Voltage-controlled Magnetism Research Articles

Non-volatile voltage-controlled magnetization in single-phase multiferroic ceramics at room temperature

Single-phase multiferroics (MFs) exhibiting ferroelectricity and ferromagnetism and the strong magnetoelectric (ME) coupling effect at room temperature are seen as key to the development of the next-generation of spintronic devices, multi-state memories, logic devices and sensors. Herein, the single-tetragonal phase (1–x) (Sr0·3Bi0·35Na0·329Li0.021)TiO3-xBiFeO3 (x = 0.2 or 0.4) system was designed to study the intrinsic ME coupling effect at room temperature and high frequencies. The polarization arises from the cooperative displacement of both Fe3+ and Ti4+ relative to the oxygen sublattice in the tetragonally distorted perovskite structure, and the magnetization stems from indirect exchange magnetic interaction between adjacent iron ions. A switchable voltage-controlled magnetization was confirmed by a change of the coercive magnetic field, Hc, and remnant magnetization, Mr, in the x = 0.4 component subjected to an external electric field at room temperature and was possibly attributed to a strain-mediated ME coupling effect. In addition, resonance behaviours of the complex magnetic permeability and complex dielectric permittivity in the GHz band indicate that this ME effect is intrinsic in nature and could broaden the applications of multiferroics to devices operating at microwave frequencies.

Fully micromagnetic analysis of voltage-controlled magnetization switching in magnetic-topological-insulator-based devices

We numerically investigate magnetization switching behavior in voltage-controlled magnetic-topological-insulator-based (VC-MTI) devices by means of the fully micromagnetic simulation. First, the influence of domain in VC-MTI devices was investigated. When the device size is larger than 1 µm, multidomain structure might appear. However, these domains disappear when the gate voltage and source-drain electric field are applied, which is the refresh operation of the actual VC-MTI device. The switching behaviors of a 100-nm-size VC-MTI device in the fully micromagnetic simulation are in agreement with those of the macrospin model although the gate pulse width is slightly different from that of the macrospin model. When the device is less than 1 µm, the macrospin model is adequate for the investigation of switching behavior in VC-MTI devices and the magnetization switching occurs in rotation mode. Therefore, for the VC-MTI device with less-than-100 nm size, the macrospin model is a good approach for the analysis of device operation and write-error rate.

Numerical analysis of voltage-controlled magnetization switching operation in magnetic-topological-insulator-based devices

We theoretically investigate influences of electronic circuit delay, noise, and temperature on write-error-rate (WER) in voltage-controlled magnetization switching operation of a magnetic-topological-insulator-based device by means of the micromagnetic simulation. This device realizes magnetization switching via spin–orbit torque (SOT) and voltage-controlled magnetic anisotropy (VCMA), which originate from the 2D-Dirac electronic structure. We reveal that the device operation is extremely robust against circuit delay and signal-to-noise ratio. We demonstrate that the WER on the order of ∼10−4 or below is achieved around room temperature due to steep change in VCMA. Also, we show that the larger SOT improves thermal stability factor. This study provides a next perspective for developing voltage-driven spintronic devices with ultra-low power consumption.

In-Situ Measurement of Magnetoelectric Coupling and Strain Transfer in Multiferroic Nanocomposites of CoFe2O4 and Hf0.5Zr0.5O2 with Residual Porosity

With increasing applications for voltage-controlled magnetism, the need to more fully understand magnetoelectric coupling and strain transfer in nanostructured multiferroic composites has also increased. Here, multiferroic nanocomposites were synthesized using block copolymer templating to create mesoporous cobalt ferrite (CFO), followed by partly filling the pores with ferroelectric zirconium-substituted hafnia (HZO) using atomic layer deposition (ALD) to produce a porous multiferroic composite with enhanced mechanical flexibility. Upon electrical poling of the nanocomposite, we observed large changes in the magnetization. These changes partly relaxed upon removing the electric field, suggesting a strain-mediated mechanism. Both the anisotropic strain transfer from HZO to CFO and the strain relaxation after the field was removed were confirmed using high-resolution X-ray diffraction measurements collected during in-situ poling. The in-situ observation of both anisotropic strain transfer and large magnetization changes allows us to directly characterize the strong multiferroic coupling that can occur in flexible, nanostructured composites.

