Magnetoelectric Heterostructures Research Articles

Magnetoelectric assisted 180° magnetization switching for electric field addressable writing in magnetoresistive random-access memory.

Magnetization-based memories, e.g., hard drive and magnetoresistive random-access memory (MRAM), use bistable magnetic domains in patterned nanomagnets for information recording. Electric field (E) tunable magnetic anisotropy can lower the energy barrier between two distinct magnetic states, promising reduced power consumption and increased recording density. However, integration of magnetoelectric heterostructure into MRAM is a highly challenging task owing to the particular architecture requirements of each component. Here, we show an epitaxial growth of self-assembled CoFe2O4 nanostripes with bistable in-plane magnetizations on Pb(Mg,Nb)O3-PbTiO3 (PMN-PT) substrates, where the magnetic switching can be triggered by E-induced elastic strain effect. An unprecedented magnetic coercive field change of up to 600 Oe was observed with increasing E. A near 180° magnetization rotation can be activated by E in the vicinity of the magnetic coercive field. These findings might help to solve the 1/2-selection problem in traditional MRAM by providing reduced magnetic coercive field in E field selected memory cells.

The electric field manipulation of magnetization in La1−xSrxCoO3/Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructures

La1−xSrxCoO3 (x = 0.18, 0.33, and 0.5) films were grown epitaxially on piezoelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 substrates by pulsed laser deposition. The magnetization of these films varies with the external electric field, showing the magnetoelectric effect. With different doping content of Sr2+ ions, the change of magnetization for these films show different behaviors with increasing temperature, which can be attributed to the competition between electric-field-induced changes of spin state and double exchange interaction. This work presents an alternative mechanism to investigate the electric field control of magnetism in magnetoelectric heterostructure by tuning the spin state.

Low-temperature spin spray deposited ferrite/piezoelectric thin film magnetoelectric heterostructures with strong magnetoelectric coupling

We report low-temperature spin spray deposited Fe3O4/ZnO thin film microwave magnetic/piezoelectric magnetoelectric heterostructures. A voltage induced effective ferromagnetic resonance field of 14 Oe was realized in Fe3O4/ZnO magnetoelectric (ME) heterostructures. Compared with most thin film magnetoelectric heterostructures prepared by high temperature (>600 °C) deposition methods, for example, pulsed laser deposition, molecular beam epitaxy, or sputtering, Fe3O4/ZnO ME heterostructures have much lower deposition temperature (<100 °C) at a much lower cost and less energy dissipation, which can be readily integrated in different integrated circuits.

Thickness-Dependent Crossover from Charge- to Strain-Mediated Magnetoelectric Coupling in Ferromagnetic/Piezoelectric Oxide Heterostructures

Magnetoelectric oxide heterostructures are proposed active layers for spintronic memory and logic devices, where information is conveyed through spin transport in the solid state. Incomplete theories of the coupling between local strain, charge, and magnetic order have limited their deployment into new information and communication technologies. In this study, we report direct, local measurements of strain- and charge-mediated magnetization changes in the La0.7Sr0.3MnO3/PbZr0.2Ti0.8O3 system using spatially resolved characterization techniques in both real and reciprocal space. Polarized neutron reflectometry reveals a graded magnetization that results from both local structural distortions and interfacial screening of bound surface charge from the adjacent ferroelectric. Density functional theory calculations support the experimental observation that strain locally suppresses the magnetization through a change in the Mn-eg orbital polarization. We suggest that this local coupling and magnetization suppression may be tuned by controlling the manganite and ferroelectric layer thicknesses, with direct implications for device applications.

Monolithic magnetoelectric heterostructure with enhanced ferroelectric and piezoelectric properties and tunable magnetic properties

This paper details fabrication of Pb(Zr0.6Ti0.4)O3 (PZT) layer on Metglas (MG) foils—a magnetoelectric monolithic herterostructure with reduced fatigue and enhanced ferroelectric and piezoelectric properties. Polarization–electric field measurements revealed reduced fatigue for PZT–MG monoliths due to reduced clamping: remnant polarization (2Pr) increased from 24µC/cm2 to 91µC/cm2 after the first 5×105 cycles. In comparison, PZT on SrTiO3 (STO) exhibited a monotonous decrease in Pr. Piezoelectric measurements demonstrated a d33 value two times larger for PZT/MG than PZT/STO. Finally, magnetic hysteresis loop measurements showed a dramatic increase of the saturation magnetic field for PZT/MG, due to elastic strain at the interface.

