now come to 2D, surface-type defects, the designing of which is the focus of the paper by Wang et al. (6). Two-dimensional defects, such as antiphase boundaries (APBs) and domain walls in certain ferroic materials, arise primarily as a consequence of symmetry mismatch in spatial coordinates, … [↵][1]1Email: rramesh{at}berkeley.edu. [1]: #xref-corresp-1-1

There is growing interest in the study of thin-film pyroelectric materials because of their potential for high performance thermal-energy conversion, thermal sensing, and beyond. Electrothermal susceptibilities, such as pyroelectricity, are known to be enhanced in proximity to polar instabilities, and this is conventionally accomplished by positioning the material close to a temperature-driven ferroelectric-to-paraelectric phase transition. The high Curie temperature (TC) for many ferroelectrics, however, limits the utility of these materials at room-temperature. Here, the nature of pyroelectric response in thin films of the widely studied multiferroic Bi1−xLaxFeO3 (x = 0–0.45) is probed. While BiFeO3 itself has a high TC, lanthanum substitution results in a chemically induced lowering of the ferroelectric-to-paraelectric and structural-phase transition. The effect of isovalent lanthanum substitution on the structural, dielectric, ferroelectric, and pyroelectric response is investigated using reciprocal-space-mapping studies; field-, frequency-, and temperature-dependent electrical measurements; and phase-sensitive pyroelectric measurements, respectively. While BiFeO3 itself has a rather small pyroelectric coefficient at room temperature (∼−40 µC/m2 K), 15% lanthanum substitution results in an enhancement of the pyroelectric coefficient by 100% which is found to arise from a systematic lowering of TC.

Cracks in solid-state materials are typically irreversible. Here we report electrically reversible opening and closing of nanoscale cracks in an intermetallic thin film grown on a ferroelectric substrate driven by a small electric field (~0.83 kV/cm). Accordingly, a nonvolatile colossal electroresistance on–off ratio of more than 108 is measured across the cracks in the intermetallic film at room temperature. Cracks are easily formed with low-frequency voltage cycling and remain stable when the device is operated at high frequency, which offers intriguing potential for next-generation high-frequency memory applications. Moreover, endurance testing demonstrates that the opening and closing of such cracks can reach over 107 cycles under 10-μs pulses, without catastrophic failure of the film., Electric-field-induced cracks are generally detrimental to functionality of ferroelectric ceramics. Liu et al. use an intermetallic alloy and ferroelectric oxide junction to mediate the reversible formation of cracks at nanoscales, resulting in colossal electroresistance modulation for memory applications.

Complex oxides provide an ideal playground for exploring the interplay among the fundamental degrees of freedom: structural (lattice), electronic (orbital and charge), and magnetic (spin). In thin films and heterostructures, new states of matter can emerge as a consequence of such interactions. Over the past decade, the ability to synthesize self-assembled nanocomposite thin films of metal oxides has provided another pathway for creating new interfaces and, thus, new physical phenomena. In this article, we describe examples of such materials systems explored to date and highlight the fascinating multifunctional properties achieved. These include enhanced flux pinning in superconductors, strain-enhanced ferroelectricity, strain- and charge-coupled magnetoelectrics, tunable magnetotransport, novel electrical/ionic transport, memristors, and tunable dielectrics.

We report a voltage controlled reversible creation and annihilation of a-axis oriented ∼10 nm wide ferroelastic nanodomains without a concurrent ferroelectric 180° switching of the surrounding c-domain matrix in archetypal ferroelectric Pb(Zr0.2Ti0.8)O3 thin films by using the piezo-response force microscopy technique. In previous studies, the coupled nature of ferroelectric switching and ferroelastic rotation has made it difficult to differentiate the underlying physics of ferroelastic domain wall movement. Our observation of distinct thresholds for ferroelectric and ferroelastic switching allows us investigate the ferroelastic switching cleanly and demonstrate a new degree of nanoscale control over the ferroelastic domains.

