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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.

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.

Compared to electrochemical energy storage, dielectric thin film-based capacitors possess the advantages of higher voltage stability and higher break-down voltage as well as lower leakage current etc. Since HfO2 films are compatible to microelectronic process and its ferroelectricity is strategically important in memory device, the realization of their excellent energy storage comparable or better than those perovskite oxides will broaden their applications in microelectronic devices. In this work, to combine the merits of relaxor-ferroelectric and antiferroelectric, we experimentally demonstrate that a superparaelectric-like relaxor antiferroelectric behavior can be realized in the HfO2-based thin films and corresponding recoverable energy density over 100 J cm-3 can be achieved at efficiency higher than 80% as well as extremely high dielectric strength > 6 MV cm-1, compared to perovskite oxide materials. This is a record high energy density in all reported HfO2-based energy storage thin films, and beyond that, we also demonstrate their superfast charging/discharging as a capacitor. Targeting at high-speed supercapacitor applications in integrated circuit, HfO2-based dielectric may win the competition with perovskite oxides in terms of dielectric breakdown strength and charging/discharging speed etc. Our fundamental understanding of the physics behind also enriches the knowledge of materials science and dielectric physics.

In this work, a ReRAM-based energy-efficient CIM accelerator is presented with two techniques for edge AI applications. Firstly, a circuit-algorithm co-design scheme is proposed to realize fully analog processing, which improves the energy efficiency and the throughput of neural network. To deal with the I-V nonlinearity of ReRAM, a nonlinear-aware training algorithm is proposed to improve the network accuracy. Secondly, a 1T2R cell is proposed to replace previous 2T2R for weight storage with 35% area saving. For evaluation, a neural network with two fully connected layers and one ReLU layer is built for the MNIST dataset. The error rate can be reduced by >46% and the energy efficiency is 99 TOPS/W@200 MHz, 2.6X improvement over the digital method.

FeCo thin films with high saturation magnetization (4 πMs) can be applied in high-frequency electronic devices such as thin film inductors and microwave noise suppressors. However, due to its large magnetocrystalline anisotropy constant and magnetostrictive coefficient of FeCo, the coercivity (Hc) of FeCo films is generally high, which is detrimental to the soft magnetic properties. Meanwhile, the thickness and deposition temperature have significant effects on the coercivity and saturation magnetization of FeCo films. In this paper, FeCo thin films with different thicknesses were prepared by magnetron sputtering at different temperatures. The effects of thickness and deposition temperature on the microstructure and magnetic properties of FeCo thin films were systematically studied. When the film thickness increases from 50 nm to 800 nm, the coercivity would decrease from 309 Oe to 160 Oe. However, the saturation magnetization decreases from 22.1 kG to 15.3 kG. After that, we try to further increase the deposition temperature from room temperature (RT) to 475 °C. It is intriguing to find that the coercivity greatly decreased from 160 Oe to 3 Oe (decreased by 98%), and the saturation magnetization increased from 15.3 kG to 23.5 kG (increased by 53%) for the film with thickness of 800 nm. For the film with thickness of 50 nm, the coercivity also greatly decreased from 309 Oe to 10 Oe (decreased by 96%), but the saturation magnetization did not change significantly. It is contributed to the increase of deposition temperature, which will lead to the increase of grain size and the decrease of the number of grain boundaries. And the coercivity decreases as the number of grain boundaries decreases. Meanwhile, for the thicker films, when increasing the deposition temperature the thermal stress increases, which changes the appearance of (200) texture, and the saturation magnetization increases. Whereas, it has a negligible effect on the orientation of thin films with small thickness (50 nm). This indicates that high-temperature deposition is beneficial to the soft magnetic properties of FeCo thin films, particularly for the films with larger thickness. This FeCo thin film with high saturation magnetization and low coercivity could be an ideal candidate for high-frequency electronic devices.

When scaling the lateral size of a ferroelectric random access memory (FeRAM) device down to the nanometer range, the polarization switching-induced displacement current becomes small and challenging to detect, which greatly limits the storage density of FeRAM. Here, we report the observation of significantly enhanced injection currents, much larger than typical switching currents, induced by polarization switching in BiFeO3 thin films via conductive atomic force microscopy. Interestingly, this injected current can be effectively modulated by applying mechanical force. As the loading force increases from ∼50 to ∼750 nN, the magnitude of the injected current increases and the critical voltage to trigger the current injection decreases. Notably, changing the loading force by an order of magnitude increases the peak current by 2-3 orders of magnitude. The mechanically boosted injected current could be useful for the development of high-density FeRAM devices. The mechanical modulation of the injected current may be attributed to the mechanical force-induced changes in the barrier height and interfacial layer width.

