Recent Progress in the Voltage-Controlled Magnetic Anisotropy Effect and the Challenges Faced in Developing Voltage-Torque MRAM

The electron spin degree of freedom can provide the functionality of “nonvolatility” in electronic devices. For example, magnetoresistive random access memory (MRAM) is expected as an ideal nonvolatile working memory, with high speed response, high write endurance, and good compatibility with complementary metal-oxide-semiconductor (CMOS) technologies. However, a challenging technical issue is to reduce the operating power. With the present technology, an electrical current is required to control the direction and dynamics of the spin. This consumes high energy when compared with electric-field controlled devices, such as those that are used in the semiconductor industry. A novel approach to overcome this problem is to use the voltage-controlled magnetic anisotropy (VCMA) effect, which draws attention to the development of a new type of MRAM that is controlled by voltage (voltage-torque MRAM). This paper reviews recent progress in experimental demonstrations of the VCMA effect. First, we present an overview of the early experimental observations of the VCMA effect in all-solid state devices, and follow this with an introduction of the concept of the voltage-induced dynamic switching technique. Subsequently, we describe recent progress in understanding of physical origin of the VCMA effect. Finally, new materials research to realize a highly-efficient VCMA effect and the verification of reliable voltage-induced dynamic switching with a low write error rate are introduced, followed by a discussion of the technical challenges that will be encountered in the future development of voltage-torque MRAM.

Related papers:

title: Novel magnetostrictive memory device
abstract: A stress-operated memory device consisting of an ellipsoidal magnetic particle array and an electrostrictive grid is proposed. In the device, the magnetic state of the particle can be controlled only by the magnetostriction effect. Each particle is located at the intersection of the grid and has an in-plane uniaxial anisotropy. A pair of electric contacts is connected to the end of each wire. In the writing process, the driving voltages are simultaneously applied to two pairs of the selected contacts. This allows to apply a local electric field whose direction and amplitude can be regulated by varying the voltage intensity and polarity. The exerting stress on the magnetic particle results in the linear magnetostriction and hence an additional anisotropy energy in the particle. The in-plane total energy minimum, corresponding to the magnetization direction, follows the local electric field. Consequently the magnetization of the single magnetic particle located at the intersection can therefore be selective...

title: High-density magnetoresistive random access memory operating at ultralow voltage at room temperature
abstract: The main bottlenecks limiting the practical applications of current magnetoresistive random access memory (MRAM) technology are its low storage density and high writing energy consumption. Although a number of proposals have been reported for voltage-controlled memory device in recent years, none of them simultaneously satisfy the important device attributes: high storage capacity, low power consumption and room temperature operation. Here we present, using phase-field simulations, a simple and new pathway towards high-performance MRAMs that display significant improvements over existing MRAM technologies or proposed concepts. The proposed nanoscale MRAM device simultaneously exhibits ultrahigh storage capacity of up to 88 Gb inch−2, ultralow power dissipation as low as 0.16 fJ per bit and room temperature high-speed operation below 10 ns.

title: Electrical control of reversible and permanent magnetization reorientation for magnetoelectric memory devices
abstract: We report giant reversible and permanent magnetic anisotropy reorientation between two perpendicular easy axes in a magnetoelectric polycrystalline Ni thin film and (011) oriented [Pb(Mg1/3Nb2/3)O3](1−x)-[PbTiO3]x (PMN-PT) heterostructure. The PMN-PT is partially poled prior to Ni film deposition to provide a remanent strain bias. Following Ni deposition and full poling of the sample, two giant remanent strains of equal and opposite values are used to reversibly and permanently reorient the magnetization state of the Ni film. These experimental results are integrated into micromagnetic simulation to demonstrate the usefulness of this approach for magnetoelectric based magnetic random access memory.

title: Electric-field control of ferromagnetism
abstract: It is often assumed that it is not possible to alter the properties of magnetic materials once they have been prepared and put into use. For example, although magnetic materials are used in information technology to store trillions of bits (in the form of magnetization directions established by applying external magnetic fields), the properties of the magnetic medium itself remain unchanged on magnetization reversal. The ability to externally control the properties of magnetic materials would be highly desirable from fundamental and technological viewpoints, particularly in view of recent developments in magnetoelectronics and spintronics. In semiconductors, the conductivity can be varied by applying an electric field, but the electrical manipulation of magnetism has proved elusive. Here we demonstrate electric-field control of ferromagnetism in a thin-film semiconducting alloy, using an insulating-gate field-effect transistor structure. By applying electric fields, we are able to vary isothermally and reversibly the transition temperature of hole-induced ferromagnetism.

title: Electrical Manipulation of Magnetization Reversal in a Ferromagnetic Semiconductor
abstract: We report electrical manipulation of magnetization processes in a ferromagnetic semiconductor, in which low-density carriers are responsible for the ferromagnetic interaction. The coercive force HC at which magnetization reversal occurs can be manipulated by modifying the carrier density through application of electric fields in a gated structure. Electrically assisted magnetization reversal, as well as electrical demagnetization, has been demonstrated through the effect. This electrical manipulation offers a functionality not previously accessible in magnetic materials and may become useful for reversing magnetization of nanoscale bits for ultrahigh-density information storage.

title: Electrically Induced Ferromagnetism at Room Temperature in Cobalt-Doped Titanium Dioxide
abstract: The magnetic properties of a magnetic insulator can be controlled by an electric field at room temperature. The electric field effect in ferromagnetic semiconductors enables switching of the magnetization, which is a key technology for spintronic applications. We demonstrated electric field–induced ferromagnetism at room temperature in a magnetic oxide semiconductor, (Ti,Co)O2, by means of electric double-layer gating with high-density electron accumulation (>1014 per square centimeter). By applying a gate voltage of a few volts, a low-carrier paramagnetic state was transformed into a high-carrier ferromagnetic state, thereby revealing the considerable role of electron carriers in high-temperature ferromagnetism and demonstrating a route to room-temperature semiconductor spintronics.

title: Electrical control of the ferromagnetic phase transition in cobalt at room temperature.
abstract: Electrical control of magnetic properties is crucial for device applications in the field of spintronics. Although the magnetic coercivity or anisotropy has been successfully controlled electrically in metals as well as in semiconductors, the electrical control of Curie temperature has been realized only in semiconductors at low temperature. Here, we demonstrate the room-temperature electrical control of the ferromagnetic phase transition in cobalt, one of the most representative transition-metal ferromagnets. Solid-state field effect devices consisting of a ultrathin cobalt film covered by a dielectric layer and a gate electrode were fabricated. We prove that the Curie temperature of cobalt can be changed by up to 12 K by applying a gate electric field of about ±2 MV cm(-1). The two-dimensionality of the cobalt film may be relevant to our observations. The demonstrated electric field effect in the ferromagnetic metal at room temperature is a significant step towards realizing future low-power magnetic applications.

