Multiferroic Heterostructures Research Articles

Ferroelectric control of electron half-metallicity in A -type antiferromagnets and its application to nonvolatile memory devices

Two-dimensional (2D) $A$-type antiferromagnetic van der Waals (vdW) materials with either intralayer ferromagnetic and/or intralayer antiferromagnetic coupling have sparked great interest due to their potential applications in spintronics. By first-principles design we predict that a heterostructure formed by sandwiching the 2D $A$-type antiferromagnet, $2\mathrm{H}\text{\ensuremath{-}}\mathrm{V}{\mathrm{Se}}_{2}$, between the 2D out-of-plane ferroelectric, ${\mathrm{Sc}}_{2}{\mathrm{CO}}_{2}$, presents a semimetal band structure with a half-metallic conduction band. The mechanism giving rise to this peculiar structure cooperatively originates from the strong built-in electric field of the double-layer ferroelectric ${\mathrm{Sc}}_{2}{\mathrm{CO}}_{2}$, and from the charge transfer selectively occurring only at one interface. Based on the so-designed ${\mathrm{Sc}}_{2}{\mathrm{CO}}_{2}\text{/}\mathrm{V}{\mathrm{Se}}_{2}$ multiferroic heterostructure, two concepts for nonvolatile memory devices are proposed, in which two states (``1'' and ``0'') are realized by switching the polarization direction of the ferroelectric layers. These results demonstrate that the combination of 2D ferroelectrics and $A$-type antiferromagnetic vdW materials provides not only a fascinating way to achieve half-metallicity in 2D materials, but also a route to the design of new types of nonvolatile ferroelectric memory devices.

Strong electric field tuning of magnetism in self-biased multiferroic structures

A new type of multiferroic heterostructure has been proposed in this work with strong electric field tuning of magnetism. It is composed of a self-biased magnetic layered structure with perpendicular magnetic anisotropy (PMA) and one piezoelectric substrate. Two configurations were investigated by a modeling approach, Ni/Ni/Ni/PMN-PT with Cu as spacer and Terfenol-D/CoFeB/Ni/PMN-PT. Magnetic multilayers at their resonance exhibit multiple absorption peaks from acoustic and optical modes of spin interaction between adjacent magnetic layers. A piezoelectric substrate transfers electric field induced strain to adjacent magnetic layer and thus shifts resonance frequencies of the multiferroic structure by tuning magnetic effective fields through magnetoelastic coupling. It has been demonstrated computationally that the resonance frequencies for the simulated structures could be up to 76 GHz under zero magnetic bias field. A larger tunability (> 100%) is achieved with applied electric field to the PMN-PT [011] substrate. Resonance mode selectivity is present in the configuration Terfenol-D/CoFeB/Ni/PMN-PT wherein one desired mode exhibits a much higher tunability compared to other modes. This enables the total mode number to be tuned by merging or diverging different modes under E-field.

Interplay between morphology and magnetoelectric coupling in Fe/PMN-PT multiferroic heterostructures studied by microscopy techniques

A ferromagnetic (FM) thin film deposited on a substrate of $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}\text{\ensuremath{-}}{\mathrm{PbTiO}}_{3}$ (PMN-PT) is an appealing heterostructure for the electrical control of magnetism, which would enable nonvolatile memories with ultralow-power consumption. Reversible and electrically controlled morphological changes at the surface of PMN-PT suggest that the magnetoelectric effects are more complex than the commonly used ``strain-mediated'' description. Here we show that changes in substrate morphology intervene in magnetoelectric coupling as a key parameter interplaying with strain. Magnetic-sensitive microscopy techniques are used to study magnetoelectric coupling in Fe/PMN-PT at different length scales, and compare different substrate cuts. The observed rotation of the magnetic anisotropy is connected to the changes in morphology, and mapped in the crack pattern at the mesoscopic scale. Ferroelectric polarization switching induces a magnetic field-free rotation of the magnetic domains at micrometer scale, with a wide distribution of rotation angles. Our results show that the relationship between the rotation of the magnetic easy axis and the rotation of the in-plane component of the electric polarization is not straightforward, as well as the relationship between ferroelectric domains and crack pattern. The understanding and control of this phenomenon is crucial to develop functional devices based on FM/PMN-PT heterostructures.

