PbTi1-xPdxO3: A New Room-temperature Magnetoelectric Multiferroic Device Material

There have been a large number of papers on bismuth ferrite (BiFeO3) over the past few years, trying to exploit its room-temperature magnetoelectric multiferroic properties. Although these are attractive, BiFeO3 is not the ideal multiferroic, due to weak magnetization and the difficulty in limiting leakage currents. Thus there is an ongoing search for alternatives, including such materials as gallium ferrite (GaFeO3). In the present work we report a comprehensive study of the perovskite PbTi1-xPdxO3 with 0 < x < 0.3. Our study includes dielectric, impedance and magnetization measurements, conductivity analysis and study of crystallographic phases present in the samples with special attention paid to minor phases, identified as PdO, PbPdO2, and Pd3Pb. The work is remarkable in two ways: Pd is difficult to substitute into ABO3 perovskite oxides (where it might be useful for catalysis), and Pd is magnetic under only unusual conditions (under strain or internal electric fields). The new material, as a PZT derivative, is expected to have much stronger piezoelectric properties than BiFeO3

High-speed domain wall racetracks in a magnetic insulator

Recent reports of current-induced switching of ferrimagnetic oxides coupled to a heavy metal layer have opened realistic prospects for implementing magnetic insulators into electrically addressable spintronic devices. However, key aspects such as the configuration and dynamics of magnetic domain walls driven by electrical currents in insulating oxides remain unexplored. Here, we investigate the internal structure of the domain walls in Tm3Fe5O12 (TmIG) and TmIG/Pt bilayers and demonstrate their efficient manipulation by spin-orbit torques with velocities of up to 400 m s$^{-1}$ and minimal current threshold for domain wall flow of 5 x 10$^{6}$ A cm$^{-2}$. Domain wall racetracks embedded in TmIG are defined by the deposition of Pt current lines, which allow us to control the domain propagation and magnetization switching in selected regions of an extended magnetic layer. Scanning nitrogen-vacancy magnetometry reveals that the domain walls of thin TmIG films are N\'eel walls with left-handed chirality, with the domain wall magnetization rotating towards an intermediate N\'eel-Bloch configuration upon deposition of Pt. These results indicate the presence of a sizable interfacial Dzyaloshinskii-Moriya interaction in TmIG, which leads to novel possibilities to control the formation of chiral spin textures in magnetic insulators. Ultimately, domain wall racetracks provide an efficient scheme to pattern the magnetic landscape of TmIG in a fast and reversible wa

Studies of multiferroic palladium perovskites

Work at St. Andrews supported by EPSRC grant EP/P024637/1; at Queens University Belfast by EPSRC Grant No. EP/J017191/1 and EP/N018389/1, at Univ. Puerto Rico by DoD AFOSR Grant FA9550-16-1-0295 and at West Virginia University by DOE (DE-SC0016176).We have studied the atomic force microscopy (AFM), X-ray Bragg reflections, X-ray absorption spectra (XAS) of the Pd L-edge, Scanning electron microscopey (SEM) and Raman spectra, and direct magnetoelectric tensor of Pd-substituted lead titanate and lead zirconate-titanate. A primary aim is to determine the percentage of Pd+4 and Pd+2 substitutional at the Ti-sites (we find that it is almost fully substitutional). The atomic force microscopy data uniquely reveal a surprise: both threefold vertical (polarized out-of-plane) and fourfold in-plane domain vertices. This is discussed in terms of the general rules for Voronoi patterns (Dirichlet tessellations) in two and three dimensions. At high pressures Raman soft modes are observed, as in pure lead titanate, and X-ray diffraction (XRD) indicates a nearly second-order displacive phase transition. However, two or three transitions are involved: First, there are anomalies in c/a ratio and Raman spectra at low pressures (P = 1 − 2 GPa); and second, the c/a ratio reaches unity at ca. P = 10 GPa, where a monoclinic (Mc) but metrically cubic transition occurs from the ambient tetragonal P4 mm structure in pure PbTiO3; whereas the Raman lines (forbidden in the cubic phase) remain until ca. 17 GPa, where a monoclinic-cubic transition is known in lead titanate.Publisher PDFPeer reviewe

