Magnetoelectric Memory Research Articles

Deterministic Switching of Perpendicular Magnetic Anisotropy by Voltage Control of Spin Reorientation Transition in (Co/Pt)3/Pb(Mg1/3Nb2/3)O3-PbTiO3 Multiferroic Heterostructures.

One of the central challenges in realizing multiferroics-based magnetoelectric memories is to switch perpendicular magnetic anisotropy (PMA) with a control voltage. In this study, we demonstrate electrical flipping of magnetization between the out-of-plane and the in-plane directions in (Co/Pt)3/(011) Pb(Mg1/3Nb2/3)O3-PbTiO3 multiferroic heterostructures through a voltage-controllable spin reorientation transition (SRT). The SRT onset temperature can be dramatically suppressed at least 200 K by applying an electric field, accompanied by a giant electric-field-induced effective magnetic anisotropy field (ΔHeff) up to 1100 Oe at 100 K. In comparison with conventional strain-mediated magnetoelastic coupling that provides a ΔHeff of only 110 Oe, that enormous effective field is mainly related to the interface effect of electric field modification of spin-orbit coupling from Co/Pt interfacial hybridization via strain. Moreover, electric field control of SRT is also achieved at room temperature, resulting in a ΔHeff of nearly 550 Oe. In addition, ferroelastically nonvolatile switching of PMA has been demonstrated in this system. E-field control of PMA and SRT in multiferroic heterostructures not only provides a platform to study strain effect and interfacial effect on magnetic anisotropy of the ultrathin ferromagnetic films but also enables the realization of power efficient PMA magnetoelectric and spintronic devices.

Multiferroic Dislocations in Ferroelectric PbTiO3.

Ultrathin multiferroics with coupled ferroelectric and ferromagnetic order parameters hold promise for novel technological paradigms, such as extremely thin magnetoelectric memories. However, these ferroic orders and their functions inevitably disappear below a fundamental size limit of several nanometers. Herein, we propose a novel design strategy for nanoscale multiferroics smaller than the critical size limit by engineering the dislocations in nonmagnetic ferroelectrics, even though these lattice defects are generally believed to be detrimental. First-principles calculations demonstrate that Ti-rich PbTiO3 dislocations exhibit magnetism due to the local nonstoichiometry intrinsic to the core structures. Highly localized spin moments in conjunction with the host ferroelectricity enable these dislocations to function as atomic-scale multiferroic channels with a pronounced magnetoelectric effect that are associated with the antiferromagnetic-ferromagnetic-nonmagnetic phase transitions in response to polarization switching. The present results thus suggest a new field of dislocation (or defect) engineering for the fabrication of ultrathin magnetoelectric multiferroics and ultrahigh density electronic devices.

Magnetoelectric properties of microwave sintered BiFeO3 and Bi0.90La0.10Fe0.95Mn0.05O3 nanoceramics

Multiferroic materials have attracted the attention of scientific community all over the world due to smart functionality and attractive applications such as in magnetoelectric memory and magnetoelectric sensors. In this paper, we report on the ferroelectric, magnetic and magnetoelectric properties of microwave sintered BiFeO3 (BFO) and Bi0.90La0.10Fe0.95Mn0.05O3 (BLFMO) ceramics. The ferromagnetic and paramagnetic components have been deconvoluted using standard magnetization equation. The ferromagnetic saturation and paramagnetic susceptibility for BFO are found to be 0.046 emu/g and 1.17 × 10−5 emu/g·Oe respectively. The magnetization in BLFMO increases enormously as compared to BFO due to suppression of cycloid spin. The magnetoelectric voltage coefficient for BFO is 3.37 mV/cm·Oe, while for BLFMO it is 10.11 mV/cm·Oe. As compared to BFO, BLFMO exhibits improved ferroelectric, magnetic and magnetoelectric properties.

Perspectives of voltage control for magnetic exchange bias in multiferroic heterostructures

Exchange bias, as an internal magnetic bias induced by a ferromagnetic–antiferromagnetic exchange coupling, is extremely important in many magnetic applications such as memories, sensors and other devices. Voltage control of exchange bias in multiferroics provides an energy-efficient way to achieve a rapidly 180° deterministic switching of magnetization, which has been considered as a key challenge in realizing next generation of fast, compact and ultra-low power magnetoelectric memories and sensors. Additionally, exchange bias can enhance dynamic magnetoelectric coupling strength in an external-field-free manner. In this paper, we provide a perspective on voltage control of exchange bias in different multiferroic heterostructures. Brief mechanization and related experiments are discussed as well as future trend and challenges that can be overcome by electrically tuning of exchange bias in state-of-the-art magnetoelectric devices.

