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Flexoelectric and photo-flexoelectricity are scientifically intriguing and hold considerable potential for various applications such as soft strain sensing, photovoltaics, energy harvesting, etc. Among flexoelectric materials, freestanding ferroelectric thin films are believed to have huge flexoelectricity and tunability due to their excellent lattice regulatory freedom and sustainability to larger strain gradients. In this work, we demonstrated a freestanding BiFeO3(BFO) thin film-based soft strain sensor and explored their flexoelectric coefficient and flexoelectric photovoltaic effect under different strain gradients. Under different bending scales, the photocurrent of the thin film exhibits a step-like variation, indicating that the sensor can measure strain gradient with high sensitivity. These results show the potential application of freestanding ferroelectric films in flexible devices.

Abstract The deterministic creation and modification of domain walls in ferroelectric films have attracted broad interest due to their unprecedented potential as the active element in non-volatile memory, logic computation and energy-harvesting technologies. However, the correlation between charged and antiphase states, and their hybridization into a single domain wall still remain elusive. Here we demonstrate the facile fabrication of antiphase boundaries in BiFeO3 thin films using a He-ion implantation process. Cross-sectional electron microscopy, spectroscopy and piezoresponse force measurement reveal the creation of a continuous in-plane charged antiphase boundaries around the implanted depth and a variety of atomic bonding configurations at the antiphase interface, showing the atomically sharp 180° polarization reversal across the boundary. Therefore, this work not only inspires a domain-wall fabrication strategy using He-ion implantation, which is compatible with the wafer-scale patterning, but also provides atomic-scale structural insights for its future utilization in domain-wall nanoelectronics.

[Introduction] As an important thrust of China's "new infrastructure", internet data centers have ushered in opportunities for vigorous development and become new areas of energy use, putting forward higher requirements for the power supply level and capacity of the local distribution network. The innovative application of flexible transmission and distribution technology and key equipment makes the power supply and distribution system more intelligent, more flexible and more reliable, and more able to cope with the challenges brought by the large proportion of DC loads and concentrated high-load energy loads such as Internet data centers, and realize the construction and operation of Internet data centers more low-carbon, more efficient, more reliable and more economical. [Method] Firstly, the basic load requirements of Internet data center were discussed, the overall classification and performance requirements of internet data centers were analyzed. The application of flexible technology in distribution network was studied, with a focus on analyzing and comparing three types of technical routes: "rectification distribution, DC distribution, and AC-DC hybrid power supply and distribution". [Result] The paper provides provides tailored solutions for issues related to system reliability, stability, power quality, power efficiency and acceptance of new energy. [Conclusion] By summarizing the existing research results, flexible transmission and distribution technology is regarded as the core technology of building internet data center. Targeted data center control scheme should be studied from different aspects such as device and algorithms. After summarizing the research results of different aspects, the paper also looks forward to the practice and popularization of head-to-head transmission and distribution technology in internet data center.

Abstract Responsible for the most significant part of the world’s burning of marine fossil fuels and shipping emissions, global maritime container shipping is under decarbonization pressure. This paper develops an integrated framework of bottom-up emission estimation and upscaling pathway analysis (BEEPA) to measure global maritime container shipping emissions from 2015 to 2020, and project possible pathways toward carbon neutrality by designing typical decarbonization scenarios. The result shows that global total seaborne container emissions fluctuated from 2015 to 2020 with a maximum value of 264 Mt, and the average annual energy consumption is 77.7 Mt (heavy fuel oil-equivalent). Container traffic to/from Asian ports generate the largest volumes of shipping emissions, accounting for about 55% of the global total. Under the most stringent scenario, container shipping emissions peak in 2025 and then quickly decline to 19.6 Mt in 2050, nearing the International Maritime Organization’s goal of reaching net zero emissions by or around 2050. Energy efficiency improvements contribute to emission reduction in the near term, but the trade growth impact still predominates in the shipping emission increase. With the maturity of infrastructural development and technological innovation, the energy transition would be the largest contributor emission reductions over the medium to long term.