We demonstrate an electrically induced, non-volatile, metal-insulator phase transition in a MoS$_2$ transistor. A single crystalline, epitaxially grown, PbZr$_{0.2}$Ti$_{0.8}$O$_3$ (PZT) was placed in the gate of a field effect transistor made of thin film MoS$_2$. When a gate voltage is applied to this ferroelectric gated transistor, a clear transition from insulator to metal and vice versa is observed. Importantly, when the gate voltage is turned off, the remnant polarization in the ferroelectric can keep the MoS$_2$ in its original phase, thereby providing a non-volatile state. Thus a metallic or insulating phase can be written, erased or retained simply by applying a gate voltage to the transistor.

To achieve high quality epitaxial thin films and heterostructures of transition metal oxides with atomically controlled interfaces, one critical requirement is the use of atomically flat single terminated oxide substrates since the atomic arrangements and the reaction chemistry of the topmost surface layer of substrates determine the growth and consequent properties of the overlying films. Achieving the atomically flat and chemically single terminated surface state of commercially available substrates, however, requires judicious efforts because the surface of as-received substrates is of chemically mixed nature and also often polar. In this review, we summarize the surface treatment procedures to accomplish atomically flat surfaces with single terminating layer for various metal oxide substrates. We particularly focus on the substrates with lattice constant ranging from 4.00 to 3.70 angstrom, as the lattice constant of most perovskite materials falls into this range. For materials outside the range, one can utilize the substrates to induce compressive or tensile strain on the films and explore new states not available in bulk. The substrates covered in this review, which have been chosen with commercial availability and, most importantly, experimental practicality as a criterion, are KTaO3, REScO3 (RE = Rare-earth elements), SrTiO3, La0.18Sr0.82Al0.59Ta0.41O3 (LSAT), NdGaO3, LaAlO3, SrLaAlO4, and YAlO3. Analyzing all the established procedures, we conclude that atomically flat surfaces with selective A- or B-site single termination would be obtained for most commercially available oxide substrates. We further note that this topmost surface layer selectivity would provide an additional degree of freedom in searching for unforeseen emergent phenomena and functional applications in epitaxial oxide thin films and heterostructures with atomically controlled interfaces., A review article

We report a giant, ~22%, electroresistance modulation for a metallic alloy above room temperature. It is achieved by a small electric field of 2 kV/cm via piezoelectric strain-mediated magnetoelectric coupling and the resulting magnetic phase transition in epitaxial FeRh/BaTiO3 heterostructures. This work presents detailed experimental evidence for an isothermal magnetic phase transition driven by tetragonality modulation in FeRh thin films, which is in contrast to the large volume expansion in the conventional temperature-driven magnetic phase transition in FeRh. Moreover, all the experimental results in this work illustrate FeRh as a mixed-phase model system well similar to phase-separated colossal magnetoresistance systems with phase instability therein., 6 pages, 3 figures

Using epitaxy and the misfit strain imposed by an underlying substrate, it is possible to elastically strain oxide thin films to percent levels—far beyond where they would crack in bulk. Under such strains, the properties of oxides can be dramatically altered. In this article, we review the use of elastic strain to enhance ferroics, materials containing domains that can be moved through the application of an electric field (ferroelectric), a magnetic field (ferromagnetic), or stress (ferroelastic). We describe examples of transmuting oxides that are neither ferroelectric nor ferromagnetic in their unstrained state into ferroelectrics, ferromagnets, or materials that are both at the same time (multiferroics). Elastic strain can also be used to enhance the properties of known ferroic oxides or to create new tunable microwave dielectrics with performance that rivals that of existing materials. Results show that for thin films of ferroic oxides, elastic strain is a viable alternative to the traditional method of chemical substitution to lower the energy of a desired ground state relative to that of competing ground states to create materials with superior properties.

Demand for visualizing nanoscale dynamics in biological and advanced materials continues to drive the development of subdiffraction optical probes. While many strategies employ scanning tips for this purpose, we instead exploit a focused electron beam to create scannable nanoscale optical excitations in an epitaxially grown thin-film of cerium-doped yttrium aluminum perovskite, whose cathodoluminescence response is bright, robust, and spatially resolved to 18 nm. We also demonstrate lithographic patterning of the film's luminescence at the nanoscale. We anticipate that converting these films into free-standing membranes will yield a powerful near-field optical microscopy without the complication of mechanical scanning.