Voltage manipulation of desired magnetization orientation in multiferroic heterostructures

We report the pure voltage manipulation of desired in-plane magnetization orientation of the nanomagnets in Ni/PMN-PT multiferroic heterostructure and systemically studies the effect of localized stress on the magnetization orientation by magnetic force microscope. Referable analysis about the dynamic magnetization process of the nanomagnet are provided by micromagnetic simulation. Both the experiments and simulations have proved that the 180 degrees switching of the nanomagnets can be induced without strict requirements of the voltage waveform. The results can be effectively applied to electrical induced logic writing of low-energy-consumption spintronic devices.

Controllable electric field tuning of anisotropic magnetic response of Ni/PMN-PT heterostructures

Epitaxial Ni films with various thicknesses were grown on (001)-oriented 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 (PMN–PT) substrates. Electric field tuning (E-tuning) of ferromagnetic resonance field (Hr) was utilized to investigate the magnetic response of Ni films along different film directions. Butterfly-like Hr vs E curves are shown when magnetic field is applied along film edge. While the combination of both loop-like and butterfly-like Hr vs E curves are observed for Ni film with thicknesses of 180 nm and 240 nm, and merely loop-like curves can be seen for film thicknesses of 330 nm and 510 nm, as magnetic field is applied along film diagonal. These interesting phenomena are assumed to be related to ferroelectric and ferroelastic domain switching in PMN-PT, film strain status, as well as magnetic anisotropy in Ni film. This work demonstrates the feasibility of control of anisotropic magnetic response via electric field in multiferroic heterostructure.

Comparison of structure and multiferroic performances of bilayer and trilayer multiferroic heterostructures

This paper reports on the comparison of structure and multiferroic performances for Ba0.85Ca0.15Zr0.1Ti0.9O3/La0.7Sr0.3MnO3 (BCZT/LSMO) bilayer and BCZT/LSMO/BCZT trilayer multiferroic heterostructures. The epitaxial growth and the strain states of the multiferroic heterostructures are confirmed by the XRD RSM, which demonstrates that both the BCZT layer and LSMO layer in the trilayer heterostructure suffer larger residual strain than the bilayer one. The room temperature multiferroic nature of the multiferroic heterostructures is proved by the simultaneous observed ferroelectric and ferromagnetic properties as well as the magnetoelectric effect, which shows strong dependence of the residual strain. As compared with the trilayer heterostructure, the bilayer heterostructure shows better electrical and ferromagnetic properties as well as the ME effect. The largest piezoelectric coefficient of 40.5 p.m./V, the highest remnant polarization of 23.3 μC/cm2 and the biggest ferromagnetic Curie temperature of 316.7 K are obtained in the bilayer heterostructure because of the smaller residual strain. And the bilayer heterostructure also shows the strongest magnetoelectric effect with the αE31 value of 215.5 mV/cm·Oe.

Tunable Magnetic Anisotropy and Dzyaloshinskii-Moriya Interaction in an Ultrathin van der Waals Fe3GeTe2/In2Se3 Heterostructure

The promise of future spintronic devices with nanoscale dimension, high-density and low-energy consumption motivates the search for van der Waals heterostructure that stabilize topologically protected whirling spin textures such as magnetic skyrmions and domain walls. To translate these compelling features into practical devices, a key challenge lies in achieving effective manipulation of the magnetic anisotropy energy and the Dzyaloshinskii-Moriya (DM) interaction, the two key parameters that determine skyrmions. Through the first-principles calculation, we demonstrate that the polarization-induced broken inversion symmetry in the two-dimensional Fe3GeTe2/In2Se3 multiferroic heterostructure does cause an interfacial DM interaction. The strong spin-orbit coupling triggers the magnetic anisotropy of the Fe3GeTe2/In2Se3 heterostructure. The magnetic anisotropy and the DM interaction in Fe3GeTe2 can be well controlled by the ferroelectric polarization of In2Se3. This work paves the way towards the spintronic devices based on van der Waals heterostructures.

