Magnetoelectric Devices Research Articles

An analytical model for the self-bias magnetoelectric effect of magnetization-graded magnetoelectric composites

This paper presents an analytical model to evaluate the self-bias magnetoelectric (ME) effect of magnetization-graded ME composites under extensional-bending coupled vibration. The distribution of the self-bias magnetic field produced inside ME composites is quantitatively analyzed based on the molecular current model. Sequentially, a multi-field coupled ME model is developed using equivalent nonlinear constitutive relations, extensional-bending coupled equations, and Maxwell’s equations simultaneously. The developed ME theory is highlighted with a direct comparison with experiments that shows their close agreement. The results indicate that an available self-bias magnetic field is produced in the magnetization-graded composites, which in turn induces a self-bias ME coupling at zero bias magnetic field. It is also demonstrated that, with applying tensile stress or appropriately increasing the thickness of the high-permeability layer, the self-bias ME coefficient can be enhanced and account for more than 55% of the maximum one, and the resonance frequency is regulated in a wide range. In addition, the competitive relationship between the influence of mechanical and magnetic properties of high-permeability materials on self-bias ME coupling is revealed. Given the significance of this study, it may be utilized in the development of high-performance self-bias ME devices.

Room-temperature ferromagnetism in two-dimensional CrBr3

High-temperature ferromagnetism and its control in ultrathin materials are of critical interest in disruptive quantum technologies. However, intrinsic ferromagnetic ordering in recently discovered two-dimensional materials is limited to cryogenic temperatures due to fundamental spin fluctuation in reduced dimensions. While chromium trihalides order below 45 K, we report a dramatic manipulation of long-range ferromagnetic order above room temperature in ${\mathrm{CrBr}}_{3}$ monolayer predicted within the first-principles long-range Heisenberg XXZ model. Gate-induced charge carrier doping externally modulates various magnetic interactions in the Mott insulating monolayer, and a nontrivial magnetic phase diagram emerges. Under varied hole density, the monolayer remains ferromagnetic with Curie temperature increasing monotonically, while electron doping triggers a complex evolution of magnetic phases. The microscopic details are investigated, and the high-temperature ferromagnetism is attributed to the substantial increase in effective ferromagnetic exchange and magnetic anisotropies under an experimentally attainable hole density. Such electrically controlled ferromagnetism in ${\mathrm{CrBr}}_{3}$ stands for experimental verification and presents enormous new possibilities for spintronic and magnetoelectric quantum devices based on atomically thin crystals.

Application of Nomogram Method for the Analysis of Magnetoelectric Interactions in Magnetostrictive–Piezoelectric Composites

The magnetoelectric (ME) effect is beginning to be extensively studied. However, the main issue in the design of new ME devices is the evaluation and selection of the ME composite and the calculation of its ME voltage coefficients. As the calculation of ME voltage coefficients based on the generalized Hooke's law is a laborious task, the well-known method of nomograms is presented to simplify it. To attract the further interest of researchers in this field toward the nomogram method, this article presents a detailed method to obtain the ME voltage coefficients of two ME composites: nickel spinel (NFO)–lead zirconate titanate (PZT) and Terfenol-D–piezoquartz (X-cut) in the low-frequency range and the longitudinal mode of electromechanical resonance (EMR). The results confirm that the nomogram method, in contrast to the general method, allows a quicker and easier determination of the ME voltage coefficients with considerable accuracy.

Vanadium trapped by oblique nano-sheets to preserve the anisotropy in Co–V thin films at high temperature

