Magnetoelectric Devices Research Articles

Evolution of Relaxor Behavior in Multiferroic Pb(Fe2/3W1/3)O3-BiFeO3 Solid Solution of Complex Perovskite Structure

Mutiferroic materials like bismuth ferrite BiFeO3 have attracted much interest in the last decade due to their promising potential for such applications as spintronics and magnetoelectric data storage devices. On the other hand, relaxor ferroelectrics have been intensively studied for their complex structures with quenched disorder and polar nanoregions which play an important role in their outstanding piezoelectric performance. Much less studied are the single-phase multiferroics that exhibit ferroelectric and/or magnetic relaxor behavior and the correlation between their structure and intricate magneto-electric interactions. In this work, we investigate the evolution of the structure and relaxor behavior in the solid solution between the complex perovskite multirelaxor Pb(Fe2/3W1/3)O3 [PFW] and canonical multiferroic BiFeO3 [BFO], (1-x)PFW-xBFO (with a solubility limit of x = 0.30). The temperature dependences of the dielectric permittivity and loss tangent measured in the frequency range from 100 Hz to 1 MHz indicate characteristic relaxor ferroelectric properties for compositions of x ≤ 0.15, with a frequency-dependent dielectric permittivity peak and its temperature, Tm, satisfying the Vogel-Fulcher law. Detailed studies of the evolution of the relaxor behavior with composition reveal that Tm decreases firstly with a small amount (x = 0.05) of BFO substitution and then increases with further increase of BFO concentration. The degree of relaxor character, as defined by ΔTm [Tm (1 MHz) - Tm (100 Hz)], increases monotonously with increasing BFO content, signifying an enhancement of relaxor behavior with BFO substitution, which is confirmed by the Lorenz-type quadratic variation of the static permittivity. A temperature - composition phase diagram is constructed in terms of the characteristic Burns temperature (TB) and freezing temperature (Tf), which delimits a paraelectric state (PE) above TB, a non-ergotic relaxor state (NR) below Tf, and an ergotic relaxor state (ER) in between. The observed enhancement of relaxor behavior is explained by an increase in the number and size distribution of polar nanoregions in the ER phase, resulting from increased compositional and charge disorders as a result of BFO substitution. The evolution of relaxor behavior and its microscopic mechanisms studied in this work are insightful for a better understanding the multirelaxor properties in multiferroics. Moreover, further substitution of BFO (x ≥ 0.2) flattens the permittivity curves and leads to a temperature-stable variation of high dielectric constant (≈ 103) in a wide temperature range, making the PFW-BFO solid solution attractive for such applications as high energy density capacitors.

Analysis of Magneto-Mechanical Response for Magnetization-Graded Ferromagnetic Material in Magnetoelectric Laminate.

This paper analyzes the dynamic magneto-mechanical response in magnetization-graded ferromagnetic materials (MGFM) comprised of high-permeability Finemet and traditional magnetostrictive materials. The theoretical modeling of the piezomagnetic coefficient that depends on the bias magnetic field of MGFM is proposed by using the nonlinear constitutive model of a piezomagnetic material, the magnetoelectric equivalent circuit method, and the simulation software Ansoft. The theoretical variation of piezomagnetic coefficients of MGFM on the bias magnetic field is in good agreement with the experiment. Using the piezomagnetic coefficient in the magnetoelectric voltage model, the theoretical longitudinal resonant magnetoelectric voltage coefficients have also been calculated, which are consistent with the experimental values. This theoretical analysis is beneficial to comprehensively understand the self-biased piezomagnetic response of MGFM, and to design magnetoelectric devices with MGFM.

Structural, multiferroic, and magnetoelectric properties of (1 − x)Bi0.85La0.15FeO3–xBaTiO3 composite ceramics