Summary The limitation of commercial single-crystal substrates and the lack of continuous strain tunability preclude the ability to take full advantage of strain engineering for further exploring novel properties and exhaustively studying fundamental physics in complex oxides. Here, we report an approach for imposing continuously tunable epitaxial strain in oxide heterostructures beyond substrate limitations by inserting an interface layer through tailoring of its gradual strain relaxation. Taking BiFeO3 as a model system, we demonstrate the introduction of an ultrathin interface layer that allows the creation of desired strain states that can induce phase transition and stabilize a super-tetragonal phase as well as morphotropic phase boundaries, overcoming substrate limitations. Continuously tunable strain from tension to compression can be generated by precisely adjusting the interface layer thickness, enabling the achievement of continuous orthorhombic–rhombohedral-like–tetragonal-like phase transition. This proposed route could be extended to other oxide heterostructures, providing a platform for creating exotic phases and emergent phenomena.

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.

In this article, a novel low-power notch-enhanced active filter is presented using the feedback notch-enhanced (FNE) technique. The active notch filter is proposed based on an area-saving second-order $LC$ series resonator. The passive devices’ loss can be effectively eliminated by the generated dc power-independent negative resistance by inserting a current feedback transistor. Moreover, the capacitive source degeneration (CSD) technique is proposed to boost the filter $Q$ by shifting the series resonant frequency to the negative-resistance region. To demonstrate the feasibility and practicability of the proposed notch filter, a 3.1–10.6-GHz IEEE 802.11a interferer rejected low-noise amplifier (IR-LNA) is fabricated in a 0.13- $\mu {\hbox {m}}$ CMOS process. The multistage structure, consisting of a common-gate (CG) stage with a dual-resonance load and a cascode stage with the proposed active notch filter, is employed for wideband input matching, gain flatness, and interferer rejection (IR) simultaneously. The measurements show that the IR-LNA provides a maximum of 16.05-dB power gain, 2.08–3.70-dB noise figure (NF) in the desired band, and a maximum of 40.9-dB IR ratio (IRR) at 5.77 GHz, with a power consumption of 10.2 mW, while the active notch filter only consumes 456- $\mu {\hbox {A}}$ current.

Ferroelectric topological objects (e.g. vortices, skyrmions) provide a fertile ground for exploring emerging physical properties that could potentially be utilized in future configurable nanoelectronic devices. Here, we demonstrate quasi-one-dimensional metallic high conduction channels along two types of exotic topological defects, i.e. the topological cores of (i) a quadrant vortex domain structure and (ii) a center domain (monopole-like) structure confined in high quality BiFeO3 nanoisland array, abbreviated as the vortex core and the center core. We unveil via phase-field simulations that the superfine (< 3 nm) metallic conduction channels along center cores arise from the screening charge carriers confined at the core whereas the high conductance of vortex cores results from a field-induced twisted state. These conducting channels can be repeatedly and reversibly created and deleted by manipulating the two topological states via an electric field, leading to an apparent electroresistance effect with an on/off ratio higher than 103. These results open up the possibility of utilizing these functional one-dimensional topological objects in high-density nanoelectronic devices such as ultrahigh density nonvolatile memory., manuscript (21 pages, 4 figures) plus supplementary information

Ferroelectrics with negative capacitance effect can amplify the gate voltage in field-effect transistors to achieve low power operation beyond the limits of the Boltzmann's Tyranny. The reduction of power consumption depends on the capacitance matching between the ferroelectric layer and gate dielectrics, which can be well controlled by adjusting the negative capacitance effect in the ferroelectrics. However, it is a great challenge to experimentally tune the negative capacitance effect. Here, the observation of tunable negative capacitance effect in ferroelectric KNbO3 through strain engineering is demonstrated. The magnitude of the voltage reduction and negative slope in polarization-electric field (P-E) curves as the symbol of negative capacitance effect can be controlled by imposing various epitaxial compressive strain. The expansion of negative curvature region in polarization-energy landscape under compressive strain is responsible for the tunable negative capacitance. Our work paves the way for fabricating low power devices and further reducing energy consumption in electronics.