The controllable manipulation of polar topological structures (e.g. skyrmion bubble) in ferroelectric materials have been considered as a cornerstone for future programmable nano-electronics. Here, we present the effective creation and erasure of polar bubble states PbTiO3 (PTO) multilayers trigged by mechanical stress and light illumination, respectively. It was found that applying atomic force microscope (AFM) tip force can induced formation of nanoscale bubble domains from the initial monodomain state. Moreover, the created bubble domain can be eliminated by exposure to ultraviolet or infrared light illumination. The above results can be understood by modulation of depolarization screening charges and bias fields, as reflected by scanning Kelvin potential microscopic (SKPM) observations, whereby the flexoelectric effect from the tip force tends to remove the screening charges on top surface and modulate the bias field that favors the formation of bubble state while light illumination tends to recover the screen charges and favor the monodomain state. The results provide a good example for multi-field manipulation of polar topologies, which might create a new avenue towards the immerging new concept electronic devices.

Memristive neural networks have extensively been investigated for their capability in handling various artificial intelligence tasks. The training performance of memristive neural networks depends on the pulse scheme applied to the constituent memristors. However, the design of the pulse scheme in most previous studies was approached in an empirical manner or through a trial-and-error method. Here, we choose ferroelectric tunnel junction (FTJ) as a model memristor and demonstrate a physics-based predictive model for the pulse design to achieve high training performance. This predictive model comprises a physical model for FTJ that can adequately describe the polarization switching and memristive switching behaviors of the FTJ and an FTJ-based neural network that uses the long-term potentiation (LTP)/long-term depression (LTD) characteristics of the FTJ for the weight update. Simulation results based on the predictive model demonstrate that the LTP/LTD characteristics with a good trade-off between ON/OFF ratio, nonlinearity, and asymmetry can lead to high training accuracies for the FTJ-based neural network. Moreover, it is revealed that an amplitude-increasing pulse scheme may be the most favorable pulse scheme as it offers the widest ranges of pulse amplitudes and widths for achieving high accuracies. This study may provide useful guidance for the pulse design in the experimental development of high-performance memristive neural networks.

Recently, there is a surge of research interest in configurable ferroelectric conductive domain walls which have been considered as possible fundamental building blocks for future electronic devices. In this work, by using piezoresponse force microscopy and conductive atomic force microscopy, we demonstrated the controlled manipulation of various conductive domain walls in epitaxial BiFeO3 thin films, e.g. neutral domain walls (NDW) and charged domain walls (CDWs). More interestingly, a specific type of nanoscale domains was also identified, which are surrounded by highly conductive circular CWDs. Similar nanoscale domains can also be controlled created and erasured by applying local field via conductive probe, which allow nondestructive current readout of different domain states with a large on/off resistance ratio up to 102. The results indicate the potential to design and develop high-density non-volatile ferroelectric memories by utilizing these programable conductive nanoscale domain walls.

As a high-k material, hafnium oxide (HfO2) has been used in gate dielectrics for decades. Since the discovery of polar phase in Si-doped HfO2 films, chemical doping has been widely demonstrated as an effective approach to stabilize the ferroelectric phase in HfO2 based thin films. However, the extra capping layer deposition, post-growth annealing and wake-up effect are usually required to arouse the ferroelectricity in HfO2 based thin films, resulting in the increase of complexity for sample synthesis and the impediment of device application. In this study, the ferroelectricity is observed in non-capped dopant-free HfO2 thin films prepared by pulsed laser deposition (PLD) without post-growth annealing. By adjusting the deposited temperature, oxygen pressure and thickness, the maximum polarization up to 14.7 μC/cm2 was obtained in 7.4 nm-thick film. The fraction of orthorhombic phase, concentrations of defects and size effects are considered as possible mechanisms for the influences of ferroelectric properties. This study indicates that PLD is an effective technique to fabricate high-quality ferroelectric HfO2 thin films in the absence of chemical doping, capping layer deposition and post-growth annealing, which may boost the process of nonvolatile memory device application.

Robust, fast, and low-power hardware platforms are desirable for the implementation of real-time machine vision. Here the authors develop a computing-in-sensor network using ferroelectric photo sensors with remanent-polarization-controlled photo responsivities.

Abstract A pole‐converging X‐band low‐noise amplifier (LNA) using 130 nm CMOS technology is proposed. An on‐chip pole‐converging capacitor CPC is added between the gate and drain node of the common‐gate (CG) stage. The capacitor CPC combines with a noise‐reducing inductor L1 to converge poles into the desired band, which results in a pole‐converging effect and wideband performance. The proposed modified broadband simultaneous noise and input‐matching technique is adopted in triple‐cascode configuration to realize good input matching and a low noise figure (NF). Measurement results exhibit a flat maximum power gain of 17.6 dB from 8 to 12 GHz and a reverse isolation over 60 dB within the desired bandwidth along with an NF ranging from 1.5 to 3.6 dB. The LNA core dissipates 17 mW from 2.4 V supply, and the chip size occupies 1.1 × 0.9 mm2 including all pads. The simulated and measured results show good agreement from 8 to 12 GHz.

