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Chemistry of Materials, 34 (14), ISSN:0897-4756

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., ACS Applied Electronic Materials, 5 (3), ISSN:2637-6113

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., Chemistry of Materials, 33 (23), ISSN:0897-4756

Magnetic skyrmions are compact chiral spin textures that exhibit a rich variety of topological phenomena and hold potential for the development of high-density memory devices and novel computing schemes driven by spin currents. Here, we demonstrate the room-temperature interfacial stabilization and current-driven control of skyrmion bubbles in the ferrimagnetic insulator Tm3Fe5O12 coupled to Pt, showing the current-induced motion of individual skyrmion bubbles. The ferrimagnetic order of the crystal together with the interplay of spin–orbit torques and pinning determine the skyrmion dynamics in Tm3Fe5O12 and result in a strong skyrmion Hall effect characterized by a negative deflection angle and hopping motion. Further, we show that the velocity and depinning threshold of the skyrmion bubbles can be modified by exchange coupling Tm3Fe5O12 to an in-plane magnetized Y3Fe5O12 layer, which distorts the spin texture of the skyrmions and leads to directional-dependent rectification of their dynamics. This effect, which is equivalent to a magnetic ratchet, is exploited to control the skyrmion flow in a racetrack-like device., Nature Nanotechnology, 17, ISSN:1748-3387, ISSN:1748-3395

Charged domain walls in ferroelectrics hold great promise for the design of novel electronic devices due to their enhanced local conductivity. In fact, charged domain walls show unique properties including the possibility of being created, moved and erased by an applied voltage. Here, we demonstrate that the charged domain walls are constituted by a core region where most of the screening charge is localized and such charge accumulation is responsible for their enhanced conductivity. In particular, the link between the local structural distortions and charge screening phenomena in 109° tail-to-tail domain walls of BiFeO3 is elucidated by a series of multiscale analysis performed by means of scanning probe techniques, including conductive atomic force microscopy (cAFM) and atomic resolution differential phase contrast scanning transmission electron microscopy (DPC-STEM). The results prove that an accumulation of oxygen vacancies occurs at the tail-to-tail domain walls as the leading charge screening process. This work constitutes a new insight in understanding the behavior of such complex systems and lays down the fundaments for their implementation into novel nanoelectronic devices.

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., Physical Sciences Reviews, 6 (2)

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., Journal of Physics: Condensed Matter, 33 (29), ISSN:0953-8984, ISSN:1361-648X

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., Scientific Reports, 9 (1), ISSN:2045-2322

The authors demonstrate room-temperature multiferroic (ferroelectric-ferromagnetic) behavior in lead titanate, in which about 30% of the titanium is substituted by palladium. Palladium is an unusual element with an instability to ferromagnetism under some special conditions. Typically, it has not been easy to incorporate it into perovskite oxides -- the most common structure for ferroelectric device materials. Although palladium is expensive, commercial devices like ferroelectric memories often use very thin films, for which the added cost of palladium is negligible. Thus, the present study combines basic new physics of the first multiferroic palladium compounds with potential commercial devices.

Advanced Materials Interfaces, 7 (14), ISSN:2196-7350