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Magnetic sensors have various applications in navigation, power distribution, robotics, factory automation, and medical diagnosis. The development of highly sensitive magnetic sensors usually requires complicated and costly fabrication process. Herein, we report giant magneto-impedance of 41036% in the commercially available ferrite core inductors. A magnetic field detection limit of 10 nT at 1 Hz has been obtained by directly measuring the impedance of the as-obtained inductor without any optimization. With a 100 pF capacitor in series connection with the inductor where lower impedance facilitates the measurement process, a limit of detection of 625 pT at 1 Hz has been obtained in the series RLC resonator. These results can be understood in terms of the magnetic field-dependent self-resonance in the inductors which act as lumped RLC resonators. Compared with the traditional electromagnetic induction sensing mode, the magneto-impedance sensing mode shows 5000 folds improvement in the magnetic field detection capability.

Piezoelectric ceramics exhibit three conventional piezoelectric coefficients, i.e., d33, d31, d15, due to their ∞ m m crystal symmetry. Unconventional piezoelectric coefficients, such as d11, d12, d13, d14, d16, etc., can only be extracted from piezoelectric single crystals of special symmetry with specific cut direction. Here we demonstrate a rotated poling method to realize unconventional piezoelectric coefficients in perovskite piezoelectric ceramics. This method is elaborated in theory and experimentally proven to be effective. Full nonzero piezoelectric coefficients in the 3 × 6 piezoelectric coefficients matrix can be obtained by combining these “quasi (effective) piezoelectric coefficients” with the conventional piezoelectric coefficients, which would expand applications in a wide variety of piezoelectric devices.

Magnetoelectric (ME) composites of bulk structures show strong ME coupling performance, and thus magnetoelectric bulk composites and their related devices have been attracting increasing attention over the last few years. Two kinds of ME coupling mechanisms can be found in ME bulk composites, i.e., direct ME coupling and converse ME coupling, where electric field control of magnetization or magnetic field control of ferroelectric polarization can be achieved. Then, ultra-fast, low-power, and miniaturized electronics can be developed based on the novel functionalities of both direct and converse ME coupling effects. Direct ME coupling is used in devices such as ME sensors and energy harvesters, while converse ME coupling is used in voltage tunable inductors, bandstop filters, ME antennas, tunable resonators, etc. Here, we present a review of bulk structure ME composite materials, as well as some notable potential applications of ME composites, with emphasis on both the opportunities and challenges for the application of ME composites. The performance of ME bulk composites, their coupling structures, state-of-the-art device applications, and perspectives on direct (magnetic to electric) and converse (electric to magnetic) ME devices are summarized.

Strain-mediated magnetoelectric effect can be utilized as an energy-efficient approach for spin manipulation. However, over 90° magnetization rotation is still challenging in un-patterned magnetic films, as the piezo-strain driven by ferroelectric domain switching is generally uniaxial rather than unidirectional, which limits the developments of non-volatile magnetic memory and logic devices. Here we demonstrate the rotation of local magnetization with a large angle of 136° by applying strains with a shear component at a fixed magnetic field of 45 Oe in FeCoSiB/PMN-PT (011) multiferroic heterostructures, revealed by a vector-resolved quantitative magneto-optical Kerr effect (MOKE) microscopy. Phase-field simulations confirm that the approximate 140° rotation of magnetization vectors is a consequence of the shear strain associated with ferroelectric/ferroelastic switching of PMN-PT (011) substrates. The visualization of over 90° magnetization rotation induced by the strain with a shear component paves the way for deterministic magnetization switching that has important implications in the energy-efficient spintronic devices.

Voltage control magnetism is one of the most energy efficient pathway towards magnetoelectric (ME) device. Ionic liquid gating (ILG) method has already shown impressive manipulation power at the IL/electrode interface to influence the structure, orbital as well as spin of the electrode materials. As key material in anisotropy magnetoresistance sensor and spin valve heterostructure, the permalloy Ni0.81Fe0.19 was utilized as the electrode to investigate the ILG induced magnetic anisotropy change. In this work, we realized magnetic anisotropy control in Au/[DEME]+[TFSI]-/Ni0.81Fe0.19 (2.5 nm)/Ta heterostructure via ILG caused electrostatic doping. This is evidenced in situ reversible ferromagnetic field (Hr) shift with electron spin resonance (ESR) spectrometer. Aiming at the question whether the charge accumulation at the ionic liquid interface is the main control mechanism at low voltage, we carefully tested the relationship between the change of resonance field and the amount of surface charge. It was found that these two had a good linear relationship between −1 V and +1 V. Defining the linear parameter as A whose value is 28.7 mT m2/Col. Unlike previously reported chemical regulation of Co, this article used ionic liquids to physically regulate NiFe, which has not been studied in the previous ionic liquid regulation. And NiFe has a narrower resonance line width for easy reference to microwave devices. In addition, It also has a stronger ferromagnetic signal than Co, which can be more easily detected as a sensor device. Therefore, this system is more promising. The ILG control NiFe may lead to a new kind of magnetoelectric sensor devices and path a new way to low energy consumption spintronics.