Interfacial control of ferroic order in oxide heterostructures

Oxide electronics have emerged as an alternative to replace the current silicon-based technology. Owing to a rich elemental composition compared to that of doped silicon, transition metal oxides can host a wide range of physical phenomena. This is especially true when oxides are integrated into ultrathin epitaxial heterostructures, in which additional properties arise from the created interfaces. Their crystal structure is, furthermore, compatible with long-range order. In particular, ferromagnetic and ferroelectric systems have gathered considerable attention due to their characteristic non-volatile response to applied external fields. While ferroic oxides are indisputable candidates for low-energy-consuming applications, there are still a few setbacks left to overcome in order to integrate them into competitive device schemes. With the work performed during the course of this thesis, we strive to provide solutions for existing limitations to the implementation of ferroic states in nanoscale devices. We place emphasis on interfacial effects in epitaxial heterostructures and their influence on ferroic order. Making use of a non-conventional approach of epitaxially combining layers with different ferroic anisotropies, we uncover novel fundamental concepts likely to benefit the ever-evolving field of oxide electronics. We identify the in-plane-polarized Aurivillius compounds as promising candidates for tuning interfacial electrostatics and achieving interfacial polar continuity in epitaxial hybrid heterostructures with ferroelectric perovskite oxides. The stabilized coalescent layer-by-layer growth mode ensures the single-crystallinity of these layered ferroelectrics, while the sub-unit-cell thickness control of the films enables detailed investigations of their polar state. For instance, the thickness-dependent in-plane-polarized domain configuration in the resulting epitaxial Aurivillius films prompts us to propose new means for ferroic domain and functional domain-wall engineering via structural defect ordering. Moving ahead with the integration of Aurivillius films into perovskite-based heterostructures, we show that the Aurivillius compounds utilized as in-plane-polarized buffer layers can overcome the notorious limitation associated with the critical thickness for ferroelectricity in canonical out-of-plane-polarized perovskite ferroelectrics. We additionally demonstrate that buffers of the Aurivillius phase can be instrumental for domain and domain-wall engineering in the room-temperature multiferroic BiFeO3. In particular, we observe a uniform chirality stabilized in Néel-like domain walls in BiFeO3 grown on our in-plane-polarized Bi5FeTi3O15 Aurivillius layer. This likely constitutes one of the first experimental signatures of the electric counterpart to the Dzyaloshinskii-Moriya interaction in magnetically ordered compounds. Lastly, we explore magnetoelectric phase control in heterostructures combining both ferroelectric and ferromagnetic order. In a proof-of-concept multiferroic heterostructure, we mimic magnetoelectric domain walls by inserting ultrathin ferromagnetic La1-xSrxMnO3 in between two ferroelectric layers. We show that its magnetization and conductivity can be controlled by changing polarization directions in the adjacent ferroelectric layers only. This opens up new possibilities for voltage-based tuning of magnetization and conductivity at the nanoscale

Design and Manipulation of Ferroic Domains in Complex Oxide Heterostructures

The current burst of device concepts based on nanoscale domain-control in magnetically and electrically ordered systems motivates us to review the recent development in the design of domain engineered oxide heterostructures. The improved ability to design and control advanced ferroic domain architectures came hand in hand with major advances in investigation capacity of nanoscale ferroic states. The new avenues offered by prototypical multiferroic materials, in which electric and magnetic orders coexist, are expanding beyond the canonical low-energy-consuming electrical control of a net magnetization. Domain pattern inversion, for instance, holds promises of increased functionalities. In this review, we first describe the recent development in the creation of controlled ferroelectric and multiferroic domain architectures in thin films and multilayers. We then present techniques for probing the domain state with a particular focus on non-invasive tools allowing the determination of buried ferroic states. Finally, we discuss the switching events and their domain analysis, providing critical insight into the evolution of device concepts involving multiferroic thin films and heterostructures.ISSN:1996-194