BiFeO3, a single-phase multiferroic material, possesses several polymorphs and exhibits a strong sensitivity to strain. Recently, emergent strain engineering in BiFeO3 thin films has attracted intense interest, which can overcome the confines of traditional strain engineering introduced through the mismatch between the film and substrate. In this review, we discuss emerging non-traditional strain engineering approaches to create new ground states and manipulate novel functionalities in multiferroic BiFeO3 thin films. Through fabricating freestanding thin films, inserting an interface layer or utilizing thermal expansion mismatch, continuously tunable strain can be imposed beyond substrate limitations. Nanostructured evolution and defect introduction are discussed as efficient routes to introduce strain, promising for the development of new nanodevices. Ultrafast optical excitation, growth conditions and chemical doping driven strain are summarized as well. We hope this review will arouse the readers’ interest in this fascinating field.

Ferroelectric topological defects show emerging exotic physical properties, which are important for future nanoelectronic devices. Here, the authors demonstrate conductive metallic conduction channels in two types of topological defects, namely vortex and center cores.

Ferroelectric tunnel junction (FTJ) has attracted considerable attention for its potential applications in nonvolatile memory and neuromorphic computing. However, the experimental exploration of FTJs with high ON/OFF ratios is a challenging task due to the vast search space comprising of ferroelectric and electrode materials, fabrication methods and conditions and so on. Here, machine learning (ML) is demonstrated to be an effective tool to guide the experimental search of FTJs with high ON/OFF ratios. A dataset consisting of 152 FTJ samples with nine features and one target attribute (i.e., ON/OFF ratio) is established for ML modeling. Among various ML models, the gradient boosting classification model achieves the highest prediction accuracy. Combining the feature importance analysis based on this model with the association rule mining, it is extracted that the utilizations of {graphene/graphite (Gra) (top), LaNiO3 (LNO) (bottom)} and {Gra (top), Ca[Formula: see text]Ce[Formula: see text]MnO3 (CCMO) (bottom)} electrode pairs are likely to result in high ON/OFF ratios in FTJs. Moreover, two previously unexplored FTJs: Gra/BaTiO3 (BTO)/LNO and Gra/BTO/CCMO, are predicted to achieve ON/OFF ratios higher than 1000. Guided by the ML predictions, the Gra/BTO/LNO and Gra/BTO/CCMO FTJs are experimentally fabricated, which unsurprisingly exhibit [Formula: see text]1000 ON/OFF ratios ([Formula: see text]8540 and [Formula: see text]7890, respectively). This study demonstrates a new paradigm of developing high-performance FTJs by using ML.

Voltage driven magnetic switching (VDMS) in multiferroic heterostructure is highly demanded for next generation energy efficient high-density memory (e.g. magnetoelectric random access memory) and spintronic devices. For practical applications in large scale integrated device, it is imperative to understand the VDMS behaviors in nanometer scale. In this work, we have investigated the effects of geometric and anisotropy parameters on 180° VDMS behaviors in a model multiferroic heterostructure system consisting of Co nano-ellipse on BiFeO3 films by using micromagnetic simulation. It was revealed that the switching behaviors can be greatly affected by geometric factors, whereby dimension shrinkage and rising aspect ratio of Co nano-ellipse can apparently increase the critical exchange coupling field (hDMcri-field) needed for triggering the VDMS, greatly reducing the watchability of VDMS. To improve the watchability, an external static perpendicular magnetoanisotropy (Kp) can be introduced to reduce the hDMcri-field, whereas too large Kp tends to reorientate the magnetization towards out-of-plane orientation. Moreover, a strategy was also proposed to assist the VDMS by applying an anisotropy pulse, which is able to reduce the hDMcri-field and expand the switching window (e.g. wider range of aspect ratio and overall dimension size) for both in-plane and out-of-plane VDMS. These results may provide some guides for further experimental modulation of VDMS for device applications.

In this study, controllable morphology evolution of hexagonal strontium ferrite (SrFe12O19) micro/nano-crystals has been demonstrated. Single phase strontium ferrite platelets with hexagonal morphology were successfully prepared by conventional ceramic process. In the hexagonal crystals, it is revealed that the anisotropic growth rate is changed, with the increasing of ball milling time, from relatively high rate along the direction (c-axis) to direction, leading to the morphology evolution. Moreover, the optimal saturation magnetization (MS) is 69.5 emu/g, which is intensely close to the theoretical value (72 emu/g). This study provides the direct evidence of the enhanced reaction activity induced by high energy ball milling in strontium hexaferrite platelets and the obtained SrFe12O19 particles are promising for the hard magnet application and the magnetoelectric electronics.