Piezoelectric devices based on d36 mode are remarkably stable that depolarization rarely occurs in d36 face shear mode, because d36 mode is completely different from d15 thickness shear mode, where the applied electric field is perpendicular to poling direction and depolarization is ineluctable due to 90° dipole rotation. However, piezoelectric ceramics conventionally possess three piezoelectric coefficients (i.e., d33, d31, and d15), while d36 only exists in single crystals of specific point groups and cut directions. In this work, we propose a method to realize d36 piezoelectric coefficient in tetragonal piezoelectric ceramics by mechanical and electric domain engineering. This method is successfully applied in bismuth scandium-lead titanate (abbreviated as BS-PT) high-temperature piezoelectric ceramics with a resultant d36 up to 160 pC/N, which broadens the application of BS-PT ceramics, such as face shear actuator, energy harvester, transducer, etc. We find that domain engineering by transversal electric poling is preferred compared with the transversal mechanical poling, due to the simpler process, higher reliability, and higher resultant d36 piezoelectric coefficient. By combining the Diffraction-Plane-Transformation (DPT) model with the domain engineering via transversal electric poling, we demonstrate a quantitative domain engineering method for the first time, which could be used for optimizing the piezoelectric properties by precise design of the domain structures in piezoelectric materials.

Ferroelectric resistive switching has recently attracted considerable attention as a promising candidate for next-generation non-volatile memory. In this work, we report an electro and photon double-driven bipolar resistive switching behavior in Pt /Bi0.9Eu0.1FeO3 (BEFO) /Nb-doped SrTiO3 (NSTO) heterostructures prepared via pulsed laser deposition. In addition to the polarization-based control of the resistive memory, a switchable photovoltaic effect is observed that can be used to detect the polarization direction non-destructively. Significantly, the electric field-modulated interfacial barrier can be further affected by photon-generated carriers. This phenomenon is attributed to the barrier modulation in the Pt /BEFO and BEFO /NSTO interfaces by electric field and photon excitation. These results indicate the feasibility of non-volatile and non-destructive readout from ferroelectric memory.

The operation mechanism of giant magnetoresistance (GMR) sensors relies on the linear response of the magnetization direction to an external magnetic field. Since the magnetic anisotropy of ferromagnetic layers can be manipulated by a strain-mediated magnetoelectric coupling effect, we propose a tunable GMR magnetic field sensor design that allows for voltage tuning of the linear range and sensitivity. A spin valve structure Ru/CoFe/Cu/CoFe/IrMn/Ru is grown on a PMN-PT (011) substrate, and the magnetization directions of ferromagnetic layers can be controlled by an electric field. An adjustable linear magnetoresistance is therefore induced. Based on the magnetoelectric coupling effect and spin valve, we prepared tunable GMR magnetic field sensors with bridge structures. The linear sensing range of a DC magnetic field is enhanced 6 times by applying an electric field of 14 kV/cm. The electrically tunable GMR sensor fulfills the requirements to work at different magnetic field ranges in the same configuration, therefore exhibiting great potential for applications in the Internet of things.

The tunable range of RF/microwave devices is limited not only due to the difficulty in growing high-quality magnetic oxide thin films but also because of their low magnetostrictive coefficient as in Y3Fe5O12 (YIG) thin films. In this work, we have fabricated flexible YIG thin films by using pulsed laser deposition (PLD) technology, which exhibit a low-damping constant (2.67 × 10−4) and well-established magnetic properties suitable for RF/microwave materials. A noticeable ferromagnetic resonance (FMR) field shift of 180 Oe was obtained along the out-of-plane direction via a flexible tunable factor f(R), which results in a frequency shift as high as 550 MHz. The result shows a greater FMR tuning than that induced by ferroelectric single crystals. At the same time, a systematic theoretical framework for tuning FMR characteristics was presented for the flexible YIG/mica thin films, which would provide an effective tuning method for other flexible magnetic oxides with low magnetostrictive coefficients. Last but not least, a flexible coplanar waveguide (CPW) integrated with the flexible YIG thin films shows excellent microwave magnetic stability and fatigue-resistant behavior. It would open a new door for further applications, such as flexible spiral detectors and high-frequency signal processors in the near future.

A single device with extensive new functionality is highly attractive for the increasing demands for complex and multifunctional optoelectronics. Multi-field coupling has been drawing considerable attention because it leads to materials that can be simultaneously operated under several external stimuli (e.g. magnetic field, electric field, electric current, light, strain, etc.), which allows each unit to store multiple bits of information and thus enhance the memory density. In this work, we report an electro–opto–mechano-driven reversible multi-state memory device based on photocurrent in Bi0.9Eu0.1FeO3 (BEFO)/La0.67Sr0.33MnO3 (LSMO)/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) heterostructures. It is found that the short-circuit current density (Jsc) can be switched by the variation of the potential barrier height and depletion region width at the Pt/BEFO interface modulated by light illumination, external strain, and ferroelectric polarization reversal. This work opens up pathways toward the emergence of novel device design features with dynamic control for developing high-performance electric–optical–mechanism integrated devices based on the BiFeO3-based heterostructures.

ITO/BNT/Pt heterojunction structure has been prepared by sol-gel method combined with dc magnetron sputtering. The BNT (Bi3.15Nd0.85Ti3O12) films were polycrystalline with typical ferroelectric hys...

Multiferroic Bi(1+x)×0.85Nd0.15FeO3 (BNF) ceramics with various Bi contents are prepared by conventional solid state reaction, and the effects of excess Bi on the structure, morphology, dielectric, ferroelectric, and magnetic properties are investigated. XRD patterns show the coexistence of rhombohedral and orthorhombic phase in all BNF ceramics due to the doping of Nd3+ ions. Bi2Fe4O9 and Bi2O3 impurity can be observed in x = 0 and x = 5%, 7.5% ceramics, respectively. Surface morphologies are closely related to the Bi content and x = 2.5% sample exhibits the largest average grain size of ∼6.72 μm. The leakage current density has been decreased by 3–4 orders of magnitudes through excess Bi. The x = 2.5% sample shows good ferroelectric property with typical hysteresis loop and a large remnant polarization of about 42 μC/cm2 at 150 kV/cm. The P-E hysteresis deteriorates in x = 5% and x = 7.5% ceramics. The PbZrO3-like superstructure related to 1/4(hh0)p diffraction spots in SAED pattern can be found in all BNF ceramics and the number of grain with superstructure observed in each sample decreases as the Bi content increases. As Bi content increases, the remnant magnetization decreases at first, and then increases. Conversion-electron Mossbauer study have ruled out the impacts of Fe2+, Fe4+, or γ-Fe2O3 on the magnetic properties, suggesting that the weak ferromagnetism comes from the destroy of spin cycloid through substitution of Bi3+ ions with Nd3+ ions. Excessive Bi3+ ions have weakened the effect of Nd3+ ions on suppression of spin cycloid and resulted in the degradation of magnetic properties. The slight improvement of magnetic properties in x = 7.5% sample is considered to attribute to Fe vacancies induced by excessive Bi.