Coupling of Lamb Waves and Spin Waves in Multiferroic Heterostructures

In this work, we investigate magneto-acoustic attenuation in thin film multiferroic Lamb wave delay lines. By leveraging magneto-acoustic interactions, multiferroics have potential to realize passive chip-scale alternatives to bulky ferrite devices. For the first time, magnetic field dependence of magneto-acoustic interactions in multiferroic Lamb wave devices is characterized. Multiferroic heterostructures of aluminum nitride and cobalt iron boron are fabricated into Lamb wave delay lines operating at 7.492 GHz to study the effect of strain nonuniformity on the multiferroic coupling. The attenuation of the Lamb waves is characterized as a function of the magnitude and angle of an applied in-plane bias magnetic field. It is found that the bias magnitude for peak attenuation is a strong function of angle, indicating that it is due to coupling between the Lamb waves and spin wave modes. This is in contrast with current models of attenuation in multiferroic SAW delay lines, where the uniform surface strains couple to ferromagnetic resonance, which has no angular dependence on the in-plane bias field magnitude. These results are the first steps towards passive chip-scale alternatives to ferrite devices utilizing Lamb wave devices, which have better coupling and scalability than their SAW counterparts. [2020-0114]

Shear-strain-induced over 90° rotation of local magnetization in FeCoSiB/PMN-PT (011) multiferroic heterostructures

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.

Electric field control of molecular magnetic state by two-dimensional ferroelectric heterostructure engineering

Two-dimensional multiferroics have attracted tremendous attention due to their intriguing physics and promising applications. However, it has been a major challenge to discover and design two-dimensional multiferroic materials with large electric polarization and strong magnetoelectric coupling. In this work, we propose a strategy to design a two-dimensional van der Waals heterostructure with strong magnetoelectric coupling by stacking a transition metal phthalocyanine (TMPc) molecule with ferroelectric monolayer In2Se3. By first-principles electronic structure calculations, we predict that the magnetic states of the TMPc molecule can be controlled by electrically switching the polarization direction of In2Se3 using an external electric field. This strong magnetoelectric coupling effect originates from the interfacial charge transfer and orbital splitting, resulting in the different magnetic states of TMPc/In2Se3 heterostructures in two opposite ferroelectric phases. Based on the TMPc/In2Se3 heterostructure, a high-density magnetic memory device is proposed for pure electric writing and magnetic reading. Our predictions may open avenues for finding and designing multiferroic heterostructures by using two-dimensional ferroelectric materials and zero-dimensional magnetic molecules with a strong proximity effect.

Multiferroic response in bismuth ferrite doped heterostructures: A buffer layer inference

Multiferroic heterostructures of doped-BiFeO3 were prepare by RF magnetron sputtering over FTO substrates. Barium-nickel co-doped BiFeO3 (Bi0.75Ba0.25Fe0.975Ni0.025O3) and Praseodymium-Manganese co-doped BiFeO3 (Bi0.85Pr0.15Fe0.97Mn0.03O3) were used as a source for ferroelectric (FE) and ferromagnetic (FM) layers in a two-layer heterostructure architecture. The arrangement of FE and FM layers were switch in order to study the effect of the previous layer consider as a buffer layer on the overall multiferroic properties of the system. X-ray diffraction in two different geometries (BBXRD and GIXRD) were implemented to study the structural effect of the heterostructures demonstrating two different tetragonal-type structures. Scanning electron microscopy showed two different morphologies and thicknesses. Magnetic hysteresis loops were obtained from both samples obtaining a major magnetization on the sample where the FE layer was on top and switching spectroscopy piezoresponse force microscopy (SSPFM) showed an improved on the ferroelectric properties for the sample where the FM layer was on top, attributing the enhancing of the multiferroic properties to the buffer layer used on top of the substrate.

Ferroelectric control of magnetic skyrmions in multiferroic heterostructures

Magnetic skyrmions with a topological particle nature are considered as potential information carriers for future spintronics memory and logic devices. The stabilization of magnetic skyrmions at zero magnetic field in nanostructured components is a prerequisite for incorporating them into advanced nonvolatile memory devices. Here, using a real-space phase field model based on Ginzburg-Landau theory, we demonstrate that ferroelectric polarization can stabilize magnetic skyrmions at zero magnetic field in multiferroic heterostructures composed of MnSi, ${\mathrm{PbTiO}}_{3}$, and ${\mathrm{SrTiO}}_{3}$. The stabilization of magnetic skyrmions in multiferroic heterostructures essentially depends on the directions of spontaneous polarizations and the size ratios of different constituents, in which polarization-induced inhomogeneous strain plays an important role. By applying an electric field, the polarization switching takes place in the ferroelectric constituent and the polarization-induced strain changes in the multiferroic heterostructures, resulting in a transition from skyrmion to helical phase in the ferromagnetic constituent. In addition, the skyrmion and helical phases can coexist in the absence of an external field and be switched reversibly by a local magnetic field with a small magnitude. Stabilization and control of magnetic skyrmions by spontaneous polarization at zero magnetic field may create additional opportunities for nonvolatile skyrmion memory devices.