A new spin on perovskites untwined: Ultrafast optical switching and tuning of spin-energy states in layered halide perovskites., Ultrafast spin manipulation for opto–spin logic applications requires material systems that have strong spin-selective light-matter interaction. Conventional inorganic semiconductor nanostructures [for example, epitaxial II to VI quantum dots and III to V multiple quantum wells (MQWs)] are considered forerunners but encounter challenges such as lattice matching and cryogenic cooling requirements. Two-dimensional halide perovskite semiconductors, combining intrinsic tunable MQW structures and large oscillator strengths with facile solution processability, can offer breakthroughs in this area. We demonstrate novel room-temperature, strong ultrafast spin-selective optical Stark effect in solution-processed (C6H4FC2H4NH3)2PbI4 perovskite thin films. Exciton spin states are selectively tuned by ~6.3 meV using circularly polarized optical pulses without any external photonic cavity (that is, corresponding to a Rabi energy of ~55 meV and equivalent to applying a 70 T magnetic field), which is much larger than any conventional system. The facile halide and organic replacement in these perovskites affords control of the dielectric confinement and thus presents a straightforward strategy for tuning light-matter coupling strength.

A controllable ferroelastic switching in ferroelectric/multiferroic oxides is highly desirable due to the non-volatile strain and possible coupling between lattice and other order parameter in heterostructures. However, a substrate clamping usually inhibits their elastic deformation in thin films without micro/nano-patterned structure so that the integration of the non-volatile strain with thin film devices is challenging. Here, we report that reversible in-plane elastic switching with a non-volatile strain of approximately 0.4% can be achieved in layered-perovskite Bi2WO6 thin films, where the ferroelectric polarization rotates by 90° within four in-plane preferred orientations. Phase-field simulation indicates that the energy barrier of ferroelastic switching in orthorhombic Bi2WO6 film is ten times lower than the one in PbTiO3 films, revealing the origin of the switching with negligible substrate constraint. The reversible control of the in-plane strain in this layered-perovskite thin film demonstrates a new pathway to integrate mechanical deformation with nanoscale electronic and/or magnetoelectronic applications., Ferroelastic switching in thin films is typically restricted by constraints from the substrate or occurs around twin-like domains. Here, the authors show reversible and non-volatile ferroelastic switching avoiding substrate constraints in layered-perovskite Bi_2WO_6 epitaxial films.

Purely voltage-driven, repeatable magnetization reversal provides a tantalizing potential for the development of spintronic devices with a minimum amount of power consumption. Substantial progress has been made in this subject especially on magnetic/ferroelectric heterostructures. Here, we report the in situ observation of such phenomenon in a NiFe thin film grown directly on a rhombohedral Pb(Mg1/3Nb2/3)0.7Ti0.3O3(PMN-PT) ferroelectric crystal. Under a cyclic voltage applied perpendicular to the PMN-PT without a magnetic field, the local magnetization of NiFe can be repetitively reversed through an out-of-plane excursion and then back into the plane. Using phase field simulations we interpret magnetization reversal as a synergistic effect of the metastable ferroelastic switching in the PMN-PT and an electrically rotatable local exchange bias field arising from the heterogeneously distributed NiO clusters at the interface.

We report the direct observation of nanoscale photovoltaic current-voltage properties associated with directional steps of the electrostatic potential at 109° domain walls in ferroelectric BiFeO3 thin films with highly ordered ferroelectric stripe domains. The unidirectional photovoltaic effect along the net-in-plane polarization direction is shown to exhibit anomalies at the micro- and nanoscale due to local differences in mesoscopic domain structure. Very high initial photovoltages for small electrode spacings are observed using a conductive AFM tip as one of the electrodes in photovoltaic I-V measurements.