Multiferroic magnetoelectric composites are beneficial in device fabrication because of their tunable ferroelectricity and magnetism. The partial substitution in bismuth ferrite (BiFeO3) is one of the best possible ways for synthesizing pure phase BiFeO3-based materials. A comprehensive study of (1 − x)Bi0.85La0.15FeO3–xBaTiO3 (x = 0–0.3) was done by coalescing ferroelectric, dielectric, magnetic, and magnetoelectric properties with structural and microstructural characterizations to explore the effect of BaTiO3 (BT) into Bi0.85La0.15FeO3 (BLFO) and forming composite ceramics. The X-ray diffraction study reveals the phase purity in BLFO and a structural transformation from rhombohedral to cubic phase with increasing content of BT. The Raman spectroscopy and scanning electron micrographs confirm the co-existence of composite formation in BLFO–xBT. The Raman modes shift towards lower wavenumber with increasing BT concentration suggests lattice compression. The room temperature M–H hysteresis curve shows the existence of weak ferromagnetism in BLFO–BT composites and superparamagnetism in BLFO–10BT ceramic. The curve fitting of M–H curve for BLFO–10BT showed the existence of superparamagnetic particles. The ferroelectric hysteresis P–E loop measurements produced unsaturated oval-shaped loops with high leakage and displayed a lossy dielectric nature. The effect of magnetic field on polarization versus electric field curve reveals the interfacial interaction due to the origin of magnetoelectric interaction in BLFO–BT composite ceramics. All the samples display peak broadening in temperature–permittivity plot and confirm relaxor behavior. The superparamagnetic behavior and magnetic field-dependent energy storage capacity of BLFO–10BT composite ceramic make this material a potential candidate for magnetoelectric devices.

Strain and phase transformation co-mediated magnetoelectric effect in epitaxial Ni/PMN-PT (0 1 1) heterostructures

E-tuning of magnetic behavior is investigated in epitaxial Ni film grown on (0 1 1)-PMN-PT single crystal substrate. Loop-like and butterfly-like Hr-E and Hme-E curves are obtained along [01-1] and [1 0 0], respectively, which is mediated by the combined effects of anisotropic strain and nonvolatile rhombohedral to orthorhombic phase transformation in PMN-PT. The results are further confirmed via E-tuning of domain in PMN-PT using piezoelectric force microscope and E-tuning of magnetic stripe domain in Ni film using magnetic force microscope, as well as E-tuning of XRD. In addition, the highest E-induced Hr shift of 1253 Oe is achieved along [01-1] under electric field of −10 kV/cm. This work is of significance for the potential application in magnetoelectric multifunctional devices.

Improved ferromagnetism and magnetodielectric properties of relaxor ferroelectric Ba4-Ca Nd2Fe1.5Co0.5Nb8O30 ceramics with tungsten bronze structure

Abstract The structural, magnetic, dielectric and magneto-dielectric properties of tungsten bronze Ba4-xCaxNd2Fe1.5Co0.5Nb8O30 (0 ≤ x ≤ 0.9) were investigated. All the samples can be indexed with the tetragonal structure and the space group of P4bm. The incorporation of Ca ions improves the structural distortion and promotes the grain’s growth. The magnetic curves at 300 K indicate the superposition of ferromagnetic (FM) and antiferomagnetic (AFM) state in all the samples, the observed FM phase can be attributed to the spin canting of AFM coupling of Fe- and Co-based sublattices. The Ca-doped samples exhibit an improved FM state due to large lattice distortion. The dielectric constant in anomaly region obeys well with Curie-Weiss law with γ = ∼1.6, implying that all the samples are relaxor ferroelectrics. The dielectric loss relaxations obey with Vogel-Fulcher relation and can be ascribed to the local polarization fluctuation induced by chemical heterogeneities. In addition, the relative change in magnetic field dependent dielectric constant follows with the change in the square of magnetization M2, suggesting that the observed magneto-dielectric is caused by the intrinsic magnetoelectric (ME) coupling effect. These results manifest that tungsten bronze compounds may be the potential candidates for the application in ME devices.