A multilevel nonvolatile magnetoelectric memory

The coexistence and coupling between magnetization and electric polarization in multiferroic materials provide extra degrees of freedom for creating next-generation memory devices. A variety of concepts of multiferroic or magnetoelectric memories have been proposed and explored in the past decade. Here we propose a new principle to realize a multilevel nonvolatile memory based on the multiple states of the magnetoelectric coefficient (α) of multiferroics. Because the states of α depends on the relative orientation between magnetization and polarization, one can reach different levels of α by controlling the ratio of up and down ferroelectric domains with external electric fields. Our experiments in a device made of the PMN-PT/Terfenol-D multiferroic heterostructure confirm that the states of α can be well controlled between positive and negative by applying selective electric fields. Consequently, two-level, four-level, and eight-level nonvolatile memory devices are demonstrated at room temperature. This kind of multilevel magnetoelectric memory retains all the advantages of ferroelectric random access memory but overcomes the drawback of destructive reading of polarization. In contrast, the reading of α is nondestructive and highly efficient in a parallel way, with an independent reading coil shared by all the memory cells.

A novel perovskite oxide chemically designed to show multiferroic phase boundary with room-temperature magnetoelectricity.

There is a growing activity in the search of novel single-phase multiferroics that could finally provide distinctive magnetoelectric responses at room temperature, for they would enable a range of potentially disruptive technologies, making use of the ability of controlling polarization with a magnetic field or magnetism with an electric one (for example, voltage-tunable spintronic devices, uncooled magnetic sensors and the long-searched magnetoelectric memory). A very promising novel material concept could be to make use of phase-change phenomena at structural instabilities of a multiferroic state. Indeed, large phase-change magnetoelectric response has been anticipated by a first-principles investigation of the perovskite BiFeO3–BiCoO3 solid solution, specifically at its morphotropic phase boundary between multiferroic polymorphs of rhombohedral and tetragonal symmetries. Here, we report a novel perovskite oxide that belongs to the BiFeO3–BiMnO3–PbTiO3 ternary system, chemically designed to present such multiferroic phase boundary with enhanced ferroelectricity and canted ferromagnetism, which shows distinctive room-temperature magnetoelectric responses.

Magnetoelectric memory effect in the Y-type hexaferrite BaSrZnMgFe12O22**Project supported by the National Natural Science Foundation of China (Grant Nos. 11534015 and 51371193).

We report on the magnetic and magnetoelectric properties of the Y-type hexaferrite BaSrZnMgFe12O22, which undergoes transitions from a collinear ferrimagnetic phase to a proper screw phase at 310 K and to a longitudinal conical phase at 45 K. Magnetic and electric measurements revealed that the magnetic structure with spiral spin order can be modified by applying a magnetic field, resulting in magnetically controllable electric polarization.It was observed that BaSrZnMgFe12O22 exhibits an anomalous magnetoelectric memory effect: the ferroelectric state can be partially recovered from the paraelectric phase with collinear spin structure by reducing magnetic field at 20 K. We ascribe this memory effect to the pinning of multiferroic domain walls, where spin chirality and structure are preserved even in the nonpolar collinear spin state.

ChemInform Abstract: Low Energy Consumption Spintronics Using Multiferroic Heterostructures

We review the recent progress in the field of multiferroic magnetoelectric heterostructures. The lack of single phase multiferroic candidates exhibiting simultaneously strong and coupled magnetic and ferroelectric orders led to an increased effort into the development of artificial multiferroic heterostructures in which these orders are combined by assembling different materials. The magnetoelectric coupling emerging from the created interface between the ferroelectric and ferromagnetic layers can result in electrically tunable magnetic transition temperature, magnetic anisotropy or magnetization reversal. The full potential of low energy consumption magnetic based devices for spintronics lies in our understanding of the magnetoelectric coupling at the scale of the ferroic domains. Although the thin film synthesis progresses resulted into the complete control of ferroic domain ordering using epitaxial strain, the local observation of magnetoelectric coupling remains challenging. The ability to imprint ferroelectric domains into ferromagnets and to manipulate those solely using electric fields suggests new technological advances for spintronics such as magnetoelectric memories or memristors.