Electric field control of dynamic spin interactions is promising to break through the limitation of the magnetostatic interaction-based magnetoelectric (ME) coupling effect. In this paper, electric field control of the two-magnon scattering (TMS) effect has been demonstrated in Ni0.5Zn0.5Fe2O4/Pb(Mn2/3Nb1/3)-PbTiO3 (001) multiferroic heterostructure. The TMS effect is an extrinsic magnetic damping mechanism that scatters the uniform spin motions excited by ferromagnetic resonance (FMR) into degenerate states of spin waves, offering a framework for voltage control of spin dynamics and further increase the ME effect. The angular dependence FMR measurement has been performed by the electron paramagnetic resonance spectrometer. A large electric field modulation of FMR field (−347 Oe) and FMR linewidth (275 Oe) is achieved at the TMS angle of $\theta _{H} = 60 {^{\circ }}$ . Particularly, the TMS effect contribute to ME coupling at the critical TMS angle is about 194% in magnitude compared with that of the strain effect-mediated ME coupling. The TMS intensity is increased by 14.5% under electric field at room temperature.

In this paper, multiferroic heterostructures have been prepared by growing MnZn ferrite films on ferroelectric single substrates using pulsed laser deposition technology. An enhanced magnetoelectric coupling has been achieved at the critical two-magnon scattering (TMS) angles, where the ferromagnetic resonance field shifts −653 and 211 Oe. The contribution of TMS effects is much larger than that of the conventional strain effect. This heterostructure provides an alternative route for novel integrated multiferroic materials and devices.

The magnetoelectric (ME) materials and related devices have been attracting increasing research attention over the last few years. They exhibit strong ME coupling effect at room temperature, and electric field control of magnetization or magnetic field control of ferroelectric polarization can be achieved. The ME coupling effect brings novel functionalities to develop ultra-fast, low-power, and miniaturized electronics. Recent progress shows the performance of ME materials is further improved and the materials are used to develop many new types of electronics such as high-speed memory, radio frequency resonator, compact ME antenna, and weak magnetic field sensor. In this review, we present the overview in those fields with emphasis on both the opportunities and challenges for the application of ME materials and devices in the cutting-edge technologies.

Phase change materials help realize voltage tunable spin dynamics., Controlling magnetization dynamics is imperative for developing ultrafast spintronics and tunable microwave devices. However, the previous research has demonstrated limited electric-field modulation of the effective magnetic damping, a parameter that governs the magnetization dynamics. Here, we propose an approach to manipulate the damping by using the large damping enhancement induced by the two-magnon scattering and a nonlocal spin relaxation process in which spin currents are resonantly transported from antiferromagnetic domains to ferromagnetic matrix in a mixed-phased metallic alloy FeRh. This damping enhancement in FeRh is sensitive to its fraction of antiferromagnetic and ferromagnetic phases, which can be dynamically tuned by electric fields through a strain-mediated magnetoelectric coupling. In a heterostructure of FeRh and piezoelectric PMN-PT, we demonstrated a more than 120% modulation of the effective damping by electric fields during the antiferromagnetic-to-ferromagnetic phase transition. Our results demonstrate an efficient approach to controlling the magnetization dynamics, thus enabling low-power tunable electronics.

Phase transition could enhance superior elasticity and flexibility in freestanding single-crystalline multiferroic oxide membranes., The integration of ferroic oxide thin films into advanced flexible electronics will bring multifunctionality beyond organic and metallic materials. However, it is challenging to achieve high flexibility in single-crystalline ferroic oxides that is considerable to organic or metallic materials. Here, we demonstrate the superior flexibility of freestanding single-crystalline BiFeO3 membranes, which are typical multiferroic materials with multifunctionality. They can endure cyclic 180° folding and have good recoverability, with the maximum bending strain up to 5.42% during in situ bending under scanning electron microscopy, far beyond their bulk counterparts. Such superior elasticity mainly originates from reversible rhombohedral-tetragonal phase transition, as revealed by phase-field simulations. This study suggests a general fundamental mechanism for a variety of ferroic oxides to achieve high flexibility and to work as smart materials in flexible electronics.

© 2020 Wiley-VCH GmbH Self-assembled membranes with periodic wrinkled patterns are the critical building blocks of various flexible electronics, where the wrinkles are usually designed and fabricated to provide distinct functionalities. These membranes are typically metallic and organic materials with good ductility that are tolerant of complex deformation. However, the preparation of oxide membranes, especially those with intricate wrinkle patterns, is challenging due to their inherently strong covalent or ionic bonding, which usually leads to material crazing and brittle fracture. Here, wrinkle-patterned BaTiO3 (BTO)/poly(dimethylsiloxane) membranes with finely controlled parallel, zigzag, and mosaic patterns are prepared. The BTO layers show excellent flexibility and can form well-ordered and periodic wrinkles under compressive in-plane stress. Enhanced piezoelectricity is observed at the sites of peaks and valleys of the wrinkles where the largest strain gradient is generated. Atomistic simulations further reveal that the excellent elasticity and the correlated coupling between polarization and strain/strain gradient are strongly associated with ferroelectric domain switching and continuous dipole rotation. The out-of-plane polarization is primarily generated at compressive regions, while the in-plane polarization dominates at the tensile regions. The wrinkled ferroelectric oxides with differently strained regions and correlated polarization distributions would pave a way toward novel flexible electronics.