Noncontact spatiotemporal strain mapping of composite multiferroic cylinders

Recently, the reinvigoration of research interest in in strain-mediated composite multiferroic structures resulted in translational magnetoelectric devices with superior performance than their traditional counterparts. Full-filed measurements and visualization of mechanical strain, with a focus at the interface, offer new possibilities for the advancement of magnetoelectric devices regardless of the material system. Here, we report spatiotemporal strain maps of composite magnetoelectric cylinders using a novel approach combining stable laser ultrasonic vibrometer and 2D motion system measurement technique. The physical measurement technique is augmented with a robust analysis algorithm that capable of effectively and consistently detect the interface. With the spatiotemporal maps, we forecast additional utility through the concurrent extraction of polarization and magnetization maps; hence highlighting the complex interactions between the involved order parameters. Understanding the converse magnetoelectric response in strain-mediated composite multiferroics is essential for the development of devices such and motors, antennas, and wireless power transferors. Given the dependence of the converse magnetoelectric response on the strain generation and transduction, mapping the mechanical response is essential for advancement of efficient devices. This mapping approach is challenging due to the dynamic nature of the response as well as the magneto-electro-mechanical coupling. Spatiotemporal maps of in-plane and out-of plane displacement and strain components were measured concurrently off the surface of a composite structure. To generate spatiotemporal strain maps, a laser-ultrasonic vibrometer mounted on a 2D gantry system was employed to map the in-plane and out-of-plane displacement components of a strain-mediated, concentric multiferroic composite cylinder. The strain maps provided insights on the behavior of the strain mediation mechanisms within the investigated composite structure including amplification of the strain due to the disparity in the mechanical properties of the constituents, which will result in a non-uniform distribution of magnetization and polarization responses. Additionally, the method to calculate the strain maps was shown to be robust for different spatial measurement resolutions.

Enhancing the soft magnetic properties of FeGa with a non-magnetic underlayer for microwave applications

An ultra-thin (∼2.5 nm) non-magnetic Cu underlayer was found to have a significant effect on the microstructure, magnetic softness, and magnetostriction of sputter-deposited Fe81Ga19 (FeGa) thin films. Compared to the experimental control where FeGa was deposited directly on Si without an underlayer, the presence of Cu increased the in-plane uniaxial anisotropy of FeGa and reduced the in-plane coercivity by nearly a factor of five. The effective Gilbert damping coefficient was also significantly reduced by a factor of four, between FeGa on Si and FeGa on a Cu underlayer. The FeGa films on Cu also retained a high saturation magnetostriction comparable to those without an underlayer. The enhancement of the desirable magnetic properties for microwave applications is attributed to the Cu underlayer, promoting the (110) film texture and increasing the compressive film strain. The results demonstrated that the structural control is viable to simultaneously achieve the necessary magnetic softness and magnetostriction in FeGa for integration in strain-mediated magnetoelectric and microwave devices.

Nanoengineering room temperature ferroelectricity into orthorhombic SmMnO3 films

Orthorhombic RMnO3 (R = rare-earth cation) compounds are type-II multiferroics induced by inversion-symmetry-breaking of spin order. They hold promise for magneto-electric devices. However, no spontaneous room-temperature ferroic property has been observed to date in orthorhombic RMnO3. Here, using 3D straining in nanocomposite films of (SmMnO3)0.5((Bi,Sm)2O3)0.5, we demonstrate room temperature ferroelectricity and ferromagnetism with TC,FM ~ 90 K, matching exactly with theoretical predictions for the induced strain levels. Large in-plane compressive and out-of-plane tensile strains (−3.6% and +4.9%, respectively) were induced by the stiff (Bi,Sm)2O3 nanopillars embedded. The room temperature electric polarization is comparable to other spin-driven ferroelectric RMnO3 films. Also, while bulk SmMnO3 is antiferromagnetic, ferromagnetism was induced in the composite films. The Mn-O bond angles and lengths determined from density functional theory explain the origin of the ferroelectricity, i.e. modification of the exchange coupling. Our structural tuning method gives a route to designing multiferroics.

Nonlinear modeling of bending-mode magnetoelectric coupling in asymmetric composite structures with magnetization-graded ferromagnetic materials

The bending-mode magnetoelectric (ME) coupling in the asymmetric laminate composite with a magnetization-graded ferromagnetic material has been investigated in this paper. Based on the nonlinear constitutive relationships of the magnetostrictive material, the dynamic piezomagnetic models have been presented for magnetostrictive materials after considering the remanence, demagnetization, and flux concentration effects. In addition, the bending-mode resonant ME coupling model is developed based on the proposed dynamic piezomagnetic model, the motion equation of the ME composite, and the ME equivalent circuit method. The composite structures FeCuNbSiB/Ni/PZT with negative magnetostrictive Ni and FeCuNbSiB/FeNi/PZT with positive magnetostrictive FeNi have been used to confirm the validity and reliability of the theoretical model. The theoretical results show that the bending-resonant ME coefficients reasonably agree with the experimental results. This theoretical model is of importance for understanding the bending-mode ME response of the asymmetric structure with magnetization-graded ferromagnetic materials and designing the ME devices.