title: Magnetoelectric switching of exchange bias.
abstract: The perpendicular exchange bias field, H(EB), of the magnetoelectric heterostructure Cr2O3(111)/(Co/Pt)(3) changes sign after field cooling to below the Néel temperature of Cr2O3 in either parallel or antiparallel axial magnetic and electric freezing fields. The switching of H(EB) is explained by magnetoelectrically induced antiferromagnetic single domains which extend to the interface, where the direction of their end spins controls the sign of H(EB). Novel applications in magnetoelectronic devices seem possible.

title: Robust isothermal electric control of exchange bias at room temperature.
abstract: Voltage-controlled spin electronics is crucial for continued progress in information technology. It aims at reduced power consumption, increased integration density and enhanced functionality where non-volatile memory is combined with high-speed logical processing. Promising spintronic device concepts use the electric control of interface and surface magnetization. From the combination of magnetometry, spin-polarized photoemission spectroscopy, symmetry arguments and first-principles calculations, we show that the (0001) surface of magnetoelectric Cr(2)O(3) has a roughness-insensitive, electrically switchable magnetization. Using a ferromagnetic Pd/Co multilayer deposited on the (0001) surface of a Cr(2)O(3) single crystal, we achieve reversible, room-temperature isothermal switching of the exchange-bias field between positive and negative values by reversing the electric field while maintaining a permanent magnetic field. This effect reflects the switching of the bulk antiferromagnetic domain state and the interface magnetization coupled to it. The switchable exchange bias sets in exactly at the bulk Néel temperature.

title: Predicted magnetoelectric effect in Fe/BaTiO3 multilayers: ferroelectric control of magnetism.
abstract: An unexplored physical mechanism which produces a magnetoelectric effect in ferroelectric-ferromagnetic multilayers is studied based on first-principles calculations. Its origin is a change in bonding at the ferroelectric-ferromagnet interface that alters the interface magnetization when the electric polarization reverses. Using Fe/BaTiO3 multilayers as a representative model, we show a sizable difference in magnetic moments of Fe and Ti atoms at the two interfaces dissimilar by the orientation of the local electric dipole moments. The predicted magnetoelectric effect opens a new direction to control magnetic properties of thin-film layered structures by electric fields.

title: Magnetoelectric coupling at metal surfaces.
abstract: Magnetoelectric coupling allows the magnetic state of a material to be changed by an applied electric field. To date, this phenomenon has mainly been observed in insulating materials such as complex multiferroic oxides. Bulk metallic systems do not exhibit magnetoelectric coupling, because applied electric fields are screened by conduction electrons. We demonstrate strong magnetoelectric coupling at the surface of thin iron films using the electric field from a scanning tunnelling microscope, and are able to write, store and read information to areas with sides of a few nanometres. Our work demonstrates that high-density, non-volatile information storage is possible in metals.

title: Deterministic switching of ferromagnetism at room temperature using an electric field
abstract: The technological appeal of multiferroics is the ability to control magnetism with electric field. For devices to be useful, such control must be achieved at room temperature. The only single-phase multiferroic material exhibiting unambiguous magnetoelectric coupling at room temperature is BiFeO3 (refs 4 and 5). Its weak ferromagnetism arises from the canting of the antiferromagnetically aligned spins by the Dzyaloshinskii–Moriya (DM) interaction. Prior theory considered the symmetry of the thermodynamic ground state and concluded that direct 180-degree switching of the DM vector by the ferroelectric polarization was forbidden. Instead, we examined the kinetics of the switching process, something not considered previously in theoretical work. Here we show a deterministic reversal of the DM vector and canted moment using an electric field at room temperature. First-principles calculations reveal that the switching kinetics favours a two-step switching process. In each step the DM vector and polarization are coupled and 180-degree deterministic switching of magnetization hence becomes possible, in agreement with experimental observation. We exploit this switching to demonstrate energy-efficient control of a spin-valve device at room temperature. The energy per unit area required is approximately an order of magnitude less than that needed for spin-transfer torque switching. Given that the DM interaction is fundamental to single-phase multiferroics and magnetoelectrics, our results suggest ways to engineer magnetoelectric switching and tailor technologically pertinent functionality for nanometre-scale, low-energy-consumption, non-volatile magnetoelectronics.

title: Electric-field-induced generation and reversal of ferromagnetic moment in ferrites
abstract: The prospect of controlling the magnetization (M) of a material solely with an external electric field (E) could enable the development of low-power spintronics. Although there has been some success towards this end, most approaches involve controlling interactions at the interface between two different materials rather than switching of a single bulk phase. Here we report the ability to exert complete control over the generation and reversal of the bulk spontaneous M of the single-component multiferroics RFeO3 (R = Dy0.70Tb0.30, Dy0.75Gd0.25) with an E alone. We achieve this by controlling the anisotropic character of rare-earth magnetism and exploiting the competition between different magnetoelectric phases. We also show that whether M is reversed or retained on the E-induced polarization reversal depends on the E modulation speed. This is ascribed to the different dynamical characteristics of ferroelectric and multiferroic domain walls governed by the reversal dynamics of rare-earth moments and iron spins, respectively. The ability to modify a material’s magnetization with an electric field could enable lower-power electronic devices. Such ‘magnetoelectric’ behaviour is usually only seen at the interface between magnetostrictive and electrostrictive materials, but has now been observed in the bulk of single-component rare-earth ferrites.

title: Electric Field-Induced Modification of Magnetism in Thin-Film Ferromagnets
abstract: A large electric field at the surface of a ferromagnetic metal is expected to appreciably change its electron density. In particular, the metal's intrinsic magnetic properties, which are commonly regarded as fixed material constants, will be affected. This requires, however, that the surface has a strong influence on the material's properties, as is the case with ultrathin films. We demonstrated that the magnetocrystalline anisotropy of ordered iron-platinum (FePt) and iron-palladium (FePd) intermetallic compounds can be reversibly modified by an applied electric field when immersed in an electrolyte. A voltage change of –0.6 volts on 2-nanometer-thick films altered the coercivity by –4.5 and +1% in FePt and FePd, respectively. The modification of the magnetic parameters was attributed to a change in the number of unpaired d electrons in response to the applied electric field. Our device structure is general and should be applicable for characterization of other thin-film magnetic systems.