Multiferroic heterostructures for spintronics

For next-generation technology, magnetic systems are of interest due to the natural ability to store information and, through spin transport, propagate this information for logic functions. Controlling the magnetization state through currents has proven energy inefficient. Multiferroic thin-film heterostructures, combining ferroelectric and ferromagnetic orders, hold promise for energy efficient electronics. The electric field control of magnetic order is expected to reduce energy dissipation by 2–3 orders of magnitude relative to the current state-of-the-art. The coupling between electrical and magnetic orders in multiferroic and magnetoelectric thin-film heterostructures relies on interfacial coupling though magnetic exchange or mechanical strain and the correlation between domains in adjacent functional ferroic layers. We review the recent developments in electrical control of magnetism through artificial magnetoelectric heterostructures, domain imprint, emergent physics and device paradigms for magnetoelectric logic, neuromorphic devices, and hybrid magnetoelectric/spin-current-based applications. Finally, we conclude with a discussion of experiments that probe the crucial dynamics of the magnetoelectric switching and optical tuning of ferroelectric states towards all-optical control of magnetoelectric switching events

Ferroelectric Thin Films for Oxide Electronics

Ferroelectric materials have set in motion numerous ultralow-energy-consuming device concepts that can be integrated into state-of-the-art complementary metal–oxide–semiconductor technology. Their nonvolatile, spontaneous electric polarization makes them promising candidates to control functionalities at the nanoscale with energy-efficient electric fields only. In this spotlight article, we start with a brief introduction to ferroelectric materials, the challenges involving the design of thin films and review the state-of-the-art of their integration into various electronic applications. Revolutionary in situ and operando diagnostic tools allowing the monitoring of the technology-relevant polarization state during the material design, or its operation will be detailed. Concepts such as chiral states in ferroelectrics and neuromorphic-type switching will be addressed to provide a comprehensive view on the evolution of ferroelectric states for the next generation of low-energy-consuming electronics. Finally, we discuss the most recent developments in the field, including the emergence of ferroelectricity at the nanoscale and in two-dimensional systems.ISSN:2637-611

In-situ monitoring of epitaxial ferroelectric thin-film growth

In ferroelectric thin films, the polarization state and the domain configuration define the macroscopic ferroelectric properties such as the switching dynamics. Engineering of the ferroelectric domain configuration during synthesis is in permanent evolution and can be achieved by a range of approaches, extending from epitaxial strain tuning over electrostatic environment control to the influence of interface atomic termination. Exotic polar states are now designed in the technologically relevant ultrathin regime. The promise of energy-efficient devices based on ultrathin ferroelectric films depends on the ability to create, probe, and manipulate polar states in ever more complex epitaxial architectures. Because most ferroelectric oxides exhibit ferroelectricity during the epitaxial deposition process, the direct access to the polarization emergence and its evolution during the growth process, beyond the realm of existing structuralin situdiagnostic tools, is becoming of paramount importance. We review the recent progress in the field of monitoring polar states with an emphasis on the non-invasive probes allowing investigations of polarization during the thin film growth of ferroelectric oxides. A particular importance is given to optical second harmonic generationin situ. The ability to determine the net polarization and domain configuration of ultrathin films and multilayers during the growth of multilayers brings new insights towards a better understanding of the physics of ultrathin ferroelectrics and further control of ferroelectric-based heterostructures for devices.ISSN:0953-8984ISSN:1361-648

Nanoscale Design of High-Quality Epitaxial Aurivillius Thin Films

Efforts for the integration of ferroelectric materials in nonvolatile, low energy consuming memories have so far been focused on perovskite oxide materials. Their down-scaling for nanodevices is, however, hindered by finite-size effects, and alternative materials offering more robust polar properties are required. Layered ferroelectrics of the Aurivillius phase have since emerged as promising candidates with robust polarization at subunit-cell thicknesses. Their controlled growth in the epitaxial thin film form has unfortunately remained elusive. Here, we demonstrate the stabilization of the coalescent layer-by-layer growth mode of the Bin+1Fen−3Ti3O3n+3 (BFTO) Aurivillius family homologues. We define the growth conditions for high-quality, single-crystalline thin films exhibiting ferroelectricity from the first half-unit-cell. We demonstrate the process to be effective for several homologous Aurivillius compositions, which highlights its general applicability. Our work thus provides the systematic framework for the integration of high-quality epitaxial layered ferroelectrics into oxide electronics.ISSN:0897-475

Ferroelectric Domain Engineering Using Structural Defect Ordering

ISSN:0897-475