Ionic liquid gating control of magnetic anisotropy in Ni0.81Fe0.19 thin films

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.

Magnetoelectric Coupling Springing Up in Molecular Ferroelectric: [N(C2H5)3CH3][FeCl4

A molecule-based ferroelectric triethylmethylammonium tetrachloroferrate(III) ([N(C2H5)3CH3][FeCl4]) powder was designed as a multifunctional material exhibiting excellent multiple bistability. Prepared by the slow evaporation method at room temperature, the compound crystallizes in the non-centrosymmetric assembly of hexagonal symmetry (P63mc space group) which undergoes a reversible temperature-triggered phase transition pinpointed at 363 K to the centrosymmetric packing within the P63/mmc space group. Aside from the inseparable role of the symmetry-breaking process smoothly unveiled from the X-ray powder diffraction data, a striking change in the dielectric permittivity observed during the paraelectric-to-ferroelectric phase transition directly discloses the bistable dielectric behavior-an exceptionally high increase in the dielectric permittivity of about 360% at 100 kHz across the heating and cooling cycles is direct proof showing the highly desirable stimuli-responsive electric ordering in this improper ferroelectric architecture. Due to the magnetically modulated physical properties resulting in the coupling of magnetic and electric orderings, the flexible assembly of [N(C2H5)3CH3][FeCl4] could be used to boost the design and development of novel magnetoelectric devices.

The influence of interface strain on the magnetization switching process in FeSi/(011)PMN-0.3PT heterostructures

The magnetization switching process of Fe80Si20/(PbMg1/3Nb2/3O3)0.7–(PbTiO3)0.3 (FeSi/PMN-0.3PT) heterostructures under electric-field-induced interface strain effects was investigated using the magneto-optical Kerr effect. In the as-deposited FeSi films with low four-fold magnetic anisotropy, the one-jump and two-jump loops corresponding to the 180° and 90° domain walls were observed and could be regularly tuned by applying saturated and converse electric fields in these crystalline regions. After 300 °C annealing treatment, the heterostructures presented enhanced crystallinity and interface strain effect. By applying an impulse electric field, we achieved a reliable and non-volatile two-jump and three-jump magnetization switching transition. These regulation results further confirm the magnetic anisotropy transition and magnetic energy superposition relationship in FeSi/PMN-0.3PT. The controllable magnetization switching process is significant for understanding the mechanism of magnetic anisotropy competition and for designing the relevant magnetoelectric devices.

Effect of particle size on magnetodielectric and magnetoelectric coupling effect of CoFe2O4@BaTiO3 composite fluids

CoFe2O4 particles with two different sizes were prepared using hydrothermal method by controlling the technological parameters. On that basis, core–shell structure CoFe2O4@BaTiO3 (CFO@BTO) particles were prepared by sol–gel process. The surfactant treated composite particles were then distributed into a highly-insulating heptane to form CFO@BTO composite fluids. Influences of CFO particle size on the magnetodielectric and magnetoelectric coupling effect were comparatively investigated. XRD, TEM and SAED results indicate that the size of CFO particles are ~ 12 and ~ 18 nm, respectively, while the prepared CFO@BTO particles show bi-phase and core–shell composite structure, and the size of CFO@BTO particles are ~ 28 and ~ 35 nm, respectively. The composite fluids show quasi super paramagnetic behavior at room temperature due to the size effect and Brovnian motion. When the size of CFO is 18 nm, the CFO@BTO composite fluids shows larger remnant magnetization and coercive field. Strong dependence of dielectric properties on external magnetic field has been observed, of which the composite fluids show negative magnetodielectric effect, and the value of permittivity was decreased by 8% at 500 Oe. When size of CFO is 12 nm, the corresponding specimen with shows more obvious magnetic response, and it has larger remnant polarization. The values of both coercive electric field and remnant polarization of the two kinds of CFO@BTO composite fluids have been enhanced distinctly under the action of external magnetic field. When the particle size of CFO is smaller, the magnetolectric coupling coefficient αE of CFO@BTO composite fluid is larger, and this value decreases with increasing external magnetic field. When the particle sizes of CFO are 12 and 18 nm, the maximal values of αE are about 7 and 10 V/cm·Oe, respectively. These values are several orders of magnitude larger than that of CFO/BTO and other magnetoelectric composite ceramics. The strong coupling effect can be explained based on the formation, movement and variation of chain-like structure in the presence of external field. These results may supply useful information for enhancing coupling effect and might play important roles in practical applications in novel magnetoelectric devices.