Voltage-controlled magnetic anisotropy enabled by resistive switching

The discovery of new mechanisms of controlling magnetic properties by electric fields or currents furthers the fundamental understanding of magnetism and has important implications for practical use. Here, we present an approach of utilizing resistive switching to control magnetic anisotropy. We study a ferromagnetic oxide that exhibits an electrically triggered metal-to-insulator phase transition producing a volatile resistive switching. The switching occurs in a characteristic spatial pattern, the formation of an insulating barrier perpendicular to the current flow, which results in an unusual ferromagnetic/paramagnetic/ferromagnetic configuration. The formation of this voltage-driven paramagnetic insulating barrier is accompanied by the emergence of a strong uniaxial magnetic anisotropy that overpowers the intrinsic material anisotropy. Our results demonstrate that resistive switching is an effective tool for manipulating magnetic properties. Because resistive switching can be induced in a broad range of materials, our findings could enable a new class of voltage-controlled magnetism systems.

Charge-mediated voltage modulation of magnetism in Hf0.5Zr0.5O2/Co multiferroic heterojunction

We construct the Hall-bar device with the size of several hundred nanometers based on the HZO/Co multiferroic heterojunction. A remarkable voltage-controlled magnetism is observed in the device that possesses both ferroelectric property and perpendicular magnetic anisotropy (PMA). The nucleation field and coercivity can be modulated by voltage pulse while saturation field keeps stable. The non-volatile and reversible voltage-controlled magnetism is ascribable to interfacial charges caused by ferroelectric polarization. Meanwhile, the effective anisotropy energy density (K u) can also be controlled by voltage pulse, a decrease of 83% and increase of 28% in K u are realized under –3-V and 3-V pulses, respectively. Because the energy barrier is directly proportional to K u under a given volume, a decreased or enhanced energy barrier can be controlled by voltage pulse. Thus, it is an effective method to realize low-power and high-stability magneto-resistive random-access memory (MRAM).

Highly efficient voltage-controlled magnetism in HfZrO/CoFeB hybrid film and Hall device

We investigate the voltage-controlled magnetism effect of HfZrO/CoFeB hybrid film and a Hall device with perpendicular magnetic anisotropy. The magnetization versus magnetic field experiments and anomalous Hall experiments before and after applying voltage are performed. The results exhibit that the coercive field of samples remain unchanged while the saturation magnetization shows a permanent increase (more than 60%), which is regardless of the direction of applied voltage. Different from conventional voltage-controlled magnetic anisotropy, in our work, only the saturation magnetization is enhanced by the applied voltage without trading off other magnetic parameters of CoFeB. Thus, such a finding proposes a more efficient voltage-controlled method to achieve a magnetic memory device with high thermal stability, high tunnel magnetoresistance and low switching current for magneto-resistive random-access memory under scaling beyond 2X nm.

Evaluating the Performances of the Ultralow Power Magnetoelectric Random Access Memory With a Physics-Based Compact Model of the Antiferromagnet/Ferromagnet Bilayer

Voltage-controlled spintronic devices are considered promising candidates for low-power applications due to the nonvolatility and the elimination of the Joule heating energy. Recently, experiments have demonstrated the voltage-controlled magnetization switching in the bismuth ferrite (BFO)/CoFe heterojunction that can achieve 180° switching at room temperature. In addition, it has been demonstrated that the ferroelectric coercive voltage of the La-doped BFO can be as low as 0.2 V by tuning the doping concentration of La and choosing a thinner BFO layer. To evaluate the potential performance of the magnetoelectric magnetic random access memory (ME-MRAM) using the BFO/CoFe heterojunction, we first build a physics-based compact model of the antiferromagnet/ferromagnet (AFM/FM) bilayer. The results from our circuit-compatible model match well with the micromagnetic simulations on the dynamics of the order parameters in both the AFM and FM layers. Next, we simulate the read and write performances of ME-MRAM using the compact model we have developed. Our results show that the write energy of the ME-MRAM can be as low as a few fJ per bit and the layout area is ~20 F², which is more energy and area efficient compared to SRAM and other spintronic memory candidates.