Single-crystalline thin films of complex oxides show a rich variety of functional properties such as ferroelectricity, piezoelectricity, ferro and antiferromagnetism and so on that have the potential for completely new electronic applications. Direct synthesis of such oxides on silicon remains challenging because of the fundamental crystal chemistry and mechanical incompatibility of dissimilar interfaces. Here we report integration of thin (down to one unit cell) single crystalline, complex oxide films onto silicon substrates, by epitaxial transfer at room temperature. In a field-effect transistor using a transferred lead zirconate titanate layer as the gate insulator, we demonstrate direct reversible control of the semiconductor channel charge with polarization state. These results represent the realization of long pursued but yet to be demonstrated single-crystal functional oxides on-demand on silicon., Synthesis of single-crystal complex-oxide films directly on silicon is difficult due to differing interfacial chemistry. Here, the authors demonstrate room-temperature integration of single-crystal lead zirconate titanate on to silicon to act as a gate insulator in a field-effect transistor.

The growth and characterization of functional oxide thin films that are ferroelectric, magnetic, or both at the same time are reviewed. The evolution of synthesis techniques and how advances in in situ characterization have enabled significant acceleration in improvements to these materials are described. Methods for enhancing the properties of functional materials or creating entirely new functionality at interfaces are covered, including strain engineering and layering control at the atomic-layer level. Emerging applications of these functional oxides such as achieving electrical control of ferromagnetism and the future of these complex functional oxides is discussed.

The fabrication of well-defined, atomically sharp substrate surfaces over a wide range of lattice parameters is reported, which is crucial for atomically regulated epitaxial growth of complex oxide heterostructures. By applying a framework for controlled selective wet etching of complex oxides on the stable rare-earth scandates (REScO3), a pseudocubic = 0.394 – 0.404 nm, the large chemical sensitivity of REScO3 to basic solutions is exploited, which results in reproducible, single-terminated surfaces. Time-of-flight mass-spectroscopy measurements show that after wet etching the surfaces are predominantly ScO2 -terminated. Moreover, the morphology study of SrRuO3 thin-film growth gives no evidence for mixed termination. Therefore, it is concluded that the REScO3 surfaces are completely ScO2-terminated.

Adv. Mater. 2009, 21, 817–823 2009 WILEY-VCH Verlag Gm Multiferroic BiFeO3 has attracted great interest due to its promising application tomagnetoelectric devices. In addition, the high remanent polarization and piezoelectric response of BiFeO3 thin films, which are comparable to those of conventional Ti-rich lead zirconia titanate, suggested BiFeO3 as a strong candidate for lead-free nonvolatile memories. BiFeO3 has a rhombohedral perovskite structure with pseudocubic lattice parameters ar1⁄4 3.96 A and ar1⁄4 0.68. Due to this low symmetry, (001)-oriented epitaxial BiFeO3 films possess the rhombohedral distortion along one of the four (111) crystallographic directions of the pseudocubic perovskite unit cell. Thus, eight possible polarization (ferroelectric) variants, which correspond to four structural (ferroelastic) domains, may form in the films, leading to complex domain patterns with both {100} and {101} twin boundaries. Such a complex domain structure can deteriorate the ferroelectric response of the system by external electric field, and complicates the examination of the coupling between magnetic and ferroelectric order parameters in BiFeO3. [3]

Multifunctional thin film nanostructures containing soft magnetic materials such as nickel ferrite are interesting for potential applications in microwave signal processing because of the possibility to shrink the size of device architecture and limit device power consumption. An essential prerequisite to future applications of such a system is a firm understanding of its magnetic properties. We show that nanostructures composed of ferrimagnetic NiFe2O4 pillars in a multiferroic BiFeO3 matrix can be tuned magnetically by altering the aspect ratio of the pillars by depositing films of varying thickness. Magnetic anisotropy is studied using ferromagnetic resonance, which shows that the uniaxial magnetic anisotropy in the growth direction changes sign upon increasing the film thickness. The magnitude of this anisotropy contribution can be explained via a combination of shape and magnetostatic effects, using the object-oriented micromagnetic framework (OOMMF). The key factors determining the magnetic properties of the films are shown to be the aspect ratio of individual pillars and magnetostatic interactions between neighboring pillars.