We have comparatively studied the dielectric, ferroelectric, conduction, and photovoltaic properties of Ti/BaTiO3 (BTO)/SrRuO3 (SRO) and Pt/BTO/SRO capacitors. The resistive switching (RS) is observed in the Pt/BTO/SRO capacitor while it is absent in the Ti/BTO/SRO capacitor, which may be attributed to the interfacial layer existing between Pt and BTO and the Ti/BTO Ohmic interface, respectively. Further analyses on the conduction mechanisms suggest that the RS may be caused by the opening/closing of conduction paths in the Pt/BTO interfacial layer, whereas the polarization is ruled out as the origin of RS because of the inconsistency between the RS switching voltages and coercive voltages. On the other hand, it is observed that the photovoltaic effects (PVEs) in both Ti/BTO/SRO and Pt/BTO/SRO capacitors are electrically unswitchable and the open-circuit voltages of the two capacitors are similar in magnitude, implying that the PVE is driven by an internal bias field rather than the polarization-induced field. The existence of such an internal bias field is indicated by the self-polarization and imprint phenomena. Our study demonstrates that the interfacial layer and the internal bias field can be the major causes for the RS and PVE in certain ferroelectric capacitors, respectively, whereas the polarization may not necessarily play a role. [ABSTRACT FROM AUTHOR]

A strain-driven orthorhombic (O) to rhombohedral (R) phase transition is reported in La-doped BiFeO3 thin films on silicon substrates. Biaxial compressive epitaxial strain is found to stabilize the rhombohedral phase at La concentrations beyond the morphotropic phase boundary (MPB). By tailoring the residual strain with film thickness, we demonstrate a mixed O/R phase structure consisting of O phase domains measuring tens of nanometers wide within a predominant R phase matrix. A combination of piezoresponse force microscopy (PFM), transmission electron microscopy (TEM), polarization-electric field hysteresis loop (P-E loop), and polarization maps reveal that the O-R structural change is an antiferroelectric to ferroelectric (AFE-FE) phase transition. Using scanning transmission electron microscopy (STEM), an atomically sharp O/R MPB is observed. Moreover, X-ray absorption spectra (XAS) and X-ray linear dichroism (XLD) measurements reveal a change in the antiferromagnetic axis orientation from out of plane (R-phase) to in plane (O-phase). These findings provide direct evidence of spin-charge-lattice coupling in La-doped BiFeO3 thin films. Furthermore, this study opens a new pathway to drive the AFE-FE O-R phase transition and provides a route to study the O/R MPB in these films. [ABSTRACT FROM AUTHOR]

A wealth of fascinating phenomena have been discovered at the BiFeO3 domain walls, examples such as domain wall conductivity, photovoltaic effects, and magnetoelectric coupling. Thus, the ability to precisely control the domain structures and accurately study their switching behaviors is critical to realize the next generation of novel devices based on domain wall functionalities. In this work, the introduction of a dielectric layer leads to the tunability of the depolarization field both in the multilayers and superlattices, which provides a novel approach to control the domain patterns of BiFeO3 films. Moreover, we are able to study the switching behavior of the first time obtained periodic 109° stripe domains with a thick bottom electrode. Besides, the precise controlling of pure 71° and 109° periodic stripe domain walls enable us to make a clear demonstration that the exchange bias in the ferromagnet/BiFeO3 system originates from 109° domain walls. Our findings provide future directions to study the room temperature electric field control of exchange bias and open a new pathway to explore the room temperature multiferroic vortices in the BiFeO3 system. [ABSTRACT FROM AUTHOR]

We investigate how epitaxial strain affects the conductivity and charge transport of Ba0.6La0.4TiO3 (BLTO) films deposited by pulsed laser deposition. Depositing BLTO films with thicknesses varying from 14.1 to 95.9 nm gives differing epitaxial strains, with a maximum strain up to ~6% for the thinnest film of 14.1 nm. Transport measurements demonstrate that film thickness (i.e. epitaxial strain) affects the conductivity, carrier mobility, and concentration. Measurements of resistivity versus temperature demonstrate that all the BLTO films undergo a distinct semiconductor-metal phase transition, and the transition temperature TSM also depends clearly on strain. For all the films, the charge transport follows the small-polaron hopping mechanism below TSM and the thermal phonon scattering mechanism above TSM. These results prove that strain in epitaxial films strongly affects the conductivity and charge transport of traditional insulating ferroelectric oxide films. [ABSTRACT FROM AUTHOR]