In spite of recent rapid development of flexible electronics, voltage-tunable spintronic structures and devices on flexible substrates have been rarely studied. Here, voltage control of magnetic anisotropy (VCMA) is demonstrated via ionic gel (IG) gating on flexible polyimide substrates with a circuit operating voltage of 1.8 V. A reversible, nonvolatile VCMA switching of 114 Oe is achieved in Pt/Fe/Pt multilayer, where the spatial magnetic anisotropy distribution is determined quantitatively by electron spin resonance technique. This IG gating process is repeatable as the substrates are under different bending conditions. The voltage modulation of magnetic anisotropy through IG gating with excellent flexibility proposes potential applications in low-power wearable spintronic devices.

The trend of mobile Internet requires portable and wearable devices as bio-device interfaces. Electric field control of magnetism is a promising approach to achieve compact, light-weight, and energy-efficient wearable devices. Within a flexible sandwich heterostructure, perpendicular magnetic anisotropy switching was achieved via low-voltage gating control of an ionic gel in mica/Ta/(Pt/Co) x/Pt/ionic gel/Pt, where (Pt/Co) x acted as a functional layer. By conducting in situ VSM, EPR, and MOKE measurements, a 1098 Oe magnetic anisotropy field change was determined at the bending state with tensile strain, corresponding to a magnetic anisotropy energy change of 3.16 × 105 J/m3 and a giant voltage tunability coefficient of 0.79 × 105 J/m3·V. The low voltage and strain dual control of magnetism on mica substrates enables tunable flexible spintronic devices with an increased degree of manipulation.

Traditional magnetostrictive/piezoelectric laminated composites rely on the two-dimensional interface that transfers stress/strain to achieve the large magnetoelectric (ME) coupling, nevertheless, they suffer from the theoretical limitation of the strain effect and of the substrate clamping effect in real ME applications. In this work, 3D NZFO/BTO-pillar nanocomposite films were grown on SrTiO3 by template-assisted pulsed laser deposition, where BaTiO3 (BTO) nanopillars appeared in an array with distinct phase transitions as the cores were covered by NiZn ferrite (NZFO) layer. The perfect 3D heteroepitaxial interface between BTO and NZFO phases can be identified without any edge dislocations, which allows effective strain transfer at the 3D interface. The 3D structure nanocomposites enable the strong two magnon scattering (TMS) effect that enhances ME coupling at the interface and reduces the clamping effect by strain relaxation. Thereby, a large FMR field shift of 1866 Oe in NZFO/BTO-pillar nanocomposite...

The ongoing demand for efficient and low-energy consumption spintronic devices has motivated the idea of manipulating magnetism by ionic liquid (IL) electrolyte gating at low voltages. Although magnetoelectric (ME) coupling has already been realized in some field-effect-transistor (FET) structures, some vital parameters such as giant ME coupling coefficient, excellent reversibility and low gating voltage seldom come at the same time, which greatly suppresses industrialization. Here we demonstrate a large 552 Oe spin dynamics modulation of Fe3O4 thin films induced at a gating voltage of Vg = +1.5 V in an IL-gated Au/[DEME]+[TFSI]−/Fe3O4/MgO heterostructure with good reversibility up to 80 cycles, giving rise to a high ME coefficient of 368 Oe V−1. Such a large ME tunability under low Vg could be attributed to the electric field (E-field) induced ionic transformation between Fe2+ and Fe3+ at the interface. The tiny thickness change (∼2 angstrom) and roughness change of Fe3O4 films under Vg = +1.5 V illustrated by in situ X-ray reflection (XRR) give a reasonable explanation of the outstanding reversible property. Interestingly, it is found that the Verwey transition temperature of Fe3O4 has a strong dependence on Vg, revealing the potential of IL gating control of the intrinsic spin ordering inside magnetic films. This work drives forward the low-voltage induced reversible ME coupling to high-performance spintronic devices.

The flexible La0.7Sr0.3MnO3 (LSMO) film shows good mechanical stretchability and property stability under bending states, which makes it a candidate material for flexible electronics and spintronics devices. Here the microwave magnetism of flexible LSMO thin film has been manipulated effectively by the ionic gel (IG) gating process. A reversible and non-volatile magnetoelectric (ME) coefficient of 50 Oe/V was observed using electron paramagnetic resonance (ESR) technology at the flat multilayer structure. This ME coupling is caused by the electrons hopping between Mn3+ and Mn4+ ions under the interfacial electric double layer. When the membranes were bent to a radius of curvature of 15 mm, a larger ME coefficient of 67 Oe/V was also observed, which indicating that this flexible structure can work effectively under mechanical deformation. This work demonstrates the voltage control of magnetism for flexible correlated materials and paves a way towards wearable low-power devices based on flexible manganite films.