Interfacial effects in magnetoelectric thin/thick composite films

This study reports the effect of interfacial structure on the magnetoelectric and ferroelectric characteristics in magnetoelectric laminate thin/thick films. Both bilayer and trilayer magnetoelectric heterostructures were deposited on a Pt/Ti/SiO2/Si substrate using pulsed laser deposition (PLD) and sol-gel deposition (SGD) respectively. BaTiO3 [BTO] / Pb(Zr0.6Ti0.4)O3 [PZT] and CoFe2O4 [CFO] were used as piezoelectric and magnetostrictive materials. The interface between CFO/PZT and CFO/BTO films was found to exhibit excellent interface without any cracks or delamination. PLD grown BTO on CFO was found to exhibit the columnar structure. SG deposited PZT/CFO/PZT tri-layer structure exhibited granular structure with high density. Ferroelectric and magnetoelectric coefficient of as-grown films was investigated to understand the effect of interface characteristics. Good ME coupling for the trilayer structure provided important insight about the nature of elastic strain coupling.

Self-biased 215 MHz magnetoelectric NEMS resonator for ultra-sensitive DC magnetic field detection.

High sensitivity magnetoelectric sensors with their electromechanical resonance frequencies < 200 kHz have been recently demonstrated using magnetostrictive/piezoelectric magnetoelectric heterostructures. In this work, we demonstrate a novel magnetoelectric nano-electromechanical systems (NEMS) resonator with an electromechanical resonance frequency of 215 MHz based on an AlN/(FeGaB/Al2O3) × 10 magnetoelectric heterostructure for detecting DC magnetic fields. This magnetoelectric NEMS resonator showed a high quality factor of 735, and strong magnetoelectric coupling with a large voltage tunable sensitivity. The admittance of the magnetoelectric NEMS resonator was very sensitive to DC magnetic fields at its electromechanical resonance, which led to a new detection mechanism for ultra-sensitive self-biased RF NEMS magnetoelectric sensor with a low limit of detection of DC magnetic fields of ~300 picoTelsa. The magnetic/piezoelectric heterostructure based RF NEMS magnetoelectric sensor is compact, power efficient and readily integrated with CMOS technology, which represents a new class of ultra-sensitive magnetometers for DC and low frequency AC magnetic fields.

Multiferroic and magnetoelectric heterostructures

We review recent developments and advances in multiferroic and magnetoelectric heterostructures. Driven by the promise of new materials functionality (i.e. electric field control of ferromagnetism), extensive on-going research is focused on the search for and characterization of new multiferroic materials. In this review we develop a comprehensive overview of multiferroic materials, including details on the nature of order parameters and coupling in these materials, the scarcity of such materials in single phase form, routes to create and control the properties of these materials, and we finish by investigating such effects in a number of model materials and heterostructures. This includes an in-depth discussion of BiFeO3, an investigation of recent advances in magnetoelectric materials, and an introduction to a variety of approaches by which one can achieve novel materials functionality.

Energy harvesting properties of all-thin-film multiferroic cantilevers

We have measured electromagnetic energy harvesting properties of all-thin-film magnetoelectric (ME) heterostructures on Si cantilevers. The devices are built on a silicon oxide/nitride/oxide stack, and the ME layers consist of a magnetostrictive Fe0.7Ga0.3 thin film and a Pb(Zr0.52Ti0.48)O3 piezoelectric thin film. The harvested peak power at 1 Oe is 0.7 mW/cm3 (RMS) at the resonant frequency (3.8 kHz) with a load of 12.5 kΩ. The resonant frequency was found to display DC bias magnetic field dependence indicative of a magnetization canting with respect to the cantilever easy axis as a result of interplay between the anisotropy and Zeeman energies.