Electroferrofluids with nonequilibrium voltage-controlled magnetism, diffuse interfaces, and patterns.

It has been recognized that driving matter to nonequilibrium states can lead to emergent behaviors and functionalities. Here, we show that uniform colloidal dispersions can be driven into dissipative nonuniform states with emerging behaviors. We experimentally demonstrate this with electrically driven weakly charged superparamagnetic iron oxide nanoparticles in a nonpolar solvent. The driving leads to formation of nonequilibrium concentration gradients that further translate to nonequilibrium magnetism, including voltage-controlled magnetization and susceptibility. The concentration gradients also serve as diffuse interfaces that respond to external magnetic fields, leading to novel dissipative patterns. We identify the closest nondissipative analogs, discuss the differences, and highlight the ability to directly quantify the dissipation and link it to the pattern formation. Beyond voltage-controlled magnetism, we foresee that the concept can be generalized to other functional colloids to create, e.g., optical, electrical, catalytic, and mechanical responses that are not possible in thermodynamic equilibrium.

Nonvolatile voltage-controlled magnetization reversal in a spin-valve multiferroic heterostructure

Pure voltage-controlled magnetism, rather than a spin current or magnetic field, is the goal for next-generation ultralow power consumption spintronic devices. To advance toward this goal, we report a voltage-controlled nonvolatile 90° magnetization rotation and voltage-assisted 180° magnetization reversal in a spin-valve multiferroic heterostructure. Here, a spin valve with a synthetic antiferromagnetic structure was grown on a (110)-cut Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) substrate, in which only the magnetic moment of the free layer can be manipulated by an electric field (E-field) via the strain-mediated magnetoelectric coupling effect. As a result of selecting a specified PMN-PT substrate with defect dipoles, nonvolatile and stable magnetization switching was achieved by using voltage impulses. Accordingly, a giant, reversible and nonvolatile magnetoresistance modulation was achieved without the assistance of a magnetic field. In addition, by adopting a small voltage impulse, the critical magnetic field required for complete 180° magnetization reversal of the free layer can be tremendously reduced. A magnetoresistance ratio as large as that obtained by a magnetic field or spin current under normal conditions is achieved. These results indicate that E-field-assisted energy-efficient in-plane magnetization switching is a feasible strategy. This work is significant to the development of ultralow-power magnetoresistive memory and spintronic devices.

Transverse barrier formation by electrical triggering of a metal-to-insulator transition

Application of an electric stimulus to a material with a metal-insulator transition can trigger a large resistance change. Resistive switching from an insulating into a metallic phase, which typically occurs by the formation of a conducting filament parallel to the current flow, is a highly active research topic. Using the magneto-optical Kerr imaging, we found that the opposite type of resistive switching, from a metal into an insulator, occurs in a reciprocal characteristic spatial pattern: the formation of an insulating barrier perpendicular to the driving current. This barrier formation leads to an unusual N-type negative differential resistance in the current-voltage characteristics. We further demonstrate that electrically inducing a transverse barrier enables a unique approach to voltage-controlled magnetism. By triggering the metal-to-insulator resistive switching in a magnetic material, local on/off control of ferromagnetism is achieved using a global voltage bias applied to the whole device.

Electrically Enhanced Exchange Bias via Solid-State Magneto-ionics

Electrically induced ionic motion offers a new way to realize voltage-controlled magnetism, opening the door to a new generation of logic, sensor, and data storage technologies. Here, we demonstrate an effective approach to magneto-ionically and electrically tune the exchange bias in Gd/Ni1-xCoxO thin films (x = 0.50 and 0.67), where neither of the layers alone is ferromagnetic at room temperature. The Gd capping layer deposited onto antiferromagnetic Ni1-xCoxO initiates a solid-state redox reaction that reduces an interfacial region of the oxide to ferromagnetic NiCo. An exchange bias is established after field cooling (FC), which can be enhanced by up to 35% after a voltage conditioning and subsequently reset with a second FC. These effects are caused by the presence of an interfacial ferromagnetic NiCo layer, which further alloys with the Gd layer upon FC and voltage application, as confirmed by electron microscopy and polarized neutron reflectometry studies. These results highlight the viability of the solid-state magneto-ionic approach to achieve electric control of exchange bias, with potential for energy-efficient magneto-ionic devices.