Deterministic reversal of single magnetic vortex circulation by an electric field

The ability to control magnetic vortex is critical for their potential applications in spintronic devices. Traditional methods including magnetic field, spin-polarized current etc. have been used to flip the core and/or reverse circulation of vortex. However, it is challenging for deterministic electric-field control of the single magnetic vortex textures with time-reversal broken symmetry and no planar magnetic anisotropy. Here it is reported that a deterministic reversal of single magnetic vortex circulation can be driven back and forth by a space-varying strain in multiferroic heterostructures, which is controlled by using a bi-axial pulsed electric field. Phase-field simulation reveals the mechanism of the emerging magnetoelastic energy with the space variation and visualizes the reversal pathway of the vortex. This deterministic electric-field control of the single magnetic vortex textures demonstrates a new approach to integrate the low-dimensional spin texture into the magnetoelectric thin film devices with low energy consumption.

Structural and magnetic properties of Ni-substituted Ba0.5Sr1.5-based Y-type hexaferrite

The Ni-doped Y-type hexaferrites Ba0.5Sr1.5Zn2−xNixFe12O22 have been prepared using the solid-state technique. The X-ray diffraction patterns indicate that the lattice constant is increased as the Ni content raises, which can be ascribed to the different ionic radius of Zn2+ and Ni2+. All samples are polycrystalline with clear grain interface by means of scanning electron microscopy. The grain size is found to increase with an increase of Ni concentration. The magnetic properties of all samples show different Ni-substitution dependence and strongly depend on Ni content. The results indicate that the value of saturation magnetization increases to a maximum of 41.5 emu/g at x = 0.8, and then decreases with increasing Ni content. For low-doped Ni-substitution (x ≤ 0.8) specimens, the magnetic transition temperature associated with a screw to ferrimagnetic state increases, suggesting that magnetically induced ferroelectric polarization will be observed at high temperature even room temperature. Especially for x = 1.2 specimen, the stabilization of the ferroelectricity phase at a low magnetic field is expected in terms of the disappearance of phase boundary from the screw to intermediate-I. At the same time, the ferroelectric polarization can be maintained in a wide range of magnetic fields for Ni-doped samples. These results confirm that Ba0.5Sr1.5-based Y-type hexaferrite with different Ni-doped content have a great application prospect in magnetoelectric devices at high temperature and under low magnetic fields.

A Material Combination Concept to Realize 4D Printed Products with Newly Emerging Property/Functionality.

4D printing is a newly emerging technique that shows the capability of additively manufacturing structures whose shape, property, or functionality can controllably vary with time under external stimuli. However, most of the existing 4D printed products only focus on the variation of physical geometries, regardless of controllable changes of their properties, as well as practical functionality. Here, a material combination concept is proposed to construct 4D printed devices whose property and functionality can controllably vary. The 4D printed devices consist of conductive and magnetic parts, enabling the integrated devices to show a piezoelectric property even neither part is piezoelectric individually. Consequently, the functionality of the devices is endowed to transfer mechanical to electrical energy based on the electromagnetic introduction principle. The working mechanism of 4D printed devices is explained by a numerical simulation method using Comsol software, facilitating further optimization of their properties by regulating diverse parameters. Due to the self‐powered, quick‐responding, and sensitive properties, the 4D printed magnetoelectric device could work as pressure sensors to warn illegal invasion. This work opens a new manufacturing method of flexible magnetoelectric devices and provides a new material combination concept for the property‐changed and functionality‐changed 4D printing.