Interface-Induced Magnetization and Exchange Bias in LSMO/BFO Multiferroic Heterostructures

Interface-induced magnetization in BiFeO3 (BFO) at the La0.67Sr0.33MnO3 (LSMO)/BFO interface is an important factor in determining the magnetoelectric coupling, exchange bias, and other phenomena i...

Anisotropic magnetoelectric functionality of ferromagnetic shape memory alloy heterostructures for MEMS magnetic sensors

In this report, the ferromagnetic shape memory alloy (Ni50Mn35In15) and Pb0.96La0.04Zr0.52Ti0.48O3 based bilayer (Ni-Mn-In/PLZT (260 nm/300 nm)), trilayer (Ni-Mn-In/PLZT/Ni-Mn-In (130 nm/300 nm/130 nm)) and four-layer (Ni-Mn-In/PLZT/Ni-Mn-In/PLZT (130 nm/150 nm/130 nm/150 nm)) multiferroic heterostructures with equal thickness ratios have been fabricated over Si substrates via DC/RF magnetron sputtering technique. The in-plane and out-of-plane magnetic hysteresis curves show the anisotropic nature of the ferromagnetic layer. The anisotropic magnetoelectric coupling characteristics have been investigated for all the fabricated devices by recording the induced magnetoelectric coupling voltages under both parallel and perpendicular applied magnetic fields to the plane of the device. Compared to all fabricated heterostructures, the trilayer structure exhibits the highest magnetoelectric coupling coefficients with 1.56 V/(cm Oe)-1 and 2.01 V/(cm Oe)-1 in longitudinal and transverse configurations, respectively. This anisotropic nature of magnetoelectric coupling can be used to measure the applied magnetic field direction. The rarely reported anisotropic AC magnetic field sensing parameters of the fabricated devices like Pearson’s r, sensitivity, and inaccuracy have been calculated. The trilayer device exhibits excellent AC magnetic field sensing parameters with inaccuracy, sensitivity, and linearity of 1.856% full-scale output (FSO), 0.63 mV cm−1 and 0.9993 in longitudinal configuration while 1.755% FSO, 1.18 mV Oe−1, and 0.9993 in the transverse configuration, respectively. Such nanoscale ferromagnetic shape memory alloys based symmetric multilayered multiferroic heterostructures pave the way for the design and development of futuristic MEMS magnetoelectric magnetic field sensor to detect the magnetic field and its spatial orientation.

Voltage control of millimeter-wave ferromagnetic resonance in multiferroic heterostructures thin films

Multiferroic heterostructures thin films, SrBi2Ta2O9/BaFe12O19 (SBT/BaM), were grown on Pt/TiO2/SiO2/Si substrates by using magnetron sputtering and Sol-gel ways. X-ray diffractometer (XRD) analysis showed that only SBT and BaM phases appeared in the multiferroic heterostructures. Magnetic hysteresis loops revealed that the saturated magnetization was 3.7 kG and the M-H characteristics of SBT/BaM were not influenced by the presence of the SBT layer. Ferromagnetic resonance (FMR) measurement showed the lowest FMR linewidth of 205 Oe at 50 GHz. Additionally, when direct-current electric field was applied to SBT layer, as a result of which mechanical deformation of the ferromagnetic layer occurred that leads to a frequency shift in ferromagnetic resonance and the magnetoelectric coupling effect (α) is 1.8 MHz*cm/kV. Our findings indicate that these SBT/BaM thin films have a significant potential for the usage in millimeter wave tunable devices.