Capacitive deionisation (CDI), as a novel electrochemical technology, has attracted a fast-growing research interest over the past years. However, the single function of salt removal limits further development of conventional CDI systems. Herein, we propose a multi-function CDI device, which can achieve the electrochemical desalination, degradation of organic pollutants and removal of heavy metal ions through the redox electrocatalytic deionisation technology. Sulphur-doped reduced graphene oxide (S@rGO) is utilised as the anode to catalyse the degradation of rhodamine B. The Pt-coated graphite paper is used as a cathode for the removal of dichromate ions by the electro-reduction process. The voltage plateau of S@rGO is reduced by ∼0.35 V compared with the pure carbon cloth electrode, indicating the effectiveness of S@rGO as the catalyser. The salt feed is desalted to ∼0.74 mS·cm−1 level at a current density of 1 mA·cm−2. The degradation efficiency of rhodamine B (RhB) and potassium dichromate are 99.1% and 55.3%, respectively. The effects of different current densities and cycling are also investigated. This work is significant for the multi-functional process of water treatment and the device design. : The single function of salt removal limits the further development of the CDI system. A multi-function CDI device is proposed to achieve electrochemical desalination, organics degradation and dichromate ion removal.

Perceiving nanoscale ferroelectric phenomena from real space is of great importance for elucidating underlying ferroelectric physics. During the past decades, nanoscale ferroelectric characterization has mainly relied on the Piezoresponse Force Microscopy (PFM) invented in 1992, however, the fundamental limitations of PFM have made the nanoscale ferroelectric studies encounter significant bottlenecks. In this study, a high-resolution non-contact ferroelectric measurement, named Non-Contact Heterodyne Electrostrain Force Microscopy (NC-HEsFM), is introduced. It is demonstrated that NC-HEsFM can operate on multiple eigenmodes to perform ideal high-resolution ferroelectric domain mapping, standard ferroelectric hysteresis loop measurement, and controllable domain manipulation. By using a quartz tuning fork (QTF) sensor, multi-frequency operation, and heterodyne detection schemes, NC-HEsFM achieves a real non-contact yet non-destructive ferroelectric characterization with negligible electrostatic force effect and hence breaks the fundamental limitations of the conventional PFM. It is believed that NC-HEsFM can be extensively used in various ferroelectric or piezoelectric studies with providing substantially improved characterization performance. Meanwhile, the QTF-based force detection makes NC-HEsFM highly compatible for high-vacuum and low-temperature environments, providing ideal conditions for investigating the intrinsic ferroelectric phenomena with the possibility of achieving an atomically resolved ferroelectric characterization.

Complex oxides with tunable structures have many fascinating properties, though high-quality complex oxide epitaxy with precisely controlled composition is still out of reach. Here we have successfully developed solution-based single-crystalline epitaxy for multiferroic (1-x)BiTi(1-y)/2FeyMg(1-y)/2O3–(x)CaTiO3 (BTFM–CTO) solid solution in large area, confirming its ferroelectricity at the atomic scale with strong spontaneous polarization. Careful compositional tuning leads to a bulk magnetization of 0.07 ± 0.035 μB/Fe at room temperature, enabling magnetically induced polarization switching exhibiting a large magnetoelectric coefficient of 2.7–3.0 × 10−7 s/m. This work demonstrates the great potential of solution processing in large-scale complex oxide epitaxy and establishes novel room-temperature magnetoelectric coupling in epitaxial BTFM–CTO film, making it possible to explore a much wider space of composition, phase, and structure that can be easily scaled up for industrial applications.

Single crystal cupric oxide (CuO) nanorod (NR) arrays are fabricated on Si substrates via a facile two-step strategy of sputter depositing Cu films followed by thermal oxidation. The Cu film with a structure of Si substrate/Cr or/and Cu fine grain buffer layer/Cu columnar crystal layer has been obtained by a step-by-step deposition process and employing different deposition parameters at each step. This structure is demonstrated to be beneficial for the NR growth and the NRs-substrate adhesion after thermal oxidation. The growth mechanism of the designed layer structure of Cu film and CuO NRs has been investigated. Both the fine grain buffer layer and Cu columnar crystal layer contribute to the diffusion of Cu atoms and the release of thermal stress during heating. To develop a Schottky nanocontact for electronic device, the nanoscale electrical properties of the p-type CuO NRs are characterized by conductive atomic force microscopy (C-AFM). The in-situ current-voltage (I V) characterization shows that the upright standing CuO NRs exhibit good rectifying characteristics, which is attributed to the high specific surface area caused surface states. This study may promote the practical applications of p-type CuO nanostructures in electrical nanodevices.