Differential-mode vibrational noise cancellation structure for Metglas/Pb(Zr,Ti)O3 fiber magnetoelectric laminates

A differential structure which has the ability to reject external vibrational noise for Metglas/Pb(Zr,Ti)O(3) (PZT) fiber-based magnetoelectric (ME) heterostructures has been studied. This type of ME structure functions better than conventional sensors as a magnetic sensor when used in an environment in which vibrational isolation is impractical. Sensors fabricated with this differential mode structure can attenuate external vibrational noise by about 10 to 20 dB at different frequencies, while simultaneously having a doubled ME voltage coefficient. Interestingly, in addition to offering a means of mitigating vibrational noise, this ME structure offers the potential to be a hybrid sensor, separating magnetic and acoustical signals.

Strain-mediated magnetoelectric coupling in magnetostrictive/piezoelectric heterostructures and resulting high-frequency effects

Magnetoelectric coupling terms are derived in piezoelectric/magnetostrictive (multiferroic) thin film heterostructures using Landau-Ginzburg free energy expansions in terms of strain and by considering strain boundary conditions between the two materials. Then, a general effective medium method for solving for the complete electromagnetic susceptibility tensor of such heterostructures is used to calculate the ferromagnetic resonance frequency in a BaTiO$_3$/NiFe$_2$O$_4$ superlattice. This method differs from existing methods for treating magnetoelectric heterostructures since the magnetic and electric dipolar fields are not assumed constant but vary from one film to another. The ferromagnetic resonance frequency shift is calculated as a function of applied electric field and is compared to some experimental results.

Design of magnetoelectric multiferroic heterostructures by topology optimization

Composite BaTiO3–CoFe2O4 nanostructures deposited on a SrTiO3 substrate are known to produce the magnetoelectric coupling effect. Although the coupling efficiency is significantly influenced by the composite layout, there appears no systematic simulation approach for configuring optimal layouts. In this study, a formulation to find optimal heterostructures using the topology optimization method is developed. For the formulation, the macroscopic extrinsic magnetoelectric coupling factor is maximized while numerical calculation is performed by finite element analysis. The proposed method yields an optimal piezoelectric and piezomagnetic material distribution. Numerical simulations are used to explain why the optimized distribution indeed maximizes the magnetoelectric coupling effect. As an application of the developed method, a magnetic read head sensor using the magnetoelectric effect is also designed.

Magnetoelectric effect in crystallographically textured BaTiO3 films deposited on ferromagnetic metallic glass foils

We demonstrate a significant control of the polarization response under an applied magnetic field for a magnetoelectric (ME) heterostructure. This structure was comprised of a 2 μm thick ferroelectric BaTiO3 (BTO) film deposited on flexible ferromagnetic metallic glass foil (25 μm thick). Au was used as a buffer layer to control BTO growth orientation, and to protect the metallic glass from oxidation. x-ray diffraction and scanning electron microscopy demonstrated the successful growth of well-crystallized BTO films with a high degree of (111) orientation on the amorphous metallic glass foils. Well-defined polarization (P-E) and magnetization (M-H) hysteresis loops confirmed the coexistence of ferroelectric and ferromagnetic properties. A ME voltage coefficient of about ∼60 mV/cm Oe was measured.

Improved Sensitivity and Noise in Magneto-Electric Magnetic Field Sensors by Use of Modulated AC Magnetostriction

A magnetic field sensor based on the nonlinear nature of the magnetostrictive response of a magneto-electric (ME) heterostructure has two orders of magnitude improvement in sensitivity and signal-to-noise ratio compared with a conventional dc-biased configuration. The sensor consists of a longitudinally magnetized and transversely poled lamination of iron-cobalt-boron (Metglas) and lead zirconate titanate (PZT). The ac-modulated sensor has enhanced environmental noise immunity and does not require a dc magnetic bias field. Combined, these advantages hold promise for the development of miniature ME sensor elements for applications with size and weight limitations.