In this study, oriented nano-sheets generated during the growth of cobalt-rich Co–V and Co–Zn thin films induced a large anisotropy in the magnetic and transport properties. The regular nano-sheets were tilted 52–54 deg. with respect to the substrate plane, ≈ 3.0–4.0 nm thick, ≈ 30–100 nm wide, and ≈ 200–300 nm long, with an inter-sheet distance of ≈ 0.9–1.2 nm. In spite of the different microstructures of the two kinds of samples where the Co–V films were amorphous, whereas the Co–Zn films showed a growth of Zn nanocrystals, the oblique nano-sheet morphology conferred noticeable shape anisotropy to both specimens. This anisotropy resulted in an in-plane uniaxial magnetic anisotropy. The changes in the nano-morphology caused by thermal treatments, and hence in their anisotropic properties, were studied. While the Co–V samples retained or increased their magnetic and transport anisotropies, this anisotropic behavior vanished for the annealed Co–Zn films. High resolution transmission electron microscopy, HRTEM, including chemical analysis at the nano-scale, and the dependence of the anisotropic resistance on temperature allowed to establish the nature and the activation energy spectra of the atomic relaxation processes during heating. These processes displayed a single peak at 1.63 eV for the Co–V and two peaks at 1.67 and 2.0 eV for the Co–Zn. These spectra and their singularities were associated to the changes induced in the nano-morphology of the films by thermal treatments. The Co–V films retained their nano-sheet morphology almost up to 500 ºC; the Co–Zn films lost their nano-sheets at 290 ºC. The thermal stability exhibited by the Co–V films makes them useful for applications in ultra high frequency, optical, magnetostrictive and magneto-electric devices.

Bias-modulated switching in Chern insulator

The Chern insulator manifests itself via the surface quantized Hall current and magnetoelectric effect. The manipulation of surface magnetizations enables a control of the dissipationless chiral transport and thus allows for potential applications of topological magnetoelectric devices with low-energy consumption. Here, we present experimental studies of bias-modulated switching the magnetic states utilizing the magnetoelectric coupling in a Chern insulator. This is achieved via applying a d.c. bias across the source and drain at various magnetic states, during which an effective magnetic field is developed to switch the quantum anomalous Hall state towards its opposite. Comprehensive transport studies show that the switch efficiency is proportional to the amplitude and applying time of the bias, depends on the initial magnetic state, but is insensitive to the electric polarity. Our results provide an efficient scheme to manipulate the Chern insulator and understanding on the electric breakdown of chiral edge states.

Effect of Cobalt Substitution on the Structural and Magnetic Properties of Bismuth Ferrite Powders

BiFeO3 (BFO) is a multiferroic material with excellent ferroelectric properties but with poor magnetic behavior. Therefore, we focused principally on the enhancement of the magnetic order of BFO. These multiferroic properties make BFO an excellent candidate for magnetoelectric devices at room temperature. Pure and Co-BiFeO3 powders were successfully synthesized via the sol–gel method at 700 °C. The effect of Co substitution on the corresponding structural and magnetic properties of BFO was studied. X-ray diffraction and Fourier Transform Infrared Spectroscopy measurements confirmed the rhombohedral perovskite structure in all samples. A secondary phase of CoFe2O4 (CFO) was detected for 9, 10, and 15% of Co doping. The scanning electron microscopy images of the Co-BFO particles showed a reduction in the particle size compared to the pure BFO powders. Vibrating sample magnetometry measurements evidenced the ferromagnetic hysteresis loop for the Co-BFO powders with values of saturation magnetization of 4.1 emu/g and a coercivity of 1083 Oe for 15% of Co doping. In this work, we report impurity free samples with notable magnetic properties at the same time, which is a difficult challenge in bismuth ferrite synthesis. This is the first step for later applications in future technology.

Structural, magnetic, magnetostrictive and optical properties of Mn and Cu codoped cobalt ferrite

Magnetostrictive materials have been a sought-after multifunctional material due to their propensity to be used in applications such as sensors, actuators, transducers, and other magnetoelectric devices. Cobalt ferrite (CoFe2O4) is one such commonly used magnetostrictive material due to its high magnetostriction and low cost. This work deals with the synthesis of CoFe2O4 (CFO) by substitution of Cu2+ at the tetrahedral site, Mn3+ at the octahedral site, and simultaneous substitution of Cu2+ and Mn3+ at both sites followed by its subsequent characterization. Hydrothermal method was adopted to synthesize the aforementioned samples. The effect of doping was then studied by analyzing their structural, magnetic, morphology, magnetostrictive, and Mössbauer properties. The obtained results reveal that doping of the transition metal ions is crucial in determining the magnetostrictive characteristics of CoFe2O4. Sequential addition of Cu2+, Mn3+ and their simultaneous addition leads to an enhancement in the piezomagnetic coefficient (dλ/dH)max of the CFO system and considerably reduces the magnetic field corresponding to the peak piezomagnetic coefficient.

Magneto-Electric-Optical Coupling in Multiferroic BiFeO3 -Based Films.

Manipulating ferroic orders and realizing their coupling in multiferroics at room temperature are promising for designing future multifunctional devices. Single external stimulation has been extensively proved to demonstrate the ability of ferroelastic switching in multiferroic oxides, which is crucial to bridge the ferroelectricity and magnetism. However, it is still challenging to directly realize multi-field-driven magnetoelectric coupling in multiferroic oxides as potential multifunctional electrical devices. Here, novel magneto-electric-optical coupling in multiferroic BiFeO3 -based thin films at room temperature mediated by deterministic ferroelastic switching using piezoresponse/magnetic force microscopy and aberration-corrected transmission electron microscopy are shown. Reversible photoinduced ferroelastic switching exhibiting magnetoelectric responses is confirmed in BiFeO3 -based films, which works at flexible strain states. This work directly demonstrates room-temperature magneto-electric-optical coupling in multiferroic films, which provides a framework for designing potential multi-field-driven magnetoelectric devices such as energy conservation memories.

Optimal magnetoelectric coupling performance of Terfenol-D/PZT composites in resonance state under bias magnetic field

Laminated magnetoelectric (ME) composites with various thickness ratios were optimized, fabricated and experimentally investigated in this work. The Terfenal-D/PZT specimens with optimal thickness ratio between the magnetostrictive phase and piezoelectric phase, and two other values were tested for their ME coupling performance. The coupling voltage output increases linearly with the increase of DC bias magnetic field. The ME voltage coefficient increases more than 100 times in the resonance state for the optimal laminate. The DC bias magnetic field affects the ME voltage coefficient significantly, and also has little effect on the resonant frequency. The strength of AC magnetic field also slightly affects the ME voltage coefficient in resonance state, but does not affect the resonant state under which the same DC magnetic field is required. The experimental results can help understand the coupling performance of ME composite under bias magnetic field and prompt the application of ME devices.

Large magnetoelectric effect in BaFe12O19-(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 particulate composite

Multiferroic composites with a large magnetoelectric response are gaining attractive interests for the design of magnetoelectric (ME) functional devices. In this work, the particulate ME composites (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3–xBaFe12O19 (x=0, 0.1, 0.2, 0.3, 0.4 and 1) were prepared, and their structural, dielectric, magnetic, ferroelectric, piezoelectric properties and magnetoelectric coupling were systematically investigated. The composites consisted of only two chemically separated phases with well-bonded interface. Dielectric and impedance analyses indicated the co-contribution of grain and grain boundary to polarization. Well-saturated ferroelectric and magnetic hysteresis loops demonstrated multiferroic nature. ME response was investigated elaborately by employing magnetically induced polarization, together with measuring ME voltage coefficient and magnetodielectric value. Specifically, a large ME coefficient of 26.78 ​mV/cm·Oe was achieved for x=0.3, which is higher than that in single-phase BaFe12O19 and its coupled composites.

Multi-field tuning of dielectric relaxation in epitaxial multiferroic hexaferrite thin film around room temperature: Property and mechanisms

Understanding the dielectric relaxation properties of multiferroic hexaferrite thin films is crucial for device applications. This work investigates the dielectric relaxation characteristics (DRC) of multiferroic hexaferrite BaFe10.2Sc1.8O19 (BFSO) thin film under multi-fields. The alternative driving voltage and biasing voltage Vb were found to have significant effects on the DRC of BFSO/electrode interface dipoles, but negligible effects on the intrinsic dipoles in BFSO film. The DRC mechanism of a BFSO film capacitor under a magnetic field is opposite to that of Vb; is dominated by intrinsic dipoles in BFSO, and is thermally activated. The temperature-dependent magnetodielectric spectra confirm that multiferroic dipoles in BFSO significantly impact the DRC of the capacitor, which is different from the DRC of nonmagnetic ceramic capacitors. The above findings can clarify the DRC mechanisms and integration of multiferroic hexaferrite thin films in magnetoelectric devices.

Physics of Composites for Low-Frequency Magnetoelectric Devices.

The article discusses the physical foundations of the application of the linear magnetoelectric (ME) effect in composites for devices in the low-frequency range, including the electromechanical resonance (EMR) region. The main theoretical expressions for the ME voltage coefficients in the case of a symmetric and asymmetric composite structure in the quasi-static and resonant modes are given. The area of EMR considered here includes longitudinal, bending, longitudinal shear, and torsional modes. Explanations are given for finding the main resonant frequencies of the modes under study. Comparison of theory and experimental results for some composites is given.

Finite element analysis of the magnetoelectric effect on hybrid magnetoelectric composites

The multi-component hybrid magnetoelectric (ME) composites obtained by using high molecular polymer as the first component and ferromagnetic or ferroelectric material as the second component by hierarchical compounding has the advantages of light weight, easy processing and flexibility. It has the characteristics of good performance and can also realize the conversion between magnetic energy and electric energy, which has potential applications in the fields of bionic robots and medical non-invasive detection. In this study, the finite element method is used to study its ME effect. Considering the distribution characteristics of the component materials and their nonlinear constitutive equations, as well as the dynamic equation and the corresponding mechanical and electrical boundary conditions, a finite element model of the ME effect of the ME composites is established. The distribution of elastic field, magnetic field and electric field of the hybrid ME composites were studied, and the influence of microstructural distribution characteristics, geometric parameters, external magnetic field, temperature and other factors on its magnetoelectric coefficient was analyzed. The calculated results can provide the theoretical guidance for the optimal design of high-performance magnetoelectric devices and render better support for the technological development in the field of magnetoelectricity.

Magnetoelectric effects in stripe- and periodic heterostructures based on nickel–lead zirconate titanate bilayers

Objectives. A topical task in the design of magnetoelectric (ME) devices based on composite ferromagnetic– piezoelectric heterostructures involves reducing their dimensions to increase their operating frequencies and optimize their integration in modern electronics. The study set out to investigate the influence of in-plane dimensions on the characteristics of ME effects in stripe and periodic nickel–lead zirconate titanate heterostructures manufactured via electrolytic deposition.Methods. Lead zirconate titanate disks with Ag-electrodes were used for manufacturing the ME heterostructures; Ni was deposited on one Ag-electrode only.Results. While a reduction in stripe size leads to an increase in the frequency of the resonant ME effect, it is followed by a decrease in ME conversion efficiency. The ME coefficient for the periodic heterostructures is about ~1 V/(Oe·cm). By increasing the angle between the magnetic field H and the Ni-stripe axis from 0° to 90°, a 2.5-fold increase in the optimal field Hm and a 4-fold drop in the maximum amplitude of ME voltage umax(Hm) was achieved.Conclusions. In periodic heterostructures, the frequency of the resonant ME effect is determined by the substrate’s size, while ME conversion efficiency depends on the width of the Ni stripes and the distance between them. The observed anisotropy of the ME effects in the investigated heterostructures is explained in terms of demagnetization effects. In the future, the anisotropic ME effect in the periodic heterostructures could be used to develop magnetic field sensors that are sensitive to field orientation.

Electric Control of Exchange Bias at Room Temperature by Resistive Switching via Electrochemical Metallization.

Electric field control of exchange bias (EB) plays an important role in spintronics due to its attractive merit of lower energy consumption. Here, we propose a novel method for electrically tunable EB at room temperature in a device with the stack of Si/SiO2/Ta/Pt/Ag/Mn-doped ZnO (MZO)/Pt/FeMn/Co/ITO by resistive switching (RS) via electrochemical metallization (ECM). The device shows enhanced and weakened EB when set at high-resistance state (HRS) and low-resistance state (LRS), respectively. For the device at LRS, the aberration-corrected scanning transmission electron microscopy (STEM) characterizations unambiguously reveal that the Ag filaments grow initially from the Ag anode and then elongate toward the ITO cathode. It is inferred that at LRS, a small portion of Ag filaments have passed through the MZO and the intervening thin Pt layer and extended into the FeMn layer. After applying reverse voltage, these Ag filaments are electrochemically dissolved and ruptured near the MZO/Pt interface. This is considered to be the main mechanism responsible for RS and switchable EB as well. This work presents a new strategy for designing low-power, nonvolatile magnetoelectric random access memory devices.

Self-biased characteristics of NZCF/BCZT layered magnetoelectric composites: A novel coupling paradigm in magnetoelectricity

Here, we investigate room temperature magnetoelectric (ME) behavior of ferroelectric Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) and ferrimagnetic Ni0.75Zn0.20Co0.05Fe2O4 (NZCF) layered composites. The ferroelectric ceramics (thickness, t, ∼0.6, 0.8, and 1.0 mm) are stacked at magnetic specimen of fixed t ∼ 1 mm, coupled with conducting silver epoxy. The phase formation, surface morphology, dielectric, magnetic, magnetoelectric and magnetodielectric characteristics of bilayer composites have been studied in the present work. The maximum value of magnetoelectric (ME) coupling coefficient (αME) is found to be 70, 90, and 110 mVcm−1Oe−1 for composite having ferroelectric layer thickness t ∼ 1, 0.8, and 0.6 mm, respectively. More importantly, αME(H→0) remains finite for composite (t ∼ 0.8, and 0.6 mm) in zero magnetic field, which is a clear evidence of self-biasing phenomenon in NZCF/BCZT layered composite. For closer insight about underlying mechanism, we estimate magnetodielectric (MD) coefficient for the layered composite. The MD coefficient is found to be significantly high ∼6.32% for the thickness 0.6 mm. MD% vs magnetic field, H, curves exhibit steeper field dependence in low field region and for H≥2kOe MD possess saturation characteristics. The high saturation characteristics suggest that the magnetoelectricity in layered composite is driven by magnetostriction mechanism and causes robust ME coupling. Further, we obtain electrical signal ∼143 mV with the Hdc ∼250 Oe, as a consequence of interplaying nature of magneto(striction)- piezo(electricity). Such a sizeable response will improve the underlying intimacy between technology and practical application. The study reveals that the pellet based layered composites may be a proven strategy for the realization of self-biased ME devices.

Inherent Spin-Polarization Coupling in a Magnetoelectric Vortex.

Solid-state materials are currently being explored as a platform for the manipulation of spins for spintronics and quantum information science. More broadly, a wide spectrum of ferroelectric materials, spanning from inorganic oxides to polymeric systems such as PVDF, present a different approach to explore quantum phenomena in which the spins are set and manipulated with electric fields. Using dilute Fe3+-doped ferroelectric PbTiO3-SrTiO3 superlattices as a model system, we demonstrate intrinsic spin-polarization control of spin directionality in complex ferroelectric vortices and skyrmions. Electron paramagnetic resonance (EPR) spectra show that the spins in the Fe3+ ion are strongly coupled to the local polarization and preferentially aligned perpendicular to the ferroelectric polar c axis in this complex vortex structure. The effect of polarization-spin directionality is corroborated by first-principles calculations, demonstrating the variation of the spin directionality with the polar texture and offering the potential for future quantum analogues of macroscopic magnetoelectric devices.

Functional properties of Yttrium Iron Garnett thin films on graphene-coated Gd3Ga5O12 for remote epitaxial transfer

Remote epitaxial growth via a graphene interlayer and subsequent mechanical exfoliation of a free-standing membrane is a recently developed technique used to transfer complex oxide thin films onto non-native substrates to form heterogeneously integrated structures for various device applications. One such oxide is Yttrium Iron Garnet (YIG), a material of choice for a wide range of magnetoelectric and spintronic devices owing to its ferromagnetism with high Curie temperature as well as high quality factor and low losses in microwave frequencies. YIG is predominantly grown on lattice matched Gadolinium Gallium Garnet (GGG) substrates, but by utilizing the remote epitaxy technique, high quality YIG films can be transferred from GGG onto another substrate such as piezoelectric Lithium Niobate (LN). Mechanical strain coupling between the layers and magnetostrictive nature of YIG would allow for the investigation of the interplay in YIG/LN structures leading to the design of novel frequency agile magneto-acoustic devices. In this study functional properties of a YIG film grown using PLD on graphene-coated GGG substrate were investigated and compared to traditional YIG on GGG. Both materials were characterized in terms of crystal structure, surface morphology, FMR and Gilbert damping, and Raman and XAS spectroscopy. It was found that YIG on graphene-coated GGG exhibits significantly higher microwave losses than standard YIG on GGG (FMR linewidth 30.9 vs 2.1 Oe at 10 GHz, and Gilbert damping coefficient 15.4 × 10-4 vs 3.4 × 10-4 respectively), which was attributed to increased concentration of Fe2+ cations in YIG/Graphene/GGG. While the damping is higher in these studied films compared to YIG grown directly on GGG, the resulting properties are still very favorable compared to many other competing materials which can be grown without the need for lattice matched substrates, such as metallic ferromagnets.

Imaging of Love Waves and Their Interaction with Magnetic Domain Walls in Magnetoelectric Magnetic Field Sensors

AbstractThe complex behavior of horizontally polarized surface shear waves in magnetoelectric surface acoustic wave based magnetic field sensor devices is revealed by time‐resolved magnetooptical microscopy with picosecond temporal and submicron spatial resolution. The imaging of the propagating waves in the magnetoelectric composites is realized through the functional soft‐magnetic layer by coupled magnetoelastic interactions. Partial surface wave reflections, wave front dephasing, and secondary wave generation occur, which originate from structures and magnetic domain walls. Closure domain structures bend and reflect the magnetic surface waves. Strain stimulated magnetic domain walls display dynamic periodic expansions, which propagate along the domain walls and change the magnetomechanical response also in the surrounding regions. The revealed spatial and temporally varying nondeterministic response restricts the noise performance of the surface acoustic wave based magnetic field sensors and thus confines the sensor's limit of detection. Magnetic time‐resolved optical imaging is shown to be a powerful method for the operando characterization of magnetoelectric devices and in‐plane displacement surface acoustic wave fields that are not accessible by other methods.

Predictions of Dynamic Multimode-Coupling and High-Frequency Vibrations in Magneto-Electro-Elastic Heterostructures.

Mode coupling between the operation mode and unwanted eigenmodes has a significant influence on the working performance of novel thin-film magnetoelectric (ME) devices operating at high frequencies. In this article, the extended frequency spectrum quantitative prediction (FSQP) method is used to investigate mode-coupling vibrations in high-frequency ME heterostructures. This method has three key procedures. First, wave propagation in ME heterostructures is studied to determine the wavenumber and frequency of the eigenmodes. Second, the variational formulation of a general ME heterostructure is constructed. Finally, frequency spectra for predicting the coupling strength among the eigenmodes are obtained by substituting the solutions consisting of all eigenmodes into the variational formulation. Two numerical examples are presented to validate the extended FSQP method. The mode shapes of the mechanical displacements are used to thoroughly describe the mode-coupling behavior in different vibration modes. The numerical results show that the mode-coupling strength is significantly affected by the structural size and number of layers in an ME heterostructure. Furthermore, structural symmetry along the thickness direction may cause specific mode-decoupling phenomena. Effective strategies for suppressing multimode-coupling vibrations in ME heterostructures by optimizing the lateral aspect ratios based on the frequency spectra are proposed to guide device design.