Local piezoresponse hysteresis loops were systematically studied on the surface of ferroelectric thin films of BiFeO3 grown on SrRuO3 and La0.7Sr0.3MnO3 electrodes and compared between ultrahigh vacuum and ambient environment. The loops on all the samples exhibited characteristic asymmetry manifested in the difference of the piezoresponse slope following local domain nucleation. Spatially resolved mapping has revealed that the asymmetry is strongly correlated with the random-field disorder inherent in the films and is not affected by the random-bond disorder component. The asymmetry thus originates from electrostatic disorder within the film, which allows using it as a unique signature of single defects or defect clusters. The electrostatic effects due to the measurement environment also contribute to the total asymmetry of the piezoresponse loop, albeit with a much smaller magnitude compared to local defects.

In this article, we review current research efforts to control the magnetic behavior of complex oxide thin films using electric fields. After providing fundamental definitions of magnetoelectric response, we survey materials, architectures, and mechanisms that exhibit promise for such electric-field control of magnetism. Finally, we mention ideas for future research and discuss prospects for the field.

We demonstrate a direct correlation between the domain structure of multiferroic BiFeO3 thin films and exchange bias of Co0.9Fe0.1/BiFeO3 heterostructures. Two distinct types of interactions, an enhancement of the coercive field (exchange enhancement) and an enhancement of the coercive field combined with large shifts of the hysteresis loop (exchange bias), have been observed in these heterostructures, which depend directly on the type and crystallography of the nanoscale (2 nm) domain walls in the BiFeO3 film. We show that the magnitude of the exchange bias interaction scales with the length of 109 degree ferroelectric domain walls in the BiFeO3 thin films which have been probed via piezoresponse force microscopy and x-ray magnetic circular dichroism., Comment: Accepted to Nano Letters May 2008

Multiferroics, materials combining multiple order parameters, offer an exciting way of coupling phenomena such as electronic and magnetic order. Using epitaxial growth and heteroepitaxy, researchers have grown high-quality thin films and heterostructures of the multiferroic BiFeO 3 . The ferroelectric and antiferromagnetic domain structure and coupling between these two order parameters in BiFeO 3 is now being studied. We describe the evolution of our understanding of the connection between structure, properties, and new functionalities (including electrical control of magnetism) using BiFeO 3 as a model system.

We write this article in honor of Professor Vitaly L. Ginzburg, truly the father of the field of ferroelectricity. This article serves as a review of the current state of research pertaining to multiferroic BiFeO 3 thin films. In this review we will delve into details of the growth of BiFeO 3 thin films and the use of piezoforce microscopy and x-ray reciprocal space mapping to characterize the crystal structure and domain structure of BiFeO 3 . We will also discuss the use of vicinal and asymmetric substrates to simplify the domain structure in BiFeO 3 . By simplifying the domain structure we can, in turn, control the ferroelectric switching mechanisms in BiFeO 3 . Finally we describe the basic ferroelectric properties of BFO films and discuss the critical issues needed to be solved in BiFeO 3 films including leakage, complex domain structure, coercivity, and reliability. Such results are promising for continued exploration for detailed multiferroic-coupling studies in the magnetoelectric BiFeO 3 system a...

Substrate clamping and inter-domain pinning limit movement of non-180° domain walls in ferroelectric epitaxial films thereby reducing the resulting piezoelectric response of ferroelectric layers. Our theoretical calculations and experimental studies of the epitaxial PbZrxTi1–xO3 films grown on single crystal SrTiO3 demonstrate that for film compositions near the morphotropic phase boundary it is possible to obtain mobile two-domain architectures by selecting the appropriate substrate orientation. Transmission electron microscopy, X-ray diffraction analysis, and piezoelectric force microscopy revealed that the PbZr0.52Ti0.48O3 films grown on (101) SrTiO3 substrates feature self-assembled two-domain structures, consisting of two tetragonal domain variants. For these films, the low-field piezoelectric coefficient measured in the direction normal to the film surface (d33) is 200 pm V–1, which agrees well with the theoretical predictions. Under external AC electric fields of about 30 kV cm–1, the (101) films exhibit reversible longitudinal strains as high as 0.35 %, which correspond to the effective piezoelectric coefficients in the order of 1000 pm V–1 and can be explained by elastic softening of the PbZrxTi1–xO3 ferroelectrics near the morphotropic phase boundary.