title: Large voltage-induced magnetic anisotropy change in a few atomic layers of iron.
abstract: In the field of spintronics, researchers have manipulated magnetization using spin-polarized currents. Another option is to use a voltage-induced symmetry change in a ferromagnetic material to cause changes in magnetization or in magnetic anisotropy. However, a significant improvement in efficiency is needed before this approach can be used in memory devices with ultralow power consumption. Here, we show that a relatively small electric field (less than 100 mV nm(-1)) can cause a large change (approximately 40%) in the magnetic anisotropy of a bcc Fe(001)/MgO(001) junction. The effect is tentatively attributed to the change in the relative occupation of 3d orbitals of Fe atoms adjacent to the MgO barrier. Simulations confirm that voltage-controlled magnetization switching in magnetic tunnel junctions is possible using the anisotropy change demonstrated here, which could be of use in the development of low-power logic devices and non-volatile memory cells.

title: Electric-Field-Controlled Magnetoelectric RAM: Progress, Challenges, and Scaling
abstract: We review the recent progress in the development of magnetoelectric RAM (MeRAM) based on electric-field-controlled writing in magnetic tunnel junctions (MTJs). MeRAM uses the tunneling magnetoresistance effect for readout in a two-terminal memory element, similar to other types of magnetic RAM. However, the writing of information is performed by voltage control of magnetic anisotropy (VCMA) at the interface of an MgO tunnel barrier and the CoFeB-based free layer, as opposed to current-controlled (e.g., spin-transfer torque or spin-orbit torque) mechanisms. We present results on voltage-induced switching of MTJs in both resonant (precessional) and thermally activated regimes, which demonstrate fast (<;1 ns) and ultralow-power (<;40 fJ/bit) write operations at voltages ~1.5-2 V. We also discuss the implications of the VCMA-based write mechanism on memory array design, highlighting the possibility of crossbar implementation for high bit density. Results are presented from a 1 kbit MeRAM test array. Endurance and voltage scaling data are presented. The scaling behavior is analyzed, and material-level requirements are discussed for the translation of MeRAM into mainstream memory applications.

title: Highly efficient voltage control of spin and enhanced interfacial perpendicular magnetic anisotropy in iridium-doped Fe/MgO magnetic tunnel junctions
abstract: Voltage control of spin enables both a zero standby power and ultralow active power consumption in spintronic devices, such as magnetoresistive random-access memory devices. A practical approach to achieve voltage control is the electrical modulation of the spin–orbit interaction at the interface between 3d-transition-ferromagnetic-metal and dielectric layers in a magnetic tunnel junction (MTJ). However, we need to initiate a new guideline for materials design to improve both the voltage-controlled magnetic anisotropy (VCMA) and perpendicular magnetic anisotropy (PMA). Here we report that atomic-scale doping of iridium in an ultrathin Fe layer is highly effective to improving these properties in Fe/MgO-based MTJs. A large interfacial PMA energy, Ki,0, of up to 3.7 mJ m−2 was obtained, which was 1.8 times greater than that of the pure Fe/MgO interface. Moreover, iridium doping yielded a huge VCMA coefficient (up to 320 fJ Vm−1) as well as high-speed response. First-principles calculations revealed that Ir atoms dispersed within the Fe layer play a considerable role in enhancing Ki,0 and the VCMA coefficient. These results demonstrate the efficacy of heavy-metal doping in ferromagnetic layers as an advanced approach to develop high-density voltage-driven spintronic devices. Researchers from Japan's AIST demonstrated a new approach to reduce the energy consumption of spintronic devices. Magnetic random-access memory requires approximately 10,000 times more energy to record data than to safely maintain it — a discrepancy that arises due to the wastefulness of electric-current-based switching of magnetic bits. Takayuki Nozaki and colleagues now report a device that enables us to write magnetic memory using electric fields, a more energy-efficient control mechanism. The team introduced an iridium-doped ultrathin iron film in magnesium oxide-based magnetic tunnel junctions, and found that the heavy-metal dopants provoked a strong voltage-controlled magnetic anisotropy change with high-speed response. Physical role of heavy metal dopants was unveiled by first-principles calculations. The developed technique can lead to a new type of non-volatile memory with ultra-low energy consumption. Highly efficient voltage control of magnetic anisotropy has been demonstrated utlizing an ultrathin Ir-doped Fe layer in MgO-based magnetic tunnel junctions. Ir adoms are dispersed inside the ultrathin Fe layer through the interdiffusion process. Large spin–orbit interaction of Ir atoms having proximity-induced magnetism is attributed to the enhancement of the voltage-controlled magnetic anisotropy (VCMA) effect. High speed response of the VCMA effect was also confirmed by voltage-induced ferromagnetic resonance. The achieved properties first satisfy the required specification for the new type of magnetoresistive random access memory (MRAM) driven by voltage.

title: Magnetoelectric Random Access Memory-Based Circuit Design by Using Voltage-Controlled Magnetic Anisotropy in Magnetic Tunnel Junctions
abstract: We introduce a magnetoelectric junction driven by voltage-controlled magnetic anisotropy (VCMA-MEJ) as a building block for a range of low-power memory applications. We present and discuss specifically two applications, magnetoelectric random access memory (MeRAM) and ternary content-addressable memory (TCAM). The MEJ differs from a magnetic tunnel junction (MTJ) in that electric field is used to induce switching in lieu of substantial current flow in MTJ. Electric field control of magnetism can dramatically enhance the performance of magnetic memory devices in terms of switching energy efficiency and switching speed. The development of such an energy-efficient and ultrafast memory has a potential to change the paradigm of a hierarchical memory system in the conventional computer architecture. By combining speed, low power, and high density, electric-field-controlled magnetic memory merges features of multiple separate memory technologies used in today's memory hierarchy. The performance of a VCMA-MEJs-based MeRAM, especially in the case of one access transistor associated with one MEJ (1T-1R) structure, is evaluated by comparing it with that of phase-change RAM, resistive RAM, and spin transfer torque RAM. MeRAM can achieve ultrafast switching (<;1 ns), low switching energy (~1 fJ), and compact cell size of 6 F2 with a shared source region, as well as nonvolatility. For another application, we propose the VCMA-MEJ-based TCAM, which will be referred to as MeTCAM, consisting of 4T-2MEJs. Since MeTCAM fully exploits the low power and high density features of the VCMA effect both in write and search operation modes, it obtains a fast searching speed (0.2 ns) with the smallest cell area (44 F2) compared to previous works.