Cross-Linking and Charging Molecular Magnetoelectronics.

Magnetoelectrics are witnessing an ever-growing success toward the voltage-controlled magnetism derived from inorganic materials. However, these inorganic materials have predominantly focused on the ferroelectromagnetism at solid-to-solid interfaces and suffered several drawbacks, including the interface-sensitive coupling mediators, high-power electric field, and limited chemical tunability. Here, we report a promising design strategy to shift the paradigm of next-generation molecular magnetoelectrics, which relies on the integration between molecular magnetism and electric conductivity though an in situ cross-linking strategy. Following this approach, we demonstrate a versatile and efficient synthesis of flexible molecular-based magnetoelectronics by cross-linking of magnetic coordination networks that incorporate conducting chain building blocks. The as-grown compounds feature an improved critical temperature up to 337 K and a room-temperature magnetism control of low-power electric field. It is envisaged that the cross-linking of molecular interfaces is a feasible method to couple and modulate magnetism and electron conducting systems.

First-principles calculations of steady-state voltage-controlled magnetism: Application to x-ray absorption spectroscopy experiments

Recent x-ray absorption experiments have demonstrated the possibility to accurately monitor the magnetism of metallic hetero-structures controlled via a time-independent perturbation caused for example by a static electric field. Using a first-principles, non-equilibrium Green function scheme, we show how the measured dichroic signal for the corresponding steady-state situation can be related to the underlying electronic structure and its response to the external stimulus. The suggested approach works from the infinitesimal limit of linear response to the regime of strong electric field effects, which is realized in present experimental high sensitivity investigations.

Voltage-induced ferromagnetism in a diamagnet.

Increasingly impressive demonstrations of voltage-controlled magnetism have been achieved recently, highlighting potential for low-power data processing and storage. Magnetoionic approaches appear particularly promising, electrolytes and ionic conductors being capable of on/off control of ferromagnetism and tuning of magnetic anisotropy. A clear limitation, however, is that these devices either electrically tune a known ferromagnet or electrically induce ferromagnetism from another magnetic state, e.g., antiferromagnetic. Here, we demonstrate that ferromagnetism can be voltage-induced even from a diamagnetic (zero-spin) state suggesting that useful magnetic phases could be electrically induced in "nonmagnetic" materials. We use ionic liquid-gated diamagnetic FeS2 as a model system, showing that as little as 1 V induces a reversible insulator-metal transition by electrostatic surface inversion. Anomalous Hall measurements then reveal electrically tunable surface ferromagnetism at up to 25 K. Density functional theory-based modeling explains this in terms of Stoner ferromagnetism induced via filling of a narrow e g band.

Simulations on the Effect of Magnetic Anisotropy on Switching of an Easy Cone Magnetized Free Layer

The easy cone state of magnetization that arises as a result of a competition between the second- and fourth-order perpendicular magnetic anisotropies has the advantage of a non-zero electric-field torque without the application of a bias magnetic field and is thus a potential candidate for purely voltage-driven magnetic storage devices. In this article, the onset of the easy cone state of magnetization is simulated in a ferromagnet film. Subsequently, the switching field and time for voltage-controlled magnetization switching process are studied as a function of the inclination angle of the easy cone state from the film normal in the range of 0°- 45°. The switching field is found to decrease with decreasing anisotropy. The switching time is found to become faster for higher inclination angle of the easy cone state due to an increase in its torque.