Electric-field-driven Deterministic and Robust 120° Magnetic Rotation in a Concave Triangular Nanomagnet

Deterministic magnetic switching driven by electric fields rather than power-dissipating currents at the nanoscale is fundamentally challenging yet promising for applications to energy-efficient and high-density spintronic devices. Here, we demonstrate an electric-field-controlled deterministic, robust and repeatable $120{}^{o}$ rotation of ``Y''-like magnetic state in a patterned nanoscale multiferroic heterostructure consisting of a concave triangular nanomagnet deposited on $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}$-${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_{3}$ film. Using phase-field simulations, we find that the rotation of ``Y''-like magnetic state is controlled by a co-action of strain-mediated electric-field-induced uniaxial magnetoelastic anisotropy, magnetic in-plane shape anisotropy of the concave-triangle-shaped nanomagnet, and an interfacial exchange-bias field from a juxtaposed antiferromagnetic layer. It is also shown that deterministic magnetic state switching can be accomplished by a pulsed strain, the duration of which can span from ten nanoseconds or longer down to a few nanoseconds, providing great design flexibility. We also discuss the dynamics of electric-field-driven switching of ``Y''-like magnetic state as well as the influence of side length, thickness, and shape variation (i.e., concave radius) of the nanomagnet on the critical strain for the switching. These results offer a technologically viable route to designing nanomagnet-based nonvolatile spin memories with high density and low power.

Enhancement of charge-mediated magnetoelectric coupling in Fe3O4/SrTiO3/Ba0.6Sr0.4TiO3 heterostructure

The in-plane magnetic hysteresis loops of Fe3O4/SrTiO3(STO) and Fe3O4/STO/Ba0.6Sr0.4TiO3(BSTO) heterostructures have been investigated at 200 K under various electric fields. The bottom BSTO layer of the STO/BSTO bilayer is used to improve the dielectric properties of the top STO layer. The polarization of the STO/BSTO bilayer is ∼78% larger than that of the STO layer at room temperature due to the improvement of surface topography and the contribution of electrostatic interlayer coupling. A significant enlargement (∼70%) in the magnetoelectric response of Fe3O4/STO/BSTO heterostructure has been achieved at 200 K and 300 kV cm−1 after introducing the BSTO layer, since the STO/BSTO bilayer with larger dielectric constant supplies more polarization charges at its interface to the Fe3O4 layer than the STO layer. It indicates that the dielectric bilayer improves the polarization and thus benefits the magnetoelectric coupling in the multiferroic heterostructure.

Spin Logical and Memory Device Based on the Nonvolatile Ferroelectric Control of the Perpendicular Magnetic Anisotropy in PbZr0.2Ti0.8O3/Co/Pt Heterostructure

AbstractIn the field of memory and spin‐logical devices, multiferroics have the potentials of low‐energy informational operation. A novel memory and logic device in a PbZr0.2Ti0.8O3/Co/Pt (PZT/Co/Pt) multiferroic heterostructure with perpendicular magnetic anisotropy (PMA) is proposed. The PMA of PZT/Co/Pt structure can be modulated via the PZT/Co interface by switching the polarization field in the PZT layer. Moreover, the anomalous Hall voltage (AHV) under downward polarization is about 63% higher than that under upward polarization at 50 K without magnetic field. Interestingly, this AHV modulation is reversible, fast, and nonvolatile. Furthermore, the multiferroic random access memory and logic device operations are demonstrated based on the ferroelectric‐modulated AHV, which can lower the operating current density. This nonvolatile manipulation via ferroelectric polarizations will offer a new pathway to improve spintronic and spin‐logical applications.

Voltage-controlled three-state magnetic memory based on anisotropic magnetoresistance in a multiferroic heterostructure

An energy efficient technique has shown to produce a three-state magnetic memory cell in a [011]-poled Ni80Co20/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) heterostructure. Via the magnetic field deposition, a 45° magnetic easy axis of the NiCo film was induced in the film plane. By using a strong converse magnetoelectric coupling between the NiCo film and the PMN-PT, the magnetic moments of NiCo can be modulated to [001] and [1-10] directions of PMN-PT by selecting an appropriate electric field (E-field). Consequently, large, medium, and small anisotropic magnetoresistance (AMR) values are obtained by fixing a measuring current along the [001] direction. The required E-field significantly reduces due to the initial direction of NiCo along the 45° direction. The tunability of the AMR ratio is as large as ∼87%. These results indicate that an energy efficient approach to generate magnetic storage by using only a small E-field rather than a magnetic field with a high energy consumption was realized. This work shows great potential for the development of ultra-low power and high-density magnetoresistive memory 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.