title: Novel voltage controlled MRAM (VCM) with fast read/write circuits for ultra large last level cache
abstract: This paper presents voltage controlled MRAM (VCM) with novel fast read/write circuits for nonvolatile ultra-large last level cache. Further, write error rate has dramatically been reduced by thermal stability factor control using “continuous read-write-verify” scheme. Read error rate has also improved with “read-disturb-free non-destructive-self-reference read” with unipolar write of VCM.

title: Analysis and Compact Modeling of Magnetic Tunnel Junctions Utilizing Voltage-Controlled Magnetic Anisotropy
abstract: A macrospin compact model of a perpendicularly magnetized voltage-controlled magnetic tunnel junction (MTJ) is introduced, for the evaluation of electronic and magnetic characteristics associated with its switching behavior. The voltage-driven precessional switching has been shown to outperform current-driven switching in terms of energy, speed, and bit density. However, to realize the device in embedded system memory applications, developing a compact model for simulating hybrid MTJ/CMOS circuits is necessary. This framework also needs to be compatible with conventional computer-aided circuit design tools and flows. In this paper, the voltage-controlled magnetic anisotropy effect is included as a component of the effective magnetic field in a Landau–Lifshitz–Gilbert (LLG) equation-based model. The compact model is described by Verilog-A, providing a numerical solution for a 3-D magnetization trajectory and the corresponding conductance change of the MTJ. We also include the thermal noise effect in the LLG equation to allow the evaluation of stochastic switching behavior under a wide variety of bias conditions, which can be quantified in terms of write error rate (WER). It is shown that the model allows for optimization of write pulse shape and design considerations to achieve the lowest WER for given MTJ parameters.

title: Self-Adaptive Write Circuit for Magnetic Tunneling Junction Memory With Voltage-Controlled Magnetic Anisotropy Effect
abstract: Currently, commercial semiconductor-based memories face the problem of static energy consumption caused by leakage currents. Magnetoelectric random access memory (MeRAM), as a type of nonvolatile memory, provides a solution to this problem and can be used to replace the entire memory hierarchy as a universal memory. In contrast to spin transfer torque RAM (STT-RAM), MeRAM uses voltage controlled magnetic anisotropy (VCMA) effect to manipulate the switching of magnetic tunnel junctions (MTJs). MeRAM or VCMA-MTJ technology is more energy efficient and less likely to break down than STT-RAM. However, VCMA-MTJ suffers from serious write problems caused by variations in the VCMA coefficient, the external in-plane magnetic field, and the CMOS process. In this paper, a compact and SPICE-compatible VCMA-MTJ model is proposed. Then, a self-adaptive write circuit is proposed in which the write process is robust to variations in the VCMA coefficient and the external in-plane magnetic field. Finally, two practical methods are proposed to make our proposed self-adaptive write circuit also robust to 3$\sigma$  CMOS process variations, as demonstrated by Monte Carlo simulations. The proposed self-adaptive write circuit can assist in the design of next-generation high-speed, low-power MeRAM chips.

title: Magnetization vector manipulation by electric fields
abstract: Conventional semiconductor devices use electric fields to control conductivity, a scalar quantity, for information processing. In magnetic materials, the direction of magnetization, a vector quantity, is of fundamental importance. In magnetic data storage, magnetization is manipulated with a current-generated magnetic field (Oersted–Ampère field), and spin current is being studied for use in non-volatile magnetic memories. To make control of magnetization fully compatible with semiconductor devices, it is highly desirable to control magnetization using electric fields. Conventionally, this is achieved by means of magnetostriction produced by mechanically generated strain through the use of piezoelectricity. Multiferroics have been widely studied in an alternative approach where ferroelectricity is combined with ferromagnetism. Magnetic-field control of electric polarization has been reported in these multiferroics using the magnetoelectric effect, but the inverse effect—direct electrical control of magnetization—has not so far been observed. Here we show that the manipulation of magnetization can be achieved solely by electric fields in a ferromagnetic semiconductor, (Ga,Mn)As. The magnetic anisotropy, which determines the magnetization direction, depends on the charge carrier (hole) concentration in (Ga, By applying an electric field using a metal–insulator–semiconductor structure, the hole concentration and, thereby, the magnetic anisotropy can be controlled, allowing manipulation of the magnetization direction.

title: Surface magnetoelectric effect in ferromagnetic metal films.
abstract: A surface magnetoelectric effect is revealed by density-functional calculations that are applied to ferromagnetic Fe(001), Ni(001), and Co(0001) films in the presence of an external electric field. The effect originates from spin-dependent screening of the electric field which leads to notable changes in the surface magnetization and the surface magnetocrystalline anisotropy. These results are of considerable interest in the area of electrically controlled magnetism and magnetoelectric phenomena.

title: Giant modification of the magnetocrystalline anisotropy in transition-metal monolayers by an external electric field.
abstract: Controlling and designing quantum magnetic properties by an external electric field is a key challenge in modern magnetic physics. Here, from first principles, the effects of an external electric field on the magnetocrystalline anisotropy (MCA) in ferromagnetic transition-metal monolayers are demonstrated which show that the MCA in an Fe(001) monolayer [but not in Co(001) and Ni(001) monolayers] can be controlled by the electric field through a change in band structure, in which small components of the p orbitals near the Fermi level, which are coupled to the d states by the electric field, play a key role. This prediction obtained opens a way to control the MCA by the electric field and invites experiments.

title: Title Finite electric field effects in the large perpendicular magnetic anisotropysurface Pt / Fe / Pt ( 001 ) : A first-principles study
abstract: *KURAに登録されているコンテンツの利用については,著作権法に規定されている私的使用や引用などの範囲内で行ってください。 *著作権法に規定されている私的使用や引用などの範囲を超える利用を行う場合には,著作権者の許諾を得てください。ただし,著作権者 から著作権等管理事業者(学術著作権協会,日本著作出版権管理システムなど)に権利委託されているコンテンツの利用手続については ,各著作権等管理事業者に確認してください。 Title Finite electric field effects in the large perpendicular magnetic anisotropy surface Pt/Fe/Pt(001): A first-principles study Author(s) Tsujikawa, Masahito; Oda, Tatsuki Citation Physical Review Letters, 102(24): 247203 Issue Date 2009-06-18 Type Journal Article Text version publisher URL http://hdl.handle.net/2297/18715 Right

title: Electric field control of interface magnetic anisotropy
abstract: We present a model for determining the Rashba spin-orbit coupling (RSOC) at magnetic surfaces or interfaces by explicitly taking into account the interaction between the inversion-symmetry-broken potential and the spin-dependent electric dipoles of the Bloch states. We show that the RSOC alone can generate a perpendicular surface magnetic anisotropy comparable to the observed values in transition metals. When an external electric field is applied across the interface, the induced screening potential modifies the RSOC and thus controls the direction of the magnetization. Our results are consistent with the existing experiments.