Multiferroic nanostructures have been attracting tremendous attention over the past decade, due to their rich cross-coupling effects and prospective electronic applications. In particular, the emergence of some exotic phenomena in size-confined multiferroic systems, including topological domain states such as vortices, center domains, and skyrmion bubble domains, has opened a new avenue to a number of intriguing physical properties and functionalities, and thus underpins a wide range of applications in future nanoelectronic devices. It is also highly appreciated that nano-domain engineering provides a pathway to control the magnetoelectric properties, which is promising for future energy-efficient spintronic devices. In recent years, this field, still in its infancy, has witnessed a rapid development and a number of challenges too. In this article, we shall review the recent advances in the emergent domain-related exotic phenomena in multiferroic nanostructures. Specific attention is paid to the topological domain structures and related novel physical behaviors as well as the electric-field-driven magnetic switching via domain engineering. This review will end with a discussion of future challenges and potential directions.

Achieving high power conversion efficiencies (PCEs) in ferroelectric photovoltaics (PVs) is a longstanding challenge. Although recently ferroelectric thick films, composite films, and bulk crystals have all been demonstrated to exhibit PCEs >1%, these systems still suffer from severe recombination because of the fundamentally low conductivities of ferroelectrics. Further improvement of PCEs may therefore rely on thickness reduction if the reduced recombination could overcompensate for the loss in light absorption. Here, a PCE of up to 2.49% (under 365-nm ultraviolet illumination) was demonstrated in a 12-nm Pb(Zr0.2Ti0.8)O3 (PZT) ultrathin film. The strategy to realize such a high PCE consists of reducing the film thickness to be comparable with the depletion width, which can simultaneously suppress recombination and lower the series resistance. The basis of our strategy lies in the fact that the PV effect originates from the interfacial Schottky barriers, which is revealed by measuring and modeling the thickness-dependent PV characteristics. In addition, the Schottky barrier parameters (particularly the depletion width) are evaluated by investigating the thickness-dependent ferroelectric, dielectric and conduction properties. Our study therefore provides an effective strategy to obtain high-efficiency ferroelectric PVs and demonstrates the great potential of ferroelectrics for use in ultrathin-film PV devices. An approach to boost the power conversion efficiencies (PCEs) of ferroelectric photovoltaics (PVs) is proposed based on the Schottky barrier effect. This approach leverages the thinning of a ferroelectric film to somewhere close to the depletion width, which can simultaneously suppress the recombination and lower the series resistance. Using this approach, we achieve a PCE up to 2.49% (under 365-nm ultraviolet illumination) in the 12-nm Pb(Zr0.2Ti0.8)O3 ultrathin films. Our study provides insightful guidance on how to design and tailor the ferroelectric films to achieve high PCEs, and also demonstrates the great potential of ferroelectrics for use in ultrathin-film PV devices. Eliminating stray electrical effects in ultra-thin films can help optimize an unconventional solar energy technology. Ferroelectric materials have internal dipoles that spontaneously move photogenerated charges toward external circuits, producing higher power outputs than predicted by theory. Zhen Fan from South China Normal University in Guangzhou and colleagues now report that the dimensions of ferroelectric thin films distinctly affect how efficiently they convert sunlight into electricity. Measurements of solar cells containing lead-zirconium-titanate ferroelectrics with different thicknesses revealed a jump in conversion efficiencies when the film reached a thickness of 12 nanometers. Further analysis showed that this thickness correlates with the solar cell’s ‘depletion width’, a zone formed when metal electrodes contact the film. The electric field in the depletion zone complements the pushing actions of the ferroelectric dipoles, lowering electrical losses compared to thicker ferroelectric films.