Magnetoelectric domain wall dynamics and its implications for magnetoelectric memory

Domain wall dynamics in a magnetoelectric antiferromagnet is analyzed, and its implications for magnetoelectric memory applications are discussed. Cr2O3 is used in the estimates of the materials parameters. It is found that the domain wall mobility has a maximum as a function of the electric field due to the gyrotropic coupling induced by it. In Cr2O3, the maximal mobility of 0.1 m/(s Oe) is reached at E≈0.06 V/nm. Fields of this order may be too weak to overcome the intrinsic depinning field, which is estimated for B-doped Cr2O3. These major drawbacks for device implementation can be overcome by applying a small in-plane shear strain, which blocks the domain wall precession. Domain wall mobility of about 0.7 m/(s Oe) can then be achieved at E = 0.2 V/nm. A split-gate scheme is proposed for the domain-wall controlled bit element; its extension to multiple-gate linear arrays can offer advantages in memory density, programmability, and logic functionality.

A novel magnetoelectric memory cell based on bilayer ferroelectric films of (1 − x)[Ba(Zr0.2Ti0.8)O3]–x(Ba0.7Ca0.3TiO3)

We demonstrate that a thin film magnetoelectric memory cell, comprised of Fe70Ga30 sputtered on top of bilayer ferroelectric films which consists of a tetragonal (T) 0.7Ba(Zr0.2Ti0.8)O3–0.3(Ba0.7Ca0.3TiO3) (70BZT–30BCT) film deposited on a rhombohedral (R) 0.3Ba(Zr0.2Ti0.8)O3–0.7(Ba0.7Ca0.3TiO3) (30BZT–70BCT) film. The insertion of rhombohedral layer improves the quality of the films and enhances the piezoelectric properties, which could be attributed to that the ferroelectric domains are tethered only by a soft R under layer, and not by the hard substrate. The working mechanism is that the 70BZT–30BCT layer acquires different strain states when an electric field is applied to it. Mechanical transduction couples this strain to the mechanically coupled Fe70Ga30 layer, which then changes its magnetic anisotropy and thus the magnetoresistance. The high (low) resistance states were realized when different voltages were applied due to the anisotropy magnetoresistance of Fe70Ga30 films. The demonstrated magnetoelectric memory device using resistance as the media and electric field as the writing field shows great promises towards exploring magnetoelectric devices for low-power and high density magnetic data storage applications.

Interface Magnetoelectric Coupling in Co/Pb(Zr,Ti)O3.

Magnetoelectric coupling at multiferroic interfaces is a promising route toward the nonvolatile electric-field control of magnetization. Here, we use optical measurements to study the static and dynamic variations of the interface magnetization induced by an electric field in Co/PbZr0.2Ti0.8O3 (Co/PZT) bilayers at room temperature. The measurements allow us to identify different coupling mechanisms. We further investigate the local electronic and magnetic structure of the interface by means of transmission electron microscopy, soft X-ray magnetic circular dichroism, and density functional theory to corroborate the coupling mechanism. The measurements demonstrate a mixed linear and quadratic optical response to the electric field, which results from a magneto-electro-optical effect. We propose a decomposition method of the optical signal to discriminate between different components involved in the electric field-induced polarization rotation of the reflected light. This allows us to extract a signal that we can ascribe to interface magnetoelectric coupling. The associated surface magnetization exhibits a clear hysteretic variation of odd symmetry with respect to the electric field and nonzero remanence. The interface coupling is remarkably stable over a wide frequency range (1-50 kHz), and the application of a bias magnetic field is not necessary for the coupling to occur. These results show the potential of exploiting interface coupling with the prospect of optimizing the performance of magnetoelectric memory devices in terms of stability, as well as fast and dissipationless operation.