title: Rashba Spin-Orbit Anisotropy and the Electric Field Control of Magnetism
abstract: The control of the magnetism of ultra-thin ferromagnetic layers using an electric field, rather than a current, has many potential technologically important applications. It is usually insisted that such control occurs via an electric field induced surface charge doping that modifies the magnetic anisotropy. However, it remains the case that a number of key experiments cannot be understood within such a scenario. Much studied is the spin-splitting of the conduction electrons of non-magnetic metals or semi-conductors due to the Rashba spin-orbit coupling. This reflects a large surface electric field. For a magnet, this same splitting is modified by the exchange field resulting in a large magnetic anisotropy energy via the Dzyaloshinskii-Moriya mechanism. This different, yet traditional, path to an electrically induced anisotropy energy can explain the electric field, thickness, and material dependence reported in many experiments.

title: Role of an interfacial FeO layer in the electric-field-driven switching of magnetocrystalline anisotropy at the Fe/MgO interface
abstract: The electric-field-induced switching of magnetocrystalline anisotropy (MCA) between in-plane and out-of-plane orientations is investigated by first-principles calculations for the prototypical Fe on MgO(001) system. Our results predict that an ideal abrupt Fe/MgO interface gives rise to a large out-of-plane MCA due to weak Fe-O hybridization at the interface, but the MCA switching by an applied electric field is found to be difficult to achieve. Instead, the existence of an interfacial FeO layer plays a key role in demonstrating the MCA switching that accompanies an electric-field-induced displacement of Fe atoms on the interfacial FeO layer.

title: Voltage-induced perpendicular magnetic anisotropy change in magnetic tunnel junctions
abstract: A voltage-induced perpendicular magnetic anisotropy change in an ultrathin FeCo layer was observed in an epitaxial magnetic tunnel junction (MTJ) structure. A spin-transfer induced ferromagnetic resonance measurement technique was used under various bias voltage applications to evaluate the anisotropy change. From the peak frequency shifts, we could estimate that a surface magnetic anisotropy change of 15 μJ/m2 was induced by an electric field application of 400 mV/nm in the MTJ with a 0.5 nm thick FeCo layer. The realization of voltage-induced anisotropy changes in an MTJ structure should have a large impact on the development of electric-field driven spintronic devices.

title: Quantitative Evaluation of Voltage-Induced Magnetic Anisotropy Change by Magnetoresistance Measurement
abstract: We investigated the voltage-induced perpendicular magnetic anisotropy change in an epitaxial magnetic tunnel junction (MTJ) with an ultrathin FeCo layer. Tunneling magnetoresistance (TMR) curves were measured under various bias voltage applications for different FeCo thicknesses. Clear changes in the shape of TMR curves were observed depending on the voltage-controlled perpendicular magnetic anisotropy. By evaluating the relative angle of two ferromagnetic layers, we could estimate the anisotropy energy change quantitatively. The realization of voltage-induced anisotropy change in the MTJ structure makes it possible to control the magnetization dynamics, leading to a new area of electric-field-based spintronics devices.

title: Electric-field-induced ferromagnetic resonance excitation in an ultrathin ferromagnetic metal layer
abstract: Ferromagnetic resonance excitations offer a means to coherently manipulate the spin dynamics of spintronic devices and systems. A demonstration of the ability to control these excitations with electric fields alone could drastically reduce the power consumption of these devices.

title: Voltage-induced ferromagnetic resonance in magnetic tunnel junctions.
abstract: We demonstrate excitation of ferromagnetic resonance in CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) by the combined action of voltage-controlled magnetic anisotropy (VCMA) and spin transfer torque (ST). Our measurements reveal that GHz-frequency VCMA torque and ST in low-resistance MTJs have similar magnitudes, and thus that both torques are equally important for understanding high-frequency voltage-driven magnetization dynamics in MTJs. As an example, we show that VCMA can increase the sensitivity of an MTJ-based microwave signal detector to the sensitivity level of semiconductor Schottky diodes.

title: High-output microwave detector using voltage-induced ferromagnetic resonance
abstract: We investigated the voltage-induced ferromagnetic resonance (FMR) with various DC bias voltage and input RF power in magnetic tunnel junctions. We found that the DC bias monotonically increases the homodyne detection voltage due to the nonlinear FMR originating in an asymmetric magnetization-potential in the free layer. In addition, the linear increase of an output voltage to the input RF power in the voltage-induced FMR is more robust than that in spin-torque FMR. These characteristics enable us to obtain an output voltage more than ten times than that of microwave detectors using spin-transfer torque.

title: Electric field-induced ferromagnetic resonance in a CoFeB/MgO magnetic tunnel junction under dc bias voltages
abstract: We measure homodyne-detected ferromagnetic resonance (FMR) induced by the electric-field effect in a CoFeB/MgO/CoFeB magnetic tunnel junction (MTJ) with perpendicular magnetic easy axis under dc bias voltages up to 0.1 V. From the bias dependence of the resonant frequency, we find that the first order perpendicular magnetic anisotropy is modulated by the applied electric field, whereas the second order component is virtually independent of the electric field. The lineshapes of the FMR spectra are bias dependent, which are explained by the combination of electric-field effect and reflection of the bias voltage from the MTJ.

title: Effect of excitation power on voltage induced local magnetization dynamics in an ultrathin CoFeB film
abstract: Voltage or electric field induced magnetization dynamics promises low power spintronics devices. For successful operation of some spintronics devices such as magnetic oscillators and magnetization switching devices a clear understanding of nonlinear magnetization dynamics is required. Here, we report a detailed experimental and micromagnetic simulation study about the effect of excitation power on voltage induced local magnetization dynamics in an ultrathin CoFeB film. Experimental results show that the resonance line-width and frequency remains constant, whereas cone angle of the magnetization precession increases linearly with square-root of excitation power below threshold value, known as linear excitation regime. Above threshold power, the dynamics enters into nonlinear regime where resonance line-width monotonically increases and resonance frequency monotonically decreases with increasing excitation power. Simulation results reveal that a strong nonlinear and incoherent magnetization dynamics are observed in our experiment above the threshold power which reduces dynamic magnetic signal by suppressing large cone angle of magnetization precession. Moreover, a significant transfer of spin angular momentum from uniform FMR mode to its degenerate spin waves outside of excitation area further restrict the cone angle of precession within only few degrees in our device. Our results will be very useful to develop all-voltage-controlled spintronics devices.