Controlling spin dynamics through modulation of spin interactions in a fast, compact, and energy-efficient way is compelling for its abundant physical phenomena and great application potential in next-generation voltage controllable spintronic devices. In this work, we report electric field manipulation of spin dynamics—the two-magnon scattering (TMS) effect in Ni0.5Zn0.5Fe2O4 (NZFO)/Pb(Mg2/3Nb1/3)–PbTiO3 (PMN–PT) multiferroic heterostructures, which breaks the bottleneck of magnetostatic interaction-based magnetoelectric (ME) coupling in multiferroics. An alternative approach allowing spin-wave damping to be controlled by external electric field accompanied by a significant enhancement of the ME effect has been demonstrated. A two-way modulation of the TMS effect with a large magnetic anisotropy change up to 688 Oe has been obtained, referring to a 24 times ME effect enhancement at the TMS critical angle at room temperature. Furthermore, the anisotropic spin-freezing behaviors of NZFO were first determin...

The magnetic field sensors based on anisotropic magnetoresistance (AMR) effect have been widely used in data storage, navigation, and medical diagnosis. However, the AMR effect of metal materials is relatively weak with an AMR ratio below 2%, which results in low voltage output. In order to improve the sensitivity of weak magnetic fields, we optimize the structure of the AMR sensor with a specific photolithographic process. We use two different designs of Hall bar and Wheatstone bridge with similar barber pole structures, and investigate the angular dependence as well as magnetic field dependence of AMR ratio and voltage output. With Wheatstone bridge, the magnetoresistance leads to a voltage output without dc components. The NiFe magnetic layer and the Au electrode are patterned into a highly conductive barber poles structure, and a high voltage output ratio of about 80% is obtained. At the same time, we achieve a high sensitivity of about 4.3 Oe−1, which implies potential applications in AMR effect-based magnetic field sensors.

Yttrium iron garnet (YIG) is of great importance in RF/microwave devices for its low loss, low intrinsic damping, and high permeability. Nevertheless, tuning of YIG-based multiferroics is still a challenge due to its near-zero magnetostriction and the difficulty of building epitaxial interface between ferromagnetic garnet and ferroelectric perovskite phases. In this work, the vertically aligned heterostructure of YIG:BTO/STO(001) with local epitaxial interface between BTO and YIG is well-constructed, where the single crystal BTO pillars are embedded in YIG matrix. A large magnetoelectric coupling effect that drives YIG’s FMR shift up to 512 and 333 Oe (1–2 order greater than those of all state-of-the-art progresses) is obtained through BTO ferroelectric phase changes induced by temperature variation at 295 and 193 K, correspondingly. This record high magnetoelectric tunability of YIG paves a way toward thermal/electrical tunable YIG devices.

Multilayer Pt/BaTiO 3 /BiFeO 3 /TiO 2 heterostructure was fabricated on FTO substrates by sol-gel technique. This heterostructure shows strong light absorption, sensitive optical switching response, and excellent photovoltaic properties, which are effectively modulated by the ferroelectric polarization. The short circuit current density and open circuit voltage of the multilayer heterostructure are significantly enhanced compared with that of the single layer BiFeO 3 thin film under the initial and positive polarization states. Analysis on the schematic energy band diagrams suggests that the excellent photovoltaic properties can be attributed to the interface Schottky barrier regulated by ferroelectric polarization, in which the TiO 2 layer acts as the electron collection layer, and the ferroelectric BaTiO 3 layer acts as a hole collection layer and also an auxiliary photovoltaic absorption layer to enhance optical absorption properties, thus improve the light absorption efficiency and the separation of electron-hole pairs. These results provide a promising method to optimize the performance of ferroelectric photovoltaic devices by using multilayer heterostructure.

Ferroelectrics are usually inflexible oxides that undergo brittle deformation. We synthesized freestanding single-crystalline ferroelectric barium titanate (BaTiO3) membranes with a damage-free lifting-off process. Our BaTiO3 membranes can undergo a ~180° folding during an in situ bending test, demonstrating a super-elasticity and ultraflexibility. We found that the origin of the super-elasticity was from the dynamic evolution of ferroelectric nanodomains. High stresses modulate the energy landscape markedly and allow the dipoles to rotate continuously between the a and c nanodomains. A continuous transition zone is formed to accommodate the variant strain and avoid high mismatch stress that usually causes fracture. The phenomenon should be possible in other ferroelectrics systems through domain engineering. The ultraflexible epitaxial ferroelectric membranes could enable many applications such as flexible sensors, memories, and electronic skins.

Wireless strain sensors have received extensive attention owing to their wide application prospects in structural health monitoring, industrial automation, human activity monitoring, and intelligent robotic systems. Here, a wireless strain sensor prototype based on the magnetoelectric heterostructure of ferromagnetic thin films on a piezoelectric substrate has been developed. The ferromagnetic resonance (FMR) frequency of the sensor is strongly dependent on external strain due to the large magnetostriction of the film. The piezoelectric substrate with a programmable voltage has been used as a strain source for the characterization of the wireless strain sensor. The limit of detection of the wireless strain sensor is 0.54 μe, which is comparable with that of commercial metal-foil sensors that need connection wires. More importantly, the FMR strain sensor shows a sensitivity of 65.46 ppm/μe, indicating more than a 60 fold improvement than that of traditional wireless strain sensors based on patch antenna and RLC resonators whose frequency shift is mainly due to the strain induced dimension change.

A highly sensitive flexible magnetic sensor based on the anisotropic magnetoresistance effect is fabricated. A limit of detection of 150 nT is observed and excellent deformation stability is achieved after wrapping of the flexible sensor, with bending radii down to 5 mm. The flexible AMR sensor is used to read a magnetic pattern with a thickness of 10 μm that is formed by ferrite magnetic inks.