Electric-Field-Control of Non-Volatile Magnetization Switching in Multiferroic CoFeB/(011)-PMN-PT Heterostructures

The giant converse magnetoelectric coupling (GME) was observed in the multiferroic Co40Fe40B20/(011)-0.7Pb (Mg1/3Nb2/3)O3-0.3PbTiO3 heterostructures at room temperature in this investigation. A tunability of magnetization by electric field along the [100] direction was up to-66.7% at-10 Oe bias magnetic fields. Moreover, the non-volatile magnetization switching was found after removal of bipolar electric field. The corresponding remanent magnetic states even without the assistance of bias magnetic fields were stable and could be modulated synchronously by a sequence of pulse electric fields. The 90o rotation of easy axis and non-symmetrical ferroelastic domain switching contributed to the above results. This work is of great significance in designing ultra-low power and non-volatile magnetoelectric memories and other spintronic devices at room temperature.

Electric field mediated non-volatile tuning magnetism in CoPt/PMN-PT heterostructure for magnetoelectric memory devices

We report a power efficient non-volatile magnetoelectric memory in the CoPt/(011)PMN-PT heterostructure. Two reversible and stable electric field induced coercivity states (i.e., high-HC or low-HC) are obtained due to the strain mediated converse magnetoelectric effect. The reading process of the different coercive field information written by electric fields is demonstrated by using a magnetoresistance read head. This result shows good prospects in the application of novel multiferroic devices.

Fabrication and characterization of a magnetoelectric memory cell of 50Ba(Zr0.2Ti0.8)O3–50Ba0.7Ca0.3TiO3/Fe70Ga30

We present a simple all-thin-film magnetoelectric memory cell with ferromagnetic film of Fe70Ga30 grown on ferroelectric film of 50Ba(Zr0.2Ti0.8)O3–50Ba0.7Ca0.3TiO3 (50BZT-50BCT) based on converse magnetoelectric effect. The heterostructure exhibits well-defined magnetic and piezoelectric behaviors. The 50BZT-50BCT layer acquires different strain states when an electric field is applied to the cell. Mechanical transduction couples this strain to the mechanically coupled Fe70Ga30 layer, which then changes its magnetic anisotropy and thus the magnetoresistance. Thus, a magnetoelectric memory cell with resistance as the media and electric field as the writing field was realized. The demonstrated electric field control of resistance states is a significant step towards exploring magnetoelectrically controlled spintronic devices for low-power and high density magnetic data storage applications.

A non-volatile four-state magnetic memory in a Co/(011)Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructure

A non-volatile four-state magnetic memory is achieved in a Co/(011)Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructure. The in-plane magnetization of ferromagnetic Co film in the heterostructure can be controlled both electrically and magnetically. Electric field mediated magnetism is caused by piezostrain effect, which displays a stable non-volatile remnant magnetization vs electric field looplike behavior. In-plane strain-electric field (S-E) behavior under different temperatures reveals a non-volatile strain switching effect, which is responsible for the non-volatile remnant magnetization switching through piezostrain mediated magnetoelectric effect. Further investigations on temperature dependence of S-E behavior suggest that the absent of the second non-180° domain switching may be responsible for the asymmetry in strain curves that causes the non-volatile strain switching, and therefore causes the non-volatile remanent magnetization switching, which is crucial for the four-state magnetoelectric memory.

Large Converse Magnetoelectric Properties Without Bias in Composite of Rosen-Type Piezoelectric Transformer and Magnetization-Graded Ferromagnetic Material

We designed a magnetoelectric (ME) composite by bonding the graded-magnetostrictive layers of FeCuNbSiB/FeNi-ferromagnetic constant elasticity (FACE) on the output section of the Rosen-type transformer, in which a much higher self-biased converse magnetoelectric (CME) coefficient is obtained. Due to the different magnetic characteristics of nanocrystalline foil FeCuNbSiB and FeNi-FACE (such as magnetic permeability and coercivity), the FeCuNbSiB/FeNi-FACE layer exhibits an internal magnetic bias field. When the ac voltage is applied on the input section of the transformer, a large strain is mechanically transferred to the graded-magnetostrictive layers due to the stress concentration effect at full-wavelength resonance frequency. Therefore, the large strain in the output section of the Rosen-type transformer associated with the internal magnetic bias field in FeCuNbSiB/FeNi-FACE leads to a large self-biased CME coefficient. The experimental results demonstrate that: 1) a large remnant CME coefficient of 0.02058 mG/V is achieved, which is $\sim 17$ times higher than the previous results; 2) two stable magnetic flux states are generated by switching a smaller electric field of 10 V/cm ON/OFF alternately without magnetic bias field, which can be used as miniature electric-field-written high-density ME memory devices with lower energy consumption; and 3) the composite shows an approximately linear relationship between applied ac voltage $V_{\rm in}$ and induced magnetic induction B .