title: Induction of coherent magnetization switching in a few atomic layers of FeCo using voltage pulses.
abstract: The magnetization direction of a metallic magnet has generally been controlled by a magnetic field or by spin-current injection into nanosized magnetic cells. Both these methods use an electric current to control the magnetization direction; therefore, they are energy consuming. Magnetization control using an electric field is considered desirable because of its expected ultra-low power consumption and coherent behaviour. Previous experimental approaches towards achieving voltage control of magnetization switching have used single ferromagnetic layers with and without piezoelectric materials, ferromagnetic semiconductors, multiferroic materials, and their hybrid systems. However, the coherent control of magnetization using voltage signals has not thus far been realized. Also, bistable magnetization switching (which is essential in information storage) possesses intrinsic difficulties because an electric field does not break time-reversal symmetry. Here, we demonstrate a coherent precessional magnetization switching using electric field pulses in nanoscale magnetic cells with a few atomic FeCo (001) epitaxial layers adjacent to a MgO barrier. Furthermore, we demonstrate the realization of bistable toggle switching using the coherent precessions. The estimated power consumption for single switching in the ideal equivalent switching circuit can be of the order of 10(4)k(B)T, suggesting a reduction factor of 1/500 when compared with that of the spin-current-injection switching process.

title: Parametric Resonance of Magnetization Excited by Electric Field.
abstract: Manipulation of magnetization by electric field is a central goal of spintronics because it enables energy-efficient operation of spin-based devices. Spin wave devices are promising candidates for low-power information processing, but a method for energy-efficient excitation of short-wavelength spin waves has been lacking. Here we show that spin waves in nanoscale magnetic tunnel junctions can be generated via parametric resonance induced by electric field. Parametric excitation of magnetization is a versatile method of short-wavelength spin wave generation, and thus, our results pave the way toward energy-efficient nanomagnonic devices.

title: Excitation of coherent propagating spin waves in ultrathin CoFeB film by voltage-controlled magnetic anisotropy
abstract: Spin waves (SWs) may be used as potential information carriers in next generation low-power spintronics devices. Here, we report an experimental study on the excitation of propagating magnetostatic surface SWs by voltage-controlled magnetic anisotropy in a 2 nm thick CoFeB film. The SWs are detected by a pico-second time-resolved longitudinal Kerr microscope with a spatial resolution of 600 nm. We found a linear increase in the SW amplitude with the applied rf voltage. We show that in this ultrathin film, the voltage excited SWs can propagate up to micrometer distances which decrease with the increase in the bias magnetic field value. This is also supported by micromagnetic simulation results. Furthermore, we show that voltage excitations are spatially localized as opposed to conventional microstrip antenna induced Oersted field excitations. We discuss about the advantage of voltage excitation compared to the Oersted field excitation. We believe that voltage excitation of SWs will be more suitable and use...

title: Large change in perpendicular magnetic anisotropy induced by an electric field in FePd ultrathin films
abstract: The magnetic properties of FePd ultrathin films and their variation under the influence of an electric field are investigated by magneto-optical Kerr effect (MOKE) measurements. L10-ordered FePd shows a spin reorientation transition when varying the thickness. The easy axis of magnetization is found to be normal to the plane at thicknesses above 9 monolayers (MLs) and in-plane below 9 ML. The coercive field, the perpendicular magnetic anisotropy and the MOKE signal at saturation vary with the applied electric field. The sensitivity of the interface magnetic anisotropy is estimated to be 602 fJ/V m.

title: Coercivity change in an FePt thin layer in a Hall device by voltage application
abstract: The coercivity (Hc) of a perpendicularly magnetized FePt layer was modulated by applying the voltage (Vapp) to a Hall device through MgO and Al–O insulating layers. A change in ∼40 Oe in Hc was observed by changing Vapp from −13 to 13 V. From the quantitative analysis of the voltage effect on Hc, the change in the anisotropy energy by voltage application was evaluated to be 18.6 fJ/V m, which was of the same order as the theoretical prediction. The role of the MgO layer for the voltage effect was also discussed.

title: Electric field effect on magnetic anisotropy for Fe-Pt-Pd alloys
abstract: The electric field effect on magnetic anisotropy was investigated for the FePt1-xPdx alloy films with perpendicular magnetic anisotropy. The polar magneto-optical Kerr (p-MOKE) loops were measured under the electric field application in order to evaluate the electric field-induced perpendicular magnetic anisotropy change per area (Δeperpt). A clear change in the saturation field of p-MOKE loop was observed for FePt by varying the applied electric field (ΔE). In the case of FePt, Δeperpt divided by ΔE was evaluated to be -129 (fJ/Vm). We found that the magnitude of Δeperpt / ΔE was significantly reduced with increasing x.

title: Enhancement of perpendicular magnetic anisotropy and its electric field-induced change through interface engineering in Cr/Fe/MgO
abstract: Recently, perpendicular magnetic anisotropy (PMA) and its voltage control (VC) was demonstrated for Cr/Fe/MgO. In this study, we shed light on the origin of large voltage-induced anisotropy change in Cr/Fe/MgO. Analysis of the chemical structure of Cr/Fe/MgO revealed the existence of Cr atoms in the proximity of the Fe/MgO interface, which can affect both magnetic anisotropy (MA) and its VC. We showed that PMA and its VC can be enhanced by controlled Cr doping at the Fe/MgO interface. For Cr/Fe (5.9 Å)/Cr (0.7 Å)/MgO with an effective PMA of 0.8 MJ/m3, a maximum value of the voltage-controlled magnetic anisotropy (VCMA) effect of 370 fJ/Vm was demonstrated due to Cr insertion.

title: Nonlinear electric field effect on perpendicular magnetic anisotropy in Fe/MgO interfaces
abstract: The electric field effect on magnetic anisotropy was studied in an ultrathin Fe(001) monocrystalline layer sandwiched between Cr buffer and MgO tunnel barrier layers, mainly through post-annealing temperature and measurement temperature dependences. A large coefficient of the electric field effect of more than 200 fJ/Vm was observed in the negative range of electric field, as well as an areal energy density of perpendicular magnetic anisotropy (PMA) of around 600 uJ/m2. More interestingly, nonlinear behavior, giving rise to a local minimum around +100 mV/nm, was observed in the electric field dependence of magnetic anisotropy, being independent of the post-annealing and measurement temperatures. The insensitivity to both the interface conditions and the temperature of the system suggests that the nonlinear behavior is attributed to an intrinsic origin such as an inherent electronic structure in the Fe/MgO interface. The present study can contribute to the progress in theoretical studies, such as ab initio calculations, on the mechanism of the electric field effect on PMA.