Rare earth (RE) permanent magnetic thin films, such as SmCo-based films, are promising candidates for future thermal-assisted magnetic recording media because of their supreme thermal stability and fine superparamagnetic critical size. Phase compositional distributions and crystallographic orientations can directly influence the magnetic domain evolutions and magnetic performance of SmCo films. However, these films often exist in multi-phases without well-defined distinct magnetic anisotropy, thereby causing difficulties in analyzing their functions. The effects of atomic diffusions induced by annealing on crystallization and magnetic anisotropy energy are poorly understood. In this work, the influence of doping Cu atoms via introducing Cu layer on magnetic anisotropy of Cr/SmCo/(Cu)/Cr films is investigated. Introducing a Cu layer and post-annealing lead to tunable magnetic anisotropy from isotropy to anisotropy and domain structure formation. Micromagnetic simulation is used to further analyze the effects of crystallization, anisotropy field and compositional distribution on the magnetization reversal behavior and coercivity variations in SmCo films. It reveals that the differences of anisotropy field of Co and amorphous phases could affect the domain wall motions and coercivity, which shows the analogous trend as an experimental result. Moreover, the elevated anisotropy constant of Sm(Co,Cu) alloy and the small fractions of SmCo5 boundary phases are also beneficial to the enhancement of coercivity derived by calculating domain wall energy. In-plane magnetic anisotropy is improved by the SmCo5 phase, which has an in-plane preferred orientation of the c axis. This work provides theoretical and experimental bases for the future fabrication of SmCo-based films by controlling atomic diffusion with optimized grain boundary phase distribution.

The controllable magnetic skyrmion motion represents a highly concerned issue in preparing advanced skyrmion-based spintronic devices. Specifically, magnon-driven skyrmion motion can be easily accessible in both metallic and insulating magnets, and thus is highly preferred over electric current control further for the ultra-low energy consumption. In this work, we investigate extensively the dynamics of skyrmion motion driven by magnon in an antiferromagnet using the collective coordinate theory, focusing on the effect of magnon polarization. It is revealed that the skyrmion Hall motion driven by circularly polarized magnon becomes inevitable generally, consistent with earlier report. Furthermore, the elastic scattering theory and numerical results unveil the strong inter-dependence between the linearly polarized magnon and skyrmion motion, suggesting the complicated dependence of the skyrmion motion on the polarization nature of driving magnon. On the reversal, the scattering from the moving skyrmion may lead to decomposition of the linearly polarized magnon into two elliptically polarized magnon bands. Consequently, a net transverse force acting on skyrmion is generated owing to the broken mirror symmetry, which in turn drives a skyrmion Hall motion. The Hall motion can be completely suppressed only in some specific condition where the mirror symmetry is preserved. The present work unveils non-trivial skyrmion-magnon scattering behavior in antiferromagnets, advancing the antiferromagnetic spintronics and benefiting to high-performance devices., 22 pages,7 figures

A physical understanding of the nature of spatially modulated phases (SMPs) in rare-earth-doped antiferrodistortive (AFD) multiferroics and how they behave close to surfaces and interfaces is lacking. Here the emergence of the antiferroelectric (AFE), ferroelectric (FE), or ferrielectric (AFE-FE) spatial modulation in the vicinity of the morphotropic phase transition in ${\mathrm{La}}_{x}{\mathrm{Bi}}_{1--x}\mathrm{Fe}{\mathrm{O}}_{3}$ ($x\ensuremath{\sim}0.2$) is explored on the atomic level using high-resolution scanning transmission electron microscopy (HRSTEM). The suppression, or ``melting,'' of the AFE-type SMP in the vicinity of the AFD twin wall/surface junction is revealed by HRSTEM in ${\mathrm{La}}_{0.22}{\mathrm{Bi}}_{0.78}\mathrm{Fe}{\mathrm{O}}_{3}$ films and explained by the hybrid approach combining Landau-Ginzburg-Devonshire (LGD) phenomenology and the semimicroscopic four-sublattice model (FSM). The LGD-FSM approach reduces the problem of AFE (or AFE-FE) SMP emergence and stability to the thermodynamic analysis of the free-energy functional with AFE, FE, and AFD long-range order parameters and two master parameters: the FE-AFE coupling strength between four neighboring A sites and the nonstoichiometry factor, which are proportional to the variations of La concentration in ${\mathrm{La}}_{x}{\mathrm{Bi}}_{1--x}\mathrm{Fe}{\mathrm{O}}_{3}$ films. We establish that the surface-induced melting of SMPs and the associated broadening of AFE AFD domain walls minimize the film free energy under certain conditions imposed on the master parameters and gradient energy below the critical value. The observed behavior provides insight into the origin of SMPs in AFD multiferroics.

Searching for new methods controlling antiferromagnetic (AFM) domain wall is one of the most important issues for AFM spintronic device operation. In this work, we study theoretically the domain wall motion of an AFM nanowire, driven by the axial anisotropy gradient generated by external electric field, allowing the electro control of AFM domain wall motion in the merit of ultra-low energy loss. The domain wall velocity depending on the anisotropy gradient magnitude and intrinsic material properties is simulated based on the Landau-Lifshitz-Gilbert equation and also deduced using the energy dissipation theorem. It is found that the domain wall moves at a nearly constant velocity for small gradient, and accelerates for large gradient due to the enlarged domain wall width. The domain wall mobility is independent of lattice dimension and types of domain wall, while it is enhanced by the Dzyaloshinskii-Moriya interaction. In addition, the physical mechanism for much faster AFM wall dynamics than ferromagnetic wall dynamics is qualitatively explained. This work unveils a promising strategy for controlling the AFM domain walls, benefiting to future AFM spintronic applications., Comment: 17 pages, 5 figures

The magnetic anisotropy of SmCo-based permanent magnetic thin films are generally controlled by introducing buffer layer or heating substrate. However, the design for the multi-layer thin film system is complicated and the thickness of SmCo is limited due to a short range of interfacial stress/strain between the buffer layer and SmCo layer. In this work, the effects of temperature gradients, generated by RTP (rapid thermal process), on the magnetic anisotropy of the SmCo-based films were systematically investigated. The results show that the as-deposited films exhibit amorphous state. The out-of-plane coercivity of RTP-treated films is strongly correlated with the heating rate, and its optimum value (2810 Oe) is larger than that of the CTA (conventional thermal annealing) treated films (1670 Oe). For the RTP-treated films, the intensity of characteristic diffraction peaks for in-plane oriented SmCo5 (200) and Sm2Co17 (300) decreases, together with reducing the roughness. Besides, the grain size is finer and the contrast of magnetic domains becomes stronger for the RTP-treated films due to that the easy axis of SmCo5/Sm2Co17 phase gradually changes from in-plane to out-of-plane direction. It is suggested that the RTP treatment with a shorter annealing time has beneficial effects on obtaining finer microstructure and improving the out-of-plane magnetic anisotropy of the SmCo-based films. This work provides a novel way to control the magnetic anisotropy via annealing temperature gradients for permanent magnetic thin films, compared with conventional methods, to induce the magnetic anisotropy.

We propose a novel method to drive phase transitions by etching strained BiFeO3 thin films to nanoislands. Atomic force microscopy (AFM) measurements reveal that the amount of rhombohedral-like (R) phase increases as the BiFeO3 thin films with tetragonal-like (T) matrix are etched to nanoislands and larger fraction of R phase can be obtained with the size reduction from 1 μm to 200 nm, indicating the T to R phase transitions induced by partial release of substrate clamping. Using piezoresponse force microscopy (PFM), it is demonstrated that rhombohedral-like (R) to tetragonal-like (T) phase transitions can be reversibly achieved under DC electric field in BFO nanoislands. Large electromechanical response has been observed in BFO nanoislands as well. This approach can be extended to other strained oxide films and provide guidance for the development of high-performance electromechanical lead-free materials.

Coupling effects among mechanical, electrical and magnetic parameters in a material provide exciting opportunity for creating novel device functionalities. Here, bilayer composite films combining magnetic shape memory alloy Ni-Mn-Ga (NMG) and antiferroelectric (AFE) Pb0.97La0.02Zr0.95Ti0.05O3 (PLZT) have been fabricated. X-ray diffraction, magnetization, and magnetic domain measurements demonstrated an austenite-martensite transformation of NMG top-layer during the annealing. Meanwhile, an AFE to ferroelectric transition of the PLZT sub-layer is observed, which can be attributed to the interfacial stress coupling. The quantitative residual stress analysis revealed that the change of residual stress in the PLZT layer is up to 300 MPa during the annealing, confirming the ferroelastic stress transfer between the NMG and PLZT layers. These results open up a new pathway for interfacial elastic coupling driven multi-functionalities in composite films.

A novel two-step nano-patterning method is proposed to fabricate epitaxial ferroelectric BiFeO 3 (BFO) nanoring array, which maintains well-epitaxial structure and possesses strong ferroelectricity demonstrated by X-ray diffraction (XRD) and piezoresponse force microscopy (PFM). The ferroelectric polarizations were examined by PFM, revealing the reversible switching behavior under an electric field. This novel method could also be extended to other oxide material systems. The fabrication of high quality ferroelectric nanoring structure provides the possibility to explore novel functionalities (e.g., ferroelectric vortices) and offers application potentials for the high-density non-volatile memory devices.

Understanding of the metal-insulator transition (MIT) in correlated transition-metal oxides is a fascinating topic in condensed matter physics and a precise control of such transitions plays a key role in developing novel electronic devices. Here we report an effective tuning of the MIT in epitaxial $\mathrm{SrV}{\mathrm{O}}_{3}$ (SVO) films by expanding the out-of-plane lattice constant without changing in-plane lattice parameters, through helium ion irradiation. Upon increase of the ion fluence, we observe a MIT with a crossover from metallic to insulating state in SVO films. A combination of transport and magnetoresistance measurements in SVO at low temperatures reveals that the observed MIT is mainly ascribed to electron-electron interactions rather than disorder-induced localization. Moreover, these results are well supported by the combination of density functional theory and dynamical mean-field theory calculations, further confirming the decrease of the bandwidth and the enhanced electron-electron interactions resulting from the expansion of the out-of-plane lattice constant. These findings provide insights into the understanding of MIT in correlated oxides and perspectives for the design of unexpected functional devices based on strongly correlated electrons.

Scanning Transmission Electron Microscopy (STEM) has enabled mapping of atomic structures of solids with sub-pm precision, providing insight to the physics of ferroic phenomena and chemical expansion. However, only a subset of information is available, due to projective nature of imaging in the beam direction. Correspondingly, the analysis often relies on the postulated known form of macroscopic Landau-Ginzburg energy, and some predefined relationship between experimentally determined atomic coordinates and the order parameter field. Here, we propose an approach for exploring the structure of ferroics using reduced order parameter models constructed based on experimental data only. We develop a four sublattices model (FSM) for the analytical description of A-cation displacement in (anti)ferroelectric-antiferrodistortive perovskites of ABO3-type. The model describes the displacements of cation A in four neighboring unit cells and determines the conditions of different structural phases appearance and stability in ABO3. We show that FSM explains the coexistence of rhombohedral (R), orthorhombic (O) and spatially modulated (SM) phases, observed by atomic-resolution STEM in La-doped BiFeO3. Using this approach, we atomically resolve and theoretically model the sublattice asymmetry inherent to the case of the A-site La/Bi cation sublattice in LaxBi1-xFeO3 polymorphs. This approach allows exploring the ferroics behaviors from experimental data only, without additional assumptions on the nature of the order parameter., 20 pages, 4 figures, supplementary materials

Recent advances in scanning transmission electron microscopy (STEM) have enabled direct visualization of the atomic structure of ferroic materials, enabling the determination of atomic column positions with ~pm precision. This, in turn, enabled direct mapping of ferroelectric and ferroelastic order parameter fields via the top-down approach, where the atomic coordinates are directly mapped on the mesoscopic order parameters. Here, we explore the alternative bottom-up approach, where the atomic coordinates derived from the STEM image are used to explore the extant atomic displacement patterns in the material and build the collection of the building blocks for the distorted lattice. This approach is illustrated for the La-doped BiFeO3 system.The full analysis procedure is available as an interactive paper in a form of a Google Colab (Jupyter) notebook where a classical paper organization is augmented with code cells that appear hidden by default (when viewed in Google Colab). This should allow a reader to retrace the analysis and, more importantly, it enables the readers to use the same codes for their data. The same paper is also available in a standard pdf format (without code).

We report a novel route to synthesize Co3O4 single nanocrystals using pulsed laser deposition by decomposition of BiCoO3. The decomposition of BiCoO3 to Bi2O3 and Co3O4 at relatively high temperature enables the formation of pure Co3O4 phase. The absence of Bi element is confirmed by both X-ray diffraction (XRD) and energy dispersive spectrometer (EDS) measurements. The effects of various PLD growth conditions, including growth rate (2–20 Hz), growth temperature (630–650 °C), O2 partial pressure (50–200 mTorr) and laser energy density (0.6–1.8 J cm−2), on the synthesis of Co3O4 nanocrystals were systematically studied. Interestingly, the orientation of Co3O4 single nanocrystals can be controlled by changing the growth parameters. It is revealed that the decrease of the laser energy density leads to the preferred orientation of Co3O4 single nanocrystals altering from (111) plane to (220), while the high temperature growth favors (400) orientation. The obtained orientation-controllable Co3O4 single nanocrystals provide the possibility to study its orientation-dependent physical and chemical properties as catalysts, anode materials and magnetic materials.