Bismuth titanate (Bi 4 Ti 3 O 12 , BIT) films were evaluated for use as lead-free piezoelectric thin films in micro-electromechanical systems. The films were grown by the polymeric precursor method on LaNiO 3 /SiO 2 /Si (1 0 0) (LNO), RuO 2 /SiO 2 /Si (1 0 0) (RuO 2 ) and Pt/Ti/SiO 2 /Si (1 0 0) (Pt) bottom electrodes in a microwave furnace at 700 °C for 10 min. The domain structure was investigated by piezoresponse force microscopy (PFM). Although the converse piezoelectric coefficient, d 33 , regardless of bottom electrode is around (∼40 pm/V), those over RuO 2 and LNO exhibit better ferroelectric properties, higher remanent polarization (15 and 10 μC/cm 2 ), lower drive voltages (2.6 and 1.3 V) and are fatigue-free. The experimental results demonstrated that the combination of the polymeric precursor method assisted with a microwave furnace is a promising technique to obtain films with good qualities for applications in ferroelectric and piezoelectric devices.

A short review of recent progress in the field of multiferroic thin films and heterostructures is given. We focus on the bismuth iron oxide system due to its desirable properties, namely high ferroelectric Curie temperature and antiferromagnetic Neel temperature. Epitaxial growth of this model system in various crystallographic orientations has conclusively demonstrated the large ferroelectric polarization in this system. Piezoforce microscopy reveal a complex domain structure, especially on (100) SrTiO3 substrates. Electrical switching experiments show the co-existence of 71°, 109° and 180° domain switching mechanisms in such films. Preliminary work has shown promise for electrically controllable exchange bias in ferromagnet–multiferroic heterostructures.

Multiferroic depicts matters in which two or more ferroic order parameters (ferroelectric, antiferromagnetic, and ferroelastic) coexist. The coexistent ferroic orders and the coupling enable multiferroics to show intriguing physical phenomena and to possess promising potentials for next-generation nanoelectronics. More recently, a number of fascinating phenomena as well as promising exploration of new device heterostructures based on multiferroic thin films have been carried out, lighting up the opportunity toward multifunctional devices and electronics. In this article, fundamental characteristics, coexistence of ferroic order parameters, and corresponding couplings will be disclosed. Furthermore, convectional types as well as promising applications of multiferriocs thin films will also be recaptured. Keywords: Multiferroics; complex oxide; magnetoelectronic; nanoelectronics; thin film

This article has been highlighted as "Hot Paper" in Nanoscale.-- Domingo, Neus et al., Layered iridates have been the subject of intense scrutiny on account of their unusually strong spin–orbit coupling, which opens up a narrow bandgap in a material that would otherwise be a metal. This insulating state is very sensitive to external perturbations. Here, we show that vertical compression at the nanoscale, delivered using the tip of a standard scanning probe microscope, is capable of inducing a five orders of magnitude change in the room temperature resistivity of Sr2IrO4. The extreme sensitivity of the electronic structure to anisotropic deformations opens up a new angle of interest on this material, with the giant and fully reversible perpendicular piezoresistance rendering iridates as promising materials for room temperature piezotronic devices., N.D wants to acknowledge the Spanish Ministerio de Ciencia e Innovación for a Ramon y Cajal research grant RYC-2010-06365. X.M. acknowledges the Grant Agency of the Czech Republic No. P204/11/P339. G.C. acknowledges ERC Starting Grant 308023. T.J. acknowledges ERC Advanced Grant 268066 and Praemium Academiae of the Academy of Sciences of the Czech Republic. Di Yi was sponsored by the National Science Foundation through the Penn State MRSEC. J.L. is supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 through the Quantum Material program in the Materials Sciences Division of Lawrence Berkeley National Laboratory. Financial support has been obtained under projects from the Spanish Ministerio de Economía y Competitividad under projects MAT2010-17771, MAT2010-20020, MAT2013-47869-C4-1-P, FIS2013-48668-C2-1-P and NANOSELECT CSD2007-00041 and Severo Ochoa Excellence Programme 2013-0295, and the Generalitat de Catalunya under projects 2014 SGR 733 and 2014 SGR 1216.