The coexistence of electric polarization and magnetization in multiferroic materials provides great opportunities for realizing magnetoelectric (ME) coupling, including electric field control of magnetism, or vice versa, through a strain-mediated ME coupling in layered magnetic/ferroelectric multiferroic heterostructures. Strong ME coupling has been the enabling factor for different multiferroic devices, which, however, has been elusive, particularly at RF/microwave frequencies. In this paper, most recent progress on new integrated multiferroic devices for sensing RF and microwave electronics will be presented, including novel RF Nano-electro-mechanical systems ME resonators with picotesla sensitivity for dc magnetic fields and novel gigahertz magnetic and multiferroic integrated inductors with a wide operation frequency range of $0.3\sim 3$ GHz, a high quality factor close to 20, and a voltage tunable inductance of 50% $\sim 150$ %. At the same time, we will also demonstrate other tunable RF devices, including integrated non-reciprocal tunable bandpass filter with ultrawideband isolation more than 13 dB. These novel magnetics and multiferroic devices show great promise for applications, such as compact, lightweight, and power-efficient sensing, memory, RF, and microwave integrated electronics.

In this paper, we demonstrate a nonvolatile modulation effect on the magnetic properties by electric (E)-field-induced ferroelectric (FE) polarizing in Bi0.95Ba0.05FeO3 single-phase multiferroics. It is found that E-field has much effect on weak ferromagnetic order, but has little effect on strong antiferromagnetic order in the sample. The change of Raman scattering spectra with different FE polar states of the sample, as well as the magnetization change under different polarization times (tp), indicates that the modulation effect on magnetism comes from E-field-induced the change of FE polarization, which couples with magnetic moments in Bi0.95Ba0.05FeO3 sample.

We report the growth of ferroelectric Pb 0.92 La 0.08 Zr 0.52 Ti 0.48 O 3 (PLZT) thick films using a poly(1-vinylpyrrolidone-co-vinyl acetate) (PVP/VA)-modified sol–gel process. A per-coating thickness of ≈0.66 μm has been demonstrated using PVP/VA-modified solution, which is more than doubled that of the PLZT films grown by PVP-modified method, and nearly 6 times the per-coating thickness of films prepared by conventional sol–gel process. PLZT thick films grown on LNO/Ni substrates exhibited denser microstructure, higher remanent polarization (11 μC/cm 2 ) and dielectric tunability (45%), lower leakage current density (≈1.2 × 10 −8 A/cm 2 ), and higher breakdown strength (≈1.6 MV/cm) than those for the samples grown on PtSi substrates. These results demonstrated great potential of using PVP/VA-modified sol–gel process for high power film capacitor applications.

In situ tuning effect on magnetism is often achieved by electric-field-induced inverse piezoelectric effect in ferromagnetic/piezoelectric structures. Here, we report on a non-volatile tuning of magnetic properties in Ni/Pb(Mg1/3Nb2/3)O3–PbTiO3 heterostructure. Both magnetization difference of up to 60% and coercivity difference of up to 23.2% are obtained by electric-field-induced the change of polar state in the substrate. This modulation effect is observed over a broad range of temperatures. The coincident changes of both magnetic properties and Raman scattering spectra with different polarization times for the heterostructure are detected, which indicates that the non-volatile tuning effect on magnetism is attributed to the change of the magnetic anisotropy result from the ferroelectric polarizing induced-strain.

High power microwave devices have been suffering from secondary electron yield (SEY), which occurs at the surface and leads to increases in energy dissipate and noise level in microwave cavities. The suppression of SEY in microwave materials is significant for space applications. In this work, we report a surface roughness evolution mechanism induced by carbon coating on a widely-used microwave cavity material Ag/Al. The competition between the preferred growth and the island growth modes leads to the roughness enhancement in the 60 nm carbon coated Ag/Al substrates. By extracting the smooth factor Ks in Vaughan model from the angular dependent SEY measurement, we attribute the suppression of SEY to the intrinsic low SEY of the carbon film and the high surface roughness. Our work demonstrates a low-cost, easy-to-scale coating process to effectively prevent the formation of secondary electron avalanche in space borne microwave devices.

The inexorable trend of next generation spintronics is to develop smaller, lighter, faster, and more energy efficient devices. Ultimately, spintronics driven by free energy, for example, solar power, is imperative. Here, a prototype photovoltaic spintronic device with an optical‐magneto‐electric tricoupled photovoltaic/magnetic thin film heterojunction, where magnetism can be manipulated directly by sunlight via interfacial effect, is proposed. The magnetic anisotropy is reduced evidenced by the out‐of‐plane ferromagnetic resonance (FMR) field change of 640.26 Oe under 150 mW cm−2 illumination via in situ electron spin resonance (ESR) method. The transient absorption analysis and the first‐principles calculation reveal that the photovoltaic electrons doping in the cobalt film alter the band filling of this ferromagnetic film. The findings provide a new path of electron doping control magnetism and demonstrate an optical‐magnetic dual controllable logical switch with limited energy supply, which may further transform the landscape of spintronics research., Visible light control of magnetism is a brand‐new area, and can offer an alternative, straightforward, and efficient way of optical control of spin dynamics, and opens an avenue for applications in low‐power, visible light controlling magnetic devices. As an outlook, the photoinduced magnetic anisotropy change can lead to visible light controllable spintronic devices without high‐power consumption or femtosecond laser instrumentation.

A magneto-mechano-electric (MME) device for energy harvesting from ambient low-frequency magnetic field is investigated in this study. Consisting of piezoelectric ceramic rings, NdFeB permanent magnets and union piece, the proposed MME energy harvester is designed with the barbell-shaped structure operating in d33 mode. NdFeB permanent magnets are attached at the free end of barbell-shaped structure, which can produce compressive stress on the piezoelectric ceramic rings via magnetic torque effect. The properties of the MME energy harvester are optimized by using the finite element analysis. Two electrode shapes, including full electrode and divided electrode, are investigated in the aspect of their influences on the device performance. The finite element analysis is consistent with the experimental results, revealing that the electrode shape of piezoelectric ceramic rings is crucial for the performance of the proposed MME device. The MME device presents a low resonant frequency ( 10 V @ 10 Oe), a maximum output power of ˜10.5 μW and a maximum power density of 3.5 μW/Oe cm3, demonstrating the potential applications for harvesting ambient low-frequency magnetic field energy.

The voltage modulation of yttrium iron garnet (YIG) is of practical and theoretical significance; due to its advantages of compactness, high-speed response, and energy efficiency, it can be used for various spintronic applications, including spin-Hall, spin-pumping, and spin-Seebeck effects. In this study, a significant ferromagnetic resonance change is achieved within the YIG/Pt bilayer heterostructures uisng ionic modulation, which is accomplished by modifying the interfacial magnetism in the deposited "capping" platinum layer. With a small voltage bias of 4.5 V, a large ferromagnetic field shift of 690 Oe is achieved in heterostructures of YIG (13 nm)/Pt (3 nm)/(ionic liquid, IL)/(Au capacitor). The remarkable magnetoelectric (ME) tunability comes from the additional and voltage-induced ferromagnetic ordering, caused by uncompensated d-orbital electrons in the Pt metal layer. Confirmed by first-principle calculations, this finding paves the way for novel voltage-tunable YIG-based spintronics.

To overcome the fundamental challenge of the weak natural response of antiferromagnetic materials under a magnetic field, voltage manipulation of antiferromagnetic interaction is developed to realize ultrafast, high-density, and power efficient antiferromagnetic spintronics. Here, we report a low voltage modulation of Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction via ionic liquid gating in synthetic antiferromagnetic multilayers of FeCoB/Ru/FeCoB and (Pt/Co)2/Ru/(Co/Pt)2. At room temperature, the distinct voltage control of transition between antiferromagnetic and ferromagnetic ordering is realized and up to 80% of perpendicular magnetic moments manage to switch with a small-applied voltage bias of 2.5 V. We related this ionic liquid gating-induced RKKY interaction modification to the disturbance of itinerant electrons inside synthetic antiferromagnetic heterostructure and the corresponding change of its Fermi level. Voltage tuning of RKKY interaction may enable the next generation of switchable spintronics between antiferromagnetic and ferromagnetic modes with both fundamental and practical perspectives. Energy efficient control of magnetization in antiferromagnetic materials is one of the key ingredients for antiferromagnetic spintronics. Here the authors demonstrate voltage control of RKKY interaction and magnetization switching in synthetic antiferromagnetic multilayers by ionic liquid gating.

To meet the demand of developing compatible and energy-efficient flexible spintronics, voltage manipulation of magnetism on soft substrates is in demand. Here, a voltage tunable flexible field-effect transistor structure by ionic gel (IG) gating in perpendicular synthetic anti-ferromagnetic nanostructure is demonstrated. As a result, the interlayer Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction can be tuned electrically at room temperature. With a circuit gating voltage, anti-ferromagnetic (AFM) ordering is enhanced or converted into an AFM-ferromagnetic (FM) intermediate state, accompanying with the dynamic domain switching. This IG gating process can be repeated stably at different curvatures, confirming an excellent mechanical property. The IG-induced modification of interlayer exchange coupling is related to the change of Fermi level aroused by the disturbance of itinerant electrons. The voltage modulation of RKKY interaction with excellent flexibility proposes an application potential for wearable spintronic devices with energy efficiency and ultralow operation voltage.

Electric field control of dynamic spin interactions is promising to break through the limitation of the magnetostatic interaction based magnetoelectric (ME) effect. In this work, electric field control of the two-magnon scattering (TMS) effect excited by in-plane lattice rotation has been demonstrated in a La0.7Sr0.3MnO3 (LSMO)/Pb(Mn2/3Nb1/3)-PbTiO3 (PMN-PT) (011) multiferroic heterostructure. Compared with the conventional strain-mediated ME effect, a giant enhancement of ME effect up to 950% at the TMS critical angle is precisely determined by angular resolution of the ferromagnetic resonance (FMR) measurement. Particularly, a large electric field modulation of magnetic anisotropy (464 Oe) and FMR line width (401 Oe) is achieved at 173 K. The electric-field-controllable TMS effect and its correlated ME effect have been explained by electric field modulation of the planar spin interactions triggered by spin–lattice coupling. The enhancement of the ME effect at various temperatures and spin dynamics contr...

Epitaxial Bi0.9Eu0.1FeO3 (BEFO) thin films are deposited on Nb-doped SrTiO3 (NSTO) substrates by pulsed laser deposition to fabricate the Pt/BEFO/NSTO (001) heterostructures. These heterostructures possess bipolar resistive switching, where the resistances versus writing voltage exhibits a distinct hysteresis loop and a memristive behavior with good retention and anti-fatigue characteristics. The local resistive switching is confirmed by the conductive atomic force microscopy (C-AFM), suggesting the possibility to scale down the memory cell size. The observed memristive behavior could be attributed to the ferroelectric polarization effect, which modulates the height of potential barrier and width of depletion region at the BEFO/NSTO interface. The continuously tunable resistive switching behavior could be useful to achieve non-volatile, high-density, multilevel random access memory with low energy consumption.

State-of-the-art compact antennas rely on electromagnetic wave resonance, which leads to antenna sizes that are comparable to the electromagnetic wavelength. As a result, antennas typically have a size greater than one-tenth of the wavelength, and further miniaturization of antennas has been an open challenge for decades. Here we report on acoustically actuated nanomechanical magnetoelectric (ME) antennas with a suspended ferromagnetic/piezoelectric thin-film heterostructure. These ME antennas receive and transmit electromagnetic waves through the ME effect at their acoustic resonance frequencies. The bulk acoustic waves in ME antennas stimulate magnetization oscillations of the ferromagnetic thin film, which results in the radiation of electromagnetic waves. Vice versa, these antennas sense the magnetic fields of electromagnetic waves, giving a piezoelectric voltage output. The ME antennas (with sizes as small as one-thousandth of a wavelength) demonstrates 1–2 orders of magnitude miniaturization over state-of-the-art compact antennas without performance degradation. These ME antennas have potential implications for portable wireless communication systems., The miniaturization of antennas beyond a wavelength is limited by designs which rely on electromagnetic resonances. Here, Nan et al. have developed acoustically actuated antennas that couple the acoustic resonance of the antenna with the electromagnetic wave, reducing the antenna footprint by up to 100.

Magnetoresistance, the dependence of resistivity on the applied magnetic field, provides the opportunity to manipulate and utilize the electronic spin degree of freedom, which is not only a long-term frontier field of solid-state physics but also the cornerstone of information storage technology. However, the negative magnetoresistance (nMR) is a relatively rare case of magnetoresistance in which the microscopic origin is still elusive and for which it is difficult to define a general interpretation. Herein, an experimental case of an intrinsic unsaturated nMR is demonstrated in van der Waals FeNbTe2 single crystal. The clear-cut evidence in angle-resolved photoemission spectroscopy (ARPES), the electronic transport measurement, and DC/AC magnetic susceptibility confirms that the intrinsic unsaturated nMR is derived from the comprehensive effect of Anderson localization and a spin glass state. Taking into consideration that intrinsic unsaturated nMR has so far been rarely reported, especially in van der Waals structures, it is anticipated that this work will not only lead to a deep understanding of the inherent microcosmic mechanism but will also serve as a guide to broaden the research of spintronics and information storage based on magnetoresistance.

Voltage control of perpendicular magnetic anisotropy (PMA) in ferromagnetic/ferroelectric multiferroic heterostructures is a promising method to enable high density and low power perpendicular magnetic information storage. In this study, we successfully achieved large voltage tuning of PMA at room temperature in (Co/Pt)3/Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) multiferroic heterostructures. Voltage tuning of magnetic anisotropy and the magnetoelectric coupling effect has been qualitatively studied by ferromagnetic resonance, and the multiferroic heterostructure could be reversibly flipped between two distinct PMA states under zero or high electric fields. During the linear piezo-strain response of PZN-PT, the multiferroic heterostructures exhibit small magnetoelectric coupling and the electric field-induced magnetic anisotropy field was about 295 Oe. During the electric field-induced phase transition of PZN-PT, voltage tuning of PMA enhanced more than two times that of the first linear region and the electric field-induced magnetic anisotropy field increased to 634 Oe. Finally, this magnetoelectric coupling was enhanced to 672 Oe by applying 12 kV/cm, corresponding to a large magnetoelectric coupling coefficient up to 56 Oe·cm/kV. Benefiting from the giant strain response during voltage-induced phase transition in PZN-PT, voltage tuning of PMA in those multiferroic heterostructures is a promising candidate for power-efficient magnetic memories.

Magnetoelectric effect, arising from the interfacial coupling between magnetic and electrical order parameters, has recently emerged as a robust means to electrically manipulate the magnetic properties in multiferroic heterostructures. Challenge remains as finding an energy efficient way to modify the distinct magnetic states in a reliable, reversible and non-volatile manner. Here we report ferroelectric switching of ferromagnetic resonance in multiferroic bilayers consisting of ultrathin ferromagnetic NiFe and ferroelectric Pb0.92La0.08Zr0.52Ti0.48O3 (PLZT) films, where the magnetic anisotropy of NiFe can be electrically modified by low voltages. Ferromagnetic resonance measurements confirm that the interfacial charge-mediated magnetoelectric effect is dominant in NiFe/PLZT heterostructures. Non-volatile modification of ferromagnetic resonance field is demonstrated by applying voltage pulses. The ferroelectric switching of magnetic anisotropy exhibits extensive applications in energy-efficient electronic devices such as magnetoelectric random access memories, magnetic field sensors and tunable radio frequency (RF)/microwave devices.

Electric field (E-field) modulation of perpendicular magnetic anisotropy (PMA) switching, in an energy-efficient manner, is of great potential to realize magnetoelectric (ME) memories and other ME devices. Voltage control of the spin-reorientation transition (SRT) that allows the magnetic moment rotating between the out-of-plane and the in-plane direction is thereby crucial. In this work, a remarkable magnetic anisotropy field change up to 1572 Oe is achieved under a small operation voltage of 4 V through ionic liquid (IL) gating control of SRT in Au/[DEME]+ [TFSI]- /Pt/(Co/Pt)2 /Ta capacitor heterostructures at room temperature, corresponding to a large ME coefficient of 378 Oe V-1 . As revealed by both ferromagnetic resonance measurements and magnetic domain evolution observation, the magnetization can be switched stably and reversibly between the out-of-plane and in-plane directions via IL gating. The key mechanism, revealed by the first-principles calculation, is that the IL gating process influences the interfacial spin-orbital coupling as well as net Rashba magnetic field between the Co and Pt layers, resulting in the modulation of the SRT and in-plane/out-of-plane magnetization switching. This work demonstrates a unique IL-gated PMA with large ME tunability and paves a way toward IL gating spintronic/electronic devices such as voltage tunable PMA memories.