Structural and multiferroic properties of trilayer green magnetoelectric Co0.8Ni0.2Fe2O4/K0.25Na0.75NbO3/Co0.8Ni0.2Fe2O4 composite structure

We have investigated the structural, dielectric, magnetic-magnetostrictive, ferroelectric-piezoelectric and magnetoelectric (ME) properties of green (Pb-free) Co0.8Ni0.2Fe2O4/K0.25Na0.75NbO3/Co0.8Ni0.2Fe2O4 (CNFO/KNN/CNFO) trilayer laminate (2-2) multiferroic composite structure. Magnetostrictive Co0.8Ni0.2Fe2O4phase was synthesized by the solution combustion technique and the piezoelectric K0.25Na0.75NbO3 phase was synthesized by a solid-state method. The trilayer ME laminate composite structure of CNFO-KNN phases fabricated by silver epoxy adhesive bonding technique. The magnetostrictive Co0.8Ni0.2Fe2O4 phase exhibits a piezomagnetic coefficient of −0.029 ppm/Oe at 1500 Oe magnetic field. The piezoelectric K0.25Na0.75NbO3 showed a piezoelectric charge coefficient of d33 ~57 pC/N. The trilayer CNFO/KNN/CNFO laminate structure reveals the ME coefficient (αME) of 3.06 mV/Oe·cm which is relatively higher than the 0–3 connectivity composite structures. Here, we have discussed the fabrication technique for multiferroic laminate structures for the green magnetoelectric device having αME ~3.06 mV/Oe·cm with their magnetic/magnetostrictive and ferroelectric/piezoelectric properties.

A Review of Thin-Film Magnetoelastic Materials for Magnetoelectric Applications.

Since the revival of multiferroic laminates with giant magnetoelectric (ME) coefficients, a variety of multifunctional ME devices, such as sensor, inductor, filter, antenna etc. have been developed. Magnetoelastic materials, which couple the magnetization and strain together, have recently attracted ever-increasing attention due to their key roles in ME applications. This review starts with a brief introduction to the early research efforts in the field of multiferroic materials and moves to the recent work on magnetoelectric coupling and their applications based on both bulk and thin-film materials. This is followed by sections summarizing historical works and solving the challenges specific to the fabrication and characterization of magnetoelastic materials with large magnetostriction constants. After presenting the magnetostrictive thin films and their static and dynamic properties, we review micro-electromechanical systems (MEMS) and bulk devices utilizing ME effect. Finally, some open questions and future application directions where the community could head for magnetoelastic materials will be discussed.

Integration of amorphous ferromagnetic oxides with multiferroic materials for room temperature magnetoelectric spintronics

A room temperature amorphous ferromagnetic oxide semiconductor can substantially reduce the cost and complexity associated with utilizing crystalline materials for spintronic devices. We report a new material (Fe0.66Dy0.24Tb0.1)3O7-x (FDTO), which shows semiconducting behavior with reasonable electrical conductivity (~500 mOhm-cm), an optical band-gap (2.4 eV), and a large enough magnetic moment (~200 emu/cc), all of which can be tuned by varying the oxygen content during deposition. Magnetoelectric devices were made by integrating ultrathin FDTO with multiferroic BiFeO3. A strong enhancement in the magnetic coercive field of FDTO grown on BiFeO3 validated a large exchange coupling between them. Additionally, FDTO served as an excellent top electrode for ferroelectric switching in BiFeO3 with no sign of degradation after ~1010 switching cycles. RT magneto-electric coupling was demonstrated by modulating the resistance states of spin-valve structures using electric fields.

Tunable Magnetoelastic Effects in Voltage-Controlled Exchange-Coupled Composite Multiferroic Microstructures.

The magnetoelectric properties of exchange-coupled Ni/CoFeB-based composite multiferroic microstructures are investigated. The strength and sign of the magnetoelastic effect are found to be strongly correlated with the ratio between the thicknesses of two magnetostrictive materials. In cases where the thickness ratio deviates significantly from one, the magnetoelastic behavior of the multiferroic microstructures is dominated by the thicker layer, which contributes more strongly to the observed magnetoelastic effect. More symmetric structures with a thickness ratio equal to one show an emergent interfacial behavior which cannot be accounted for simply by summing up the magnetoelastic effects occurring in the two constituent layers. This aspect is clearly visible in the case of ultrathin bilayers, where the exchange coupling drastically affects the magnetic behavior of the Ni layer, making the Ni/CoFeB bilayer a promising next-generation synthetic magnetic system entirely. This study demonstrates the richness and high tunability of composite multiferroic systems based on coupled magnetic bilayers compared to their single magnetic layer counterparts. Furthermore, because of the compatibility of CoFeB with present magnetic tunnel junction-based spintronic technologies, the reported findings are expected to be of great interest for the development of ultralow-power magnetoelectric memory devices.