Multiferroic Vacancies at FerroelectricPbTiO3Surfaces

Multiferroics in nanoscale dimensions are promising for novel functional device paradigms, such as magnetoelectric memories, due to an intriguing cross-coupling between coexisting ferroelectric and (anti)ferromagnetic order parameters. However, the ferroic order is inevitably destroyed below the critical dimension of several nanometers. Here, we demonstrate a new path towards atomic-size multiferroics while resolving the controversial origin of dilute ferromagnetism that unexpectedly emerges in nanoparticles of nonmagnetic ferroelectric PbTiO(3). Systematic exploration using predictive quantum-mechanical calculations demonstrates that oxygen vacancies formed at surfaces induce ferromagnetism due to local nonstoichiometry and orbital symmetry breaking. The localized character of the emerged magnetization allows an individual oxygen vacancy to act as an atomic-scale multiferroic element with a nonlinear magnetoelectric effect that involves rich ferromagnetic-antiferromagnetic-nonmagnetic phase transitions in response to switching of the spontaneous polarization.

All-Electric Access to the Magnetic-Field-Invariant Magnetization of Antiferromagnets.

The rich physics of thin film antiferromagnets can be harnessed for prospective spintronic devices given that all-electric assessment of the tiny uncompensated magnetic moment is achieved. On the example of magnetoelectric antiferromagnetic Cr2O3, we prove that spinning-current anomalous Hall magnetometry serves as an all-electric method to probe the field-invariant uncompensated magnetization of antiferromagnets. We obtain direct access to the surface magnetization of magnetoelectric antiferromagnets providing a read-out method for ferromagnet-free magnetoelectric memory. Owing to the great sensitivity, the technique bears a strong potential to address the physics of antiferromagnets. Exemplarily, we apply the method to access the criticality of the magnetic transition for an antiferromagnetic thin film. We reveal the presence of field-invariant uncompensated magnetization even in 6-nm-thin IrMn films and clearly distinguish two contributions, of which only the minor one is involved in interfacial magnetic coupling. This approach is likely to advance the fundamental understanding of the anomalous Hall and magnetic proximity effects.

Electric field-controlled magnetization in bilayered magnetic films for magnetoelectric memory

Bilayered magnetic films (Co50Fe50 (CoFe)/Metglas) were RF sputtered on both (001)-oriented and (011)-oriented PMN-PT (lead magnesium niobate-lead titanate) substrates. Electric field-controlled magnetization changes were observed in all these samples: 65 nm CoFe/24 nm Metglas/(001) PMN-PT, 65 nm CoFe/24 nm Metglas/(011) PMN-PT, and 30 nm CoFe/12 nm Metglas/(011) PMN-PT. The maximum magnetic remanence ratio change (ΔMr/Ms) was 46% for CoFe/Metglas/(001) PMN-PT. In this heterostructure, the electric-field created two new non-volatile switchable remanence states and the as-grown remanence state was altered permanently. High-resolution transmission electron microscopy images show a sharp and smooth interface between Metglas and substrate and conversely a rougher interface was observed between Metglas and CoFe films. In the 30 nm CoFe/12 nm Metglas/(011) PMN-PT sample, a large ΔMr/Ms of 80% along the [100] direction was measured, while the ΔMr/Ms along the [01-1] direction was 60% at the applied electric field of 5 kV/cm, corresponding to a giant magnetoelectric coupling constant α = μoΔMr/E = 2.9 × 10−6 s/m.

Electric field tuning of non-volatile three-state magnetoelectric memory in FeCo-NiFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructures

We demonstrate electric field impulse-induced reversible tristable magnetization switching in FeCo-NiFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) heterostructures at room temperature. The magnetic properties of the FeCo-NiFe2O4 film can be changed reversibly by the strain-mediated magnetoelectric coupling effect. Three piezostrain-mediated reversible and stable electric resistance states were obtained in the FeCo-NiFe2O4 film when different electric field impulses were applied, including large positive and negative fields and an impulse that was smaller than the electric coercive field. Consequently, reversible electric field impulse tuning of the tristable resistance state, which is related to the different magnetization switching properties of the materials, was realized. These results provide a promising approach for low loss multistate magnetoelectric memory devices for information storage applications.