RF voltage-controlled magnetization switching in a nano-disk

Nanomagnetic oscillators are key components for radio-frequency (RF) signal generation in nanoscale devices. However, these oscillators are primarily electric current-based, which is energy inefficient at the nanoscale due to ohmic losses. In this study, we present an actuation mechanism for magnetization switching using a multiferroic structure that relies on an RF voltage input instead of electrical current. An AC voltage with a DC bias is applied to the piezoelectric substrate and the magnetic nanodisk with perpendicular magnetic anisotropy that is attached onto the substrate, which can achieve steady magnetic oscillation when the driven voltage is at ferromagnetic resonance (FMR) of the nanodisk. Changing the DC bias changes the magnetic anisotropy of the magnetoelastic nanodisk, hence changes the FMR and oscillation frequency. The frequency modulation is quantified using the Kittel equation. Parametric studies are conducted to investigate the influence of voltage amplitude, frequency, waveform, and the thickness of the magnetoelastic nanodisk. This multiferroic approach opens possibilities for designing energy efficient nanomagnetic oscillators that have both large amplitude and broad frequency range.

GdFe0.8Ni0.2O3: A Multiferroic Material for Low-Power Spintronic Devices with High Storage Capacity.

Multiferroic materials are strong candidates for reducing the energy consumption of voltage-controlled spintronic devices because of the coexistence of ferroelectric (FE) and magnetic orders in a single phase. In this article, we present a new multiferroic perovskite, GdNixFe1-xO3 (GFNO), produced via sputtering on a SrTiO3 substrate. The proposed GFNO is FE and canted antiferromagnetic (AFM) within a monoclinic framework at room temperature. The FE polarization of the GFNO is up to 37 μC/cm2. When capped with a Co layer, the resulting heterostructure exhibits voltage-controlled magnetism (VCM). The heterostructured device exhibits two distinct features. First, its VCM depends on the magnitude as well as the polarity of the applied bias, thereby doubling the number of available magnetic readout states under a fixed voltage. Furthermore, the magnetic order of the device can be controlled very effectively within ±1 V. These two characteristics satisfy the requirements for low-power and high-storage technology. Theoretical analysis and experimental results indicate the importance of Ni dopant in regulating the polarity-dependent multiferroicity of this gadolinium ferrite system.

Electric field modulated ferromagnetism in ZnO films deposited at room temperature

The ZnO film deposited at room temperature, which is composed of the amorphous-phase background plus a few nanograins or nanoclusters (about 1–2 nm), exhibits room temperature ferromagnetism (FM). Such FM is found to be connected with oxygen vacancies. For the Ta/ZnO/Pt device based on the medium layer ZnO deposited at room temperature, the saturation magnetization not only is modulated between high and low resistive states by electric voltage with DC loop electric current but also increases/decreases through adjusting the magnitudes of positive/negative DC sweeping voltage. Meanwhile, the voltage-controlled conductance quantization is observed in Ta/ZnO/Pt, accompanying the voltage-controlled magnetization. However, the saturation magnetization of the Ta/ZnO/Pt device becomes smaller under positive electric voltage and returns in some extent under negative electric voltage, when the DC loop electric current is not applied.

Voltage-controlled magnetization switching in MRAMs in conjunction with spin-transfer torque and applied magnetic field

Voltage-controlled magnetic anisotropy (VCMA) effect has attracted a significant amount of attention in recent years because of its low cell power consumption during the anisotropy modulation of a thin ferromagnetic film. However, the applied voltage or electric field alone is not enough to completely and reliably reverse the magnetization of the free layer of a magnetic random access memory (MRAM) cell from anti-parallel to parallel configuration or vice versa. An additional symmetry-breaking mechanism needs to be employed to ensure the deterministic writing process. Combinations of voltage-controlled magnetic anisotropy together with spin-transfer torque (STT) and with an applied magnetic field (Happ) were evaluated for switching reliability, time taken to switch with low error rate, and energy consumption during the switching process. In order to get a low write error rate in the MRAM cell with VCMA switching mechanism, a spin-transfer torque current or an applied magnetic field comparable to the critical current and field of the free layer is necessary. In the hybrid processes, the VCMA effect lowers the duration during which the higher power hungry secondary mechanism is in place. Therefore, the total energy consumed during the hybrid writing processes, VCMA + STT or VCMA + Happ, is less than the energy consumed during pure spin-transfer torque or applied magnetic field switching.