Using Scanning Nonlinear Dielectric Microscopy (SNDM) which has attained sub-nanometer resolution, we measured the linear dielectric constant of a-domains and c-domains in the (1,0,0) and (0,0,1) oriented PbZr 0.2 Ti 0.8 O 3 thin film and confirmed that the dielectric constant of a-domain is higher than that of c-domain. Next, we observed 90° domain walls (a-c domain walls) and 180° domain walls (c-c domain walls) and we obtained the minimum value of 180° c-c domain wall thickness was 1.87 nm and 90° a-c domain wall was 2.52 nm.

Microstructure in the SrTiO3/Si system has been studied using high-resolution transmission electron microscopy and image simulations. SrTiO3 grows heteroepitaxially on Si with the orientation relationship given by (001)STO//(001)Si and [100]STO//[110]Si. The lattice misfit between the SrTiO3 thin films and the Si substrate is accommodated by the presence of interfacial dislocations at the Si substrate side. The interface most likely consists of Si bonded to O in SrTiO3. The alternative presentation of Sr and Si atoms along the interface leads to the formation of 2× and 3× Sr configurations. Structural defects in the SrTiO3 thin film mainly consist of tilted domains and dislocations.

We have used x-ray diffraction to study the structure of strained, epitaxial ${\text{BiFeO}}_{3}$ (BFO) films, which exhibit ordered arrays of stripelike ferroelectric domains, in which the polarization vector P alternates by either 109${}^{\ensuremath{\circ}}$ or 71${}^{\ensuremath{\circ}}$. Diffraction maps exhibit an intricate satellite structure that arises from coherent, gratinglike diffraction from the domain structure. In the case of the 109${}^{\ensuremath{\circ}}$ arrays, the domain structure was found to exert a strain modulation on the ${\text{DyScO}}_{3}$ substrate, with the same periodicity, indicating that domains in BFO can have an influence on the substrate structure. In the case of the 71${}^{\ensuremath{\circ}}$ arrays, in which there is no contrast between neighboring domains and coherent scattering is not expected, weak scattering is nonetheless observed, which we interpret as evidence for previously unobserved, internal strains in these domain walls. To understand the x-ray data, we introduce a simple, single-scattering model that incorporates Gaussian disorder and fits the diffraction maps, providing domain periods and surface ``puckering'' angles that are in good agreement with atomic force microscopy and piezoresponse force microscopy measurements. Our study demonstrates a simple, computationally inexpensive technique for semiquantitatively interpreting coherent domain scattering in ferroelectric films, and suggests that tuning domain structures is a potential route to engineering the near-surface properties of perovskite oxides.

Precise control of composition and microstructure of multicomponent oxide thin films is critical for the production of ferroelectric and high dielectric constant thin film devices. In addition, the integration of film-based capacitors with semiconductor substrates, for device fabrication, requires good control of the composition and structureof the dielectric/substrate and top electrode / dielectric interfaces to control the capacitor properties. In order to understand the processes described above, we are using a variety of integrated complementary in situ analytical techniques including time-of-flight ion scattering and recoil spectroscopy, X-ray photoelectron spectroscopy, spectroscopic ellipsometry, and ex situ methods such as transmission electron microscopy, scanning force microscopy, and scanning electron microscopy. Examples of studies recently performed by our group that are reviewed here include: (a) effects of microstructure on the oxidation of Ti-Al layers that can be used in a dual f...

In thin film ferroelectric devices, switching of ferroelastic domains can significantly enhance electromechanical response. Previous studies have shown disagreement regarding the mobility or immobility of ferroelastic domain walls, indicating that switching behaviour strongly depends on specific microstructures in ferroelectric systems. Here we study the switching dynamics of individual ferroelastic domains in thin Pb(Zr0.2,Ti0.8)O3 films under electrical and mechanical excitations by using in situ transmission electron microscopy and phase-field modelling. We find that ferroelastic domains can be effectively and permanently stabilized by dislocations at the substrate interface while similar domains at free surfaces without pinning dislocations can be removed by either electric or stress fields. For both electrical and mechanical switching, ferroelastic switching is found to occur most readily at the highly active needle points in ferroelastic domains. Our results provide new insights into the understanding of polarization switching dynamics as well as the engineering of ferroelectric devices.