La-doped BaTiO3 thin films (BLTO) were deposited by pulsed laser deposition, and a comprehensive investigation on their conductivity, charge transport, and ferroelectricity was carried out. Resistivity-temperature (T) measurements demonstrate that the conductivity of Ba1−xLaxTiO3 (x  =  0.05, 0.1, 0.2, 0.3, and 0.4) films can be largely regulated in a wide range. The BLTO film exhibits typical semiconductor conduction behavior for x  ⩽  0.3, whereas a typical metallic conduction behavior was observed for x  =  0.4 at T  >  240 K. The La doping concentration x also plays an important role on the charge transport mechanisms. For x  ⩽  0.3, the charge transport follows small-polaron hopping at low temperature (T  <  300 K) and thermal excitation at high temperature (T  ⩾  300 K). While the charge transport for x  =  0.4 changes from small-polaron hopping to thermal phonon scattering with the increase of temperature. Piezo-force microscopy measurements reveal that BLTO films exhibit ferroelectricity for La doping concentration x up to 0.4. These results demonstrated that La doping plays an important role in regulating the conductivity and charge transport in BLTO films. More importantly, we revealed experimentally that macro metallic conduction and ferroelectricity can coexist in the La-doped BaTiO3 epitaxial thin films. [ABSTRACT FROM AUTHOR]

Magnetic skyrmions in multilayer stack structures are promising magnetic memory bits for applications in race-track storage devices or random-access memories. For practical implementation in energy efficient memory devices, it is essential to achieve a deterministic switch of skyrmion states, e.g. nucleation and elimination, triggered by electric fields. In this work, by means of micromagnetic simulation, we demonstrate the strain-mediated electric field driven switching of magnetic skyrmion and other topological states without the presence of a magnetic field in a proposed device consisting of nanodiscs of trilayer Pt/Co/Ta stacks on a piezoelectric substrate. It is revealed that the multiple magnetic state switching behaviors, including the nucleation and elimination of skyrmion or labyrinth stripe domain states, as well as topological transitions between skyrmion, vortex, skyrmionium and vertical single domain states, can be triggered by applying an external electric field mediated by strains. The corresponding critical strains and their dependence on the geometric parameters of the nanodiscs (e.g. size and number of stacks) for triggering such magnetic switching are also explored. The stability of these topological domain states and possible switching behaviors (e.g. volatile and non-volatile) between the different states are discussed as well. The capability for controlled switching of these topological states provides a new pathway to energy efficient high-density magnetoelectric memory and logic devices. [ABSTRACT FROM AUTHOR]

The ferroelectric properties of 20 nm Hf0.5Zr0.5O2 (HZO) thin films has been investigated in a wide temperature range from 100 K to 450 K. The remnant polarization of HZO thin films decreases slightly from 24.6 μC cm−2 (100 K) to 17.9 μC cm−2 (450 K), indicating a robust temperature stability. The capacitors also exhibit excellent endurance properties up to 109 cycles at 100 K and 300 K, and their endurance cycles slightly degrades to 108 at an elevated temperature of 450 K. The results show that HZO ferroelectric thin films have great potential for future emerging memory applications in various harsh temperature environments. [ABSTRACT FROM AUTHOR]

Efficient and cost-competitive fabrication of high-quality ferroelectric and multiferroic nanostructures is of general interest. In this work, a top-down nano-patterning technique is developed by the Ar+ ion beam etching in combination with the sacrificed ultrathin anodic alumina (AAO) mask. This technique is demonstrated by preparation of the epitaxial BiFeO3 (BFO) nanostructures of various geometries, including nanodot and anti-nanodot arrays. The lateral dot size is as small as ∼60 nm and an ultrahigh dot density of ∼60 Gbit/inch2 is achieved. It is revealed that the etching process involves sequential shape evolution of both the AAO mask and the underlying BFO film, resulting in the nanodots and anti-nanodots arrays of various geometries. The as-etched BFO nanodots array exhibits well-established ferroelectric domain structures and reversible polarization switching, as examined by piezoresponse force microscopy (PFM). It is suggested that this technique is extendable to fabrication of a wide range of functional oxide nanostructures for potential nanoelectronic applications. [ABSTRACT FROM AUTHOR]