Electric-field control of ferromagnetic resonance in monolithic BaFe12O19–Ba0.5Sr0.5TiO3 heterostructures

This paper demonstrates an electric-field tuning of the ferromagnetic resonance (FMR) responses at millimeter wave frequencies for a monolithic magneto-electric heterostructure. The layered stack is comprised of c-axis oriented and low loss barium hexaferrite (BaM) and (111) oriented ferroelectric barium strontium titanate (BSTO) layers along with embedded platinum electrode layers, all fabricated by pulsed laser deposition technique. A tunability of the FMR frequency as large as 3.5 MHz/V has been observed at 60 GHz due to application of bias voltages in the range of several volts. The realization of such a large tunability relies on the quasi-lattice-to-lattice contact between the BaM and BSTO layers as well as the high quality of those layers.

Magnetic and Dielectric Properties of Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;/LiNbO&lt;sub&gt;3&lt;/sub&gt;/Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Multi Layer

We proposed a new type of multi-ferroic system using magnetoelectric (ME:Cr2O3) material/ultra-thin ferroelectric materials (FE)/a magnetoelectric material hetero structure. This ME and FE heterosystem showed a general Polarization – Electric field hysteresis curve. When a magnetic and electric field (ME field) was applied at the same time, a decrease of polarization was observed. This might be due to the antiferromagnetic domain alignment of Cr2O3 by the ME field.

Control of magnetic and electric responses with electric and magnetic fields in magnetoelectric heterostructures

This paper reports on the tuning of both magnetic and electric responses with electric and magnetic fields for metglas-Pb (Zr,Ti)O3 based magnetoelectric (ME) heterostructures that can be promising for communication and sensor applications. The hysteresis loop results indicate a change in the in-plane magnetization due to application of voltages that leads to a tuning of the ferromagnetic resonance frequency by up to about 210 MHz with electric field. Furthermore, these structures show a high ME voltage coefficient that results in the detection of a 2 nT ac magnetic field and a low noise floor.

Enhancement in the field sensitivity of magnetoelectric laminate heterostructures

The effect of magnetostrictive layer thickness on the magnetoelectric (ME) response and field sensitivity of Pb(Zr,Ti)O3-metglas based sandwiched ME heterostructures has been studied. Such structures hold promise for sensor applications. The increase in metglas thickness results in a significant increase in the ME response and magnetic field sensitivity. The ME coefficient and field sensitivity increase by about 1.5–1.75 and 2.7 times, respectively, for a structure with 150 μm thick six metglas layers on both sides of the Pb(Zr,Ti)O3, in comparison to a 50 μm thick two layered structure.

Converse magnetoelectric coupling in multilayer capacitors

We report electrically induced changes of 20% in both the remnant magnetization and the coercive field of a magnetoelectric heterostructure. This heterostructure is an industrially produced multilayer capacitor comprising Ni-based magnetostrictive electrodes sandwiching a BaTiO3-based dielectric that is ferroelectric and therefore piezoelectric. Both magnetization and strain are shown to be hysteretic with applied electric field. These inexpensive capacitors might find use as electrically controlled ferromagnets.

Electrical and magnetic properties of multiferroic BiFeO3/CoFe2O4 heterostructure

To realize a magnetoelectric heterostructure with desired ferroelectric and magnetic properties, a heterostructure consisting of BiFeO3 (BFO)/CoFe2O4 (CFO) layers has been grown on SrRuO3 buffered Pt/TiO2/SiO2/Si substrate by rf sputtering. X-ray diffraction shows that the BFO and CFO phases have been successfully retained in the heterostructure. Grain growth of the CFO phase was enhanced on top of the BFO layer. The heterostructure exhibits both ferroelectric and magnetic behaviors at room temperature. Its remanent polarization (2Pr) is measured to be ∼146 μC/cm2 and the coercive field (2Ec) is ∼1803 kV/cm, while the saturation magnetization (2Ms) is 140 emu/cm3 and the coercive field (2Hc) is ∼2.7 kOe. The leakage current behavior of the heterostructure is consistent with space charge limited conduction mechanism. While the heterostructure is promising for multiferroic behavior, it would be necessary to control the structural defects such that the leakage is minimized.