A review is presented of approaches to integrate thin film Pb(Zr, Ti)O3-based ferroelectric capacitor structures on a filled poly-Si plug, which is a critical requirement for a high-density memory technology. Key materials issues relevant to the development of conducting diffusion barrier layers for integration of these materials on Si wafers are discussed. Specific attention in this paper is on the use of conducting perovskite oxide electrodes to contact the ferroelectric thin film. The second part of this review focuses on some novel materials that we have investigated for use as diffusion barriers. Finally, we present data on the scaling of ferroelectric properties with lateral dimensions of the capacitor.

The science and technology of ferroelectric thin films has experienced an explosive development during the last ten years. Low-density non-volatile ferroelectric random access memories (NVFRAMs) are now incorporated in commercial products such as “smart cards”, while high permittivity capacitors are incorporated in cellular phones. However, substantial work is still needed to develop materials integration strategies for high-density memories. We have demonstrated that the implementation of complementary in situ characterization techniques is critical to understand film growth and interface processes, which play critical roles in film microstructures and properties. We are using uniquely integrated time of flight ion scattering and recoil spectroscopy (TOF-ISARS) and spectroscopic ellipsometry (SE) techniques to perform in situ, real-time studies of film growth processes in the high background gas pressure required to growth ferroelectric thin films. TOF-ISARS provides information on surface proce...

Current-voltage $(I\ensuremath{-}V)$ behaviors of ${\mathrm{P}\mathrm{b}(\mathrm{Z}\mathrm{r},\mathrm{}\mathrm{T}\mathrm{i})\mathrm{O}}_{3}$-based capacitors with ${(\mathrm{L}\mathrm{a},\mathrm{}\mathrm{S}\mathrm{r})\mathrm{C}\mathrm{o}\mathrm{O}}_{3}$ electrodes were studied to investigate the dominant leakage mechanism. Epitaxial ${(\mathrm{L}\mathrm{a},\mathrm{S}\mathrm{r})\mathrm{CoO}}_{3}{/\mathrm{P}\mathrm{b}(\mathrm{Z}\mathrm{r},\mathrm{T}\mathrm{i})\mathrm{O}}_{3}{/(\mathrm{L}\mathrm{a},\mathrm{S}\mathrm{r})\mathrm{CoO}}_{3}$ capacitors were fabricated to simplify the analysis and eliminate any effects of granularity. The $I\ensuremath{-}V$ characteristics were almost symmetric and temperature dependent with a positive temperature coefficient. The leakage current at low fields (0.5 V or 10 kV/cm) shows Ohmic behavior with a slope of nearly 1 and is nonlinear at higher voltages and temperatures. Further analysis suggests that at higher fields and temperatures, bulk-limited field-enhanced thermal ionization of trapped carriers (i.e., Poole-Frenkel emission) is the controlling mechanism. The activation energies calculated for the films are in the range 0.5--0.6 eV. These energies are compatible with ${\mathrm{Ti}}^{4+}$ ion acting as the Poole-Frenkel centers.

We report the results of systematic studies of the effect of thin film deposition conditions, such as deposition temperature, oxygen pressure during deposition, etc., on the microstructural, magnetic, and microwave properties of pulsed laser deposited epitaxial thin films of barium ferrite on single crystal sapphire substrates. Ferromagnetic resonance (FMR) linewidths are very sensitive to the presence of defects and inhomogeneities and therefore change markedly with the variation of deposition parameters. After careful optimization of the deposition conditions, relatively narrow resonance lines were realized in these films. For further improvement in the film quality, these films were annealed at elevated temperatures in flowing oxygen. As a result of the high degree of epitaxy, good stoichiometry, and reduced concentration of defects, FMR linewidths as small as 37 Oe were obtained in films deposited at 920 °C and subsequently annealed at 1000 °C in an oxygen atmosphere.