title: Strong Bias Effect on Voltage-Driven Torque at Epitaxial Fe-MgO Interface
abstract: Shinji Miwa, Junji Fujimoto, Philipp Risius, Kohei Nawaoka, Minori Goto, and Yoshishige Suzuki Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan Center for Spintronics Research Network, Osaka University, Toyonaka, Osaka 560-8531, Japan Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan (Received 12 February 2017; revised manuscript received 7 June 2017; published 26 July 2017)

title: Magnetic Tunnel Junctions with a Nearly Zero Moment Manganese Nanolayer with Perpendicular Magnetic Anisotropy.
abstract: A magnetic nanolayer with a perpendicular magnetic easy axis and negligible magnetization is demonstrated. Even though a manganese metal is antiferromagnetic in bulk form, a few manganese monolayers grown on a paramagnetic ordered alloy template and capped by an oxide layer exhibit a strong perpendicular magnetic anisotropy field exceeding 19 T as well as a negligible magnetization of 25 kA/m. The nanolayer shows tunnel magnetoresistance. Moreover, the perpendicular magnetic anisotropy for the nanolayer can be reduced by applying an electric voltage. These findings will provide new insight into a creation of new nanolayer magnets.

title: Electric-field-control of magnetic anisotropy of Co0.6Fe0.2B0.2/oxide stacks using reduced voltage
abstract: We have demonstrated purely electrical manipulation of the magnetic anisotropy of a Co0.6Fe0.2B0.2 film by applying only 8 V across the CoFeB/oxide stack. A clear transition from in-plane to perpendicular anisotropy was observed. The quantitative relationship between interface anisotropy energy and the applied electric-field was determined from the linear voltage dependence of the saturation field. By comparing the dielectric stacks of MgO/Al2O3 and MgO/HfO2/Al2O3, enhanced voltage control was also demonstrated, due to the higher dielectric constant of the HfO2. These results suggest the feasibility of purely electrical control of magnetization with small voltage bias for spintronics applications.

title: Magnetic field sensor with voltage-tunable sensing properties
abstract: We report on a magnetic field sensor based on CoFeB/MgO/CoFeB magnetic tunnel junctions. By taking advantage of the perpendicular magnetic anisotropy of the MgO/CoFeB interface, the magnetization of the sensing layer is tilted out-of-plane which results in a linear magnetoresistance response to in-plane magnetic fields. The application of a bias voltage across the MgO tunnel barrier of the sensor affects the magnetic anisotropy and thereby its sensing properties. We propose a voltage-tunable magnetic field sensor design that allows for active control of the sensitivity and the operating field range by the strength and polarity of the applied bias voltage.

title: Voltage-Induced Magnetic Anisotropy Changes in an Ultrathin FeB Layer Sandwiched between Two MgO Layers
abstract: We investigated the effect of voltage on the perpendicular magnetic anisotropy in an ultrathin Fe80B20 layer sandwiched between two MgO barrier layers with different thicknesses, in which the bias voltage is predominantly applied to one of the MgO layers. The application of both positive and negative bias voltages enhanced the perpendicular anisotropy, in contrast with the odd function dependence previously observed in a MgO/ferromagnetic metal/non-magnetic metal structure. Moreover, for positive bias voltages, the large anisotropy change slope of 108 fJ/Vm was demonstrated. These results indicate that the MgO-sandwich structure has high potential to extend the availability of the voltage effect.

title: Opposite signs of voltage-induced perpendicular magnetic anisotropy change in CoFeB|MgO junctions with different underlayers
abstract: We report a voltage-induced perpendicular magnetic anisotropy (PMA) change in sputter-deposited Ta|CoFeB|MgO and Ru|CoFeB|MgO junctions. The PMA change is quantitatively evaluated by the field dependence of the tunneling magnetoresistance for various bias voltages. We find that both the sign and amplitude of the voltage effect depend on the underlayer, Ta or Ru, below the CoFeB layer. The rf voltage-induced ferromagnetic resonance spectra also support the underlayer-material-dependent direction of the voltage torque. The present study shows that the underlayer is one of the key parameters for controlling the voltage effect.

title: Electric field control of spin re-orientation in perpendicular magnetic tunnel junctions—CoFeB and MgO thickness dependence
abstract: We report an investigation of electric-field (EF) control of spin re-orientation as functions of the thicknesses of CoFeB free layer (FL) and MgO layer in synthetic-antiferromagnetic pinned magnetic tunnel junctions with perpendicular magnetic anisotropy. It is found that the EF modulates the coercivity (Hc) of the FL almost linearly for all FL thicknesses, while the EF efficiency, i.e., the slope of the linearity, increases as the FL thickness increases. This linear variation in Hc is also observed for larger MgO thicknesses (≥1.5 nm), while the EF efficiency increases only slightly from 370 to 410 Oe nm/V when MgO thickness increases from 1.5 to 1.76 nm. We have further observed the absence of quasi-DC unipolar switching. We discuss its origin and highlight the underlying challenges to implement the EF controlled switching in a practical magnetic memory.

title: Electric-field effects on magnetic anisotropy and damping constant in Ta/CoFeB/MgO investigated by ferromagnetic resonance
abstract: We investigate electric-field effects on the effective magnetic anisotropy energy density Keff and the Gilbert damping constant α in Ta/CoFeB/MgO structures with CoFeB thickness t ranging from 1.4 to 1.8 nm by ferromagnetic resonance. The electric field-induced modulation ratio of the areal energy density Kefft does not depend on the CoFeB thickness, indicating that the electric-field effect on the magnetic anisotropy originates from the modulation of CoFeB/MgO-interfacial magnetic anisotropy. A clear electric-field modulation of α is observed for the structure with t = 1.4 nm, and almost no modulation for the structures with t ≥ 1.5 nm.

title: Thermally stable voltage-controlled perpendicular magnetic anisotropy in Mo|CoFeB|MgO structures
abstract: We study voltage-controlled magnetic anisotropy (VCMA) and other magnetic properties in annealed Mo|CoFeB|MgO layered structures. The interfacial perpendicular magnetic anisotropy (PMA) is observed to increase with annealing over the studied temperature range, and a VCMA coefficient of about 40 fJ/V-m is sustained after annealing at temperatures as high as 430 °C. Ab initio electronic structure calculations of interfacial PMA as a function of strain further show that strain relaxation may lead to the increase of interfacial PMA at higher annealing temperatures. Measurements also show that there is no significant VCMA and interfacial PMA dependence on the CoFeB thickness over the studied range, which illustrates the interfacial origin of the anisotropy and its voltage dependence, i.e., the VCMA effect. The high thermal annealing stability of Mo|CoFeB|MgO structures makes them compatible with advanced CMOS back-end-of-line processes, and will be important for integration of magnetoelectric random access memory into on-chip embedded applications.

title: Perpendicular magnetic anisotropy of Ir/CoFeB/MgO trilayer system tuned by electric fields
abstract: The perpendicular magnetic anisotropy of an Ir/CoFeB/MgO trilayer was investigated after annealing at temperatures ranging from 200 to 350 °C. In the trilayer system annealed at 300 °C, we measured an interface anisotropy energy of 1.9 mJ/m2. Further annealing led to mixing of the buffer and ferromagnet, degrading the properties of the latter. In addition, we show the dependence of the magnetic anisotropy on the bias voltage. The presented system is important for the development of perpendicular magnetic tunnel junctions for storage applications.

title: Size and voltage dependence of effective anisotropy in sub-100-nm perpendicular magnetic tunnel junctions
abstract: Magnetic tunnel junctions with perpendicular magnetic anisotropy are investigated using a conductive atomic force microscope. The 1.23 nm Co40Fe40B20 recording layer coercivity exhibits a size dependence which suggests single domain behavior for diameters ≤ 100 nm. Focusing on devices with diameters smaller than 100 nm, we determine the effect of voltage and size on the effective device anisotropy Keff using two different techniques. Keff is extracted both from distributions of the switching fields of the recording and reference layers, and from measurement of thermal fluctuations of the recording layer magnetization when a field close to the switching field is applied. The results from both sets of measurements reveal that Keff increases monotonically with decreasing junction diameter, consistent with the size dependence of the demagnetization energy density. We demonstrate that Keff can be controlled with a voltage down to the smallest size measured, 64 nm. PACS numbers: 85.75.-d,73.40.Gk,75.78.-n,75.70.-i

title: Enhancement of voltage-controlled magnetic anisotropy through precise control of Mg insertion thickness at CoFeB|MgO interface
abstract: We studied the impact of different insertion layers (Ta, Pt, and Mg) at the CoFeB|MgO interface on voltage-controlled magnetic anisotropy (VCMA) effect and other magnetic properties. Inserting a very thin Mg layer of 0.1–0.3 nm yielded a VCMA coefficient of 100 fJ/V-m, more than 3 times higher than the average values of around 30 fJ/V-m reported in Ta|CoFeB|MgO-based structures. Ta and Pt insertion layers also showed a small improvement, yielding VCMA coefficients around 40 fJ/V-m. Electrical, magnetic, and X-ray diffraction results reveal that a Mg insertion layer of around 1.2 nm gives rise to the highest perpendicular magnetic anisotropy, saturation magnetization, as well as the best CoFe and MgO crystallinity. Other Mg insertion thicknesses give rise to either under- or over-oxidation of the CoFe|MgO interface; a strong over-oxidation of the CoFe layer leads to the maximum VCMA effect. These results show that precise control over the Mg insertion thickness and CoFe oxidation level at the CoFeB|MgO int...

title: Electric-field-induced magnetic anisotropy in a nanomagnet investigated on the atomic scale.
abstract: Magnetoelectric coupling is studied using the electric field between the tip of a spin-polarized scanning tunneling microscope and a nanomagnet. Our experiments show that a negative (positive) electric field stabilizes (destabilizes) in-plane magnetization against thermal agitation, whereas it destabilizes (stabilizes) out-of-plane magnetization. We conclude that the electric field E induces a uniaxial anisotropy that favors in-plane magnetization for E<0 and out-of-plane magnetization for E>0. Our experiments demonstrate magnetic manipulation on the atomic scale without exploiting spin or charge currents.

title: Manipulation of magnetism by electrical field in a real recording system
abstract: We report an electrically controlled magnetism in a real recording system with CoCrPt–TiO2 nanocomposite thin films as recording medium. We show that in a spin-stand test, with a small voltage of 3 V applied across the head-media gap during recording, a 13% reduction of saturation current was observed and the read back waveforms showed sharper transitions. These account for the 3 dB improvement in read back signal-noise-ratio of the written magnetic information. The improved recording performance is mainly attributed to the reduction of anisotropy of the recording medium in the presence of electrical field. Simulations were carried out to understand the magnetization reversal process under applied electric and magnetic fields.We report an electrically controlled magnetism in a real recording system with CoCrPt–TiO2 nanocomposite thin films as recording medium. We show that in a spin-stand test, with a small voltage of 3 V applied across the head-media gap during recording, a 13% reduction of saturation current was observed and the read back waveforms showed sharper transitions. These account for the 3 dB improvement in read back signal-noise-ratio of the written magnetic information. The improved recording performance is mainly attributed to the reduction of anisotropy of the recording medium in the presence of electrical field. Simulations were carried out to understand the magnetization reversal process under applied electric and magnetic fields.

title: Voltage control of magnetic anisotropy in Fe films with quantum well states
abstract: Uwe Bauer,1,* Marek Przybylski,2,3 and Geoffrey S. D. Beach1 1Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA 2Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle, Germany 3Faculty of Physics and Applied Computer Science, and Academic Centre for Materials and Nanotechnology, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Krakow, Poland (Received 20 January 2014;  May 2014)

title: Electric-field control of magnetic domain-wall velocity in ultrathin cobalt with perpendicular magnetization.
abstract: Controlling the displacement of a magnetic domain wall is potentially useful for information processing in magnetic non-volatile memories and logic devices. A magnetic domain wall can be moved by applying an external magnetic field and/or electric current, and its velocity depends on their magnitudes. Here we show that the applying an electric field can change the velocity of a magnetic domain wall significantly. A field-effect device, consisting of a top-gate electrode, a dielectric insulator layer, and a wire-shaped ferromagnetic Co/Pt thin layer with perpendicular anisotropy, was used to observe it in a finite magnetic field. We found that the application of the electric fields in the range of ± 2-3 MV cm(-1) can change the magnetic domain wall velocity in its creep regime (10(6)-10(3) m s(-1)) by more than an order of magnitude. This significant change is due to electrical modulation of the energy barrier for the magnetic domain wall motion.

title: Electric-field control of domain wall motion in perpendicularly magnetized materials.
abstract: Domain wall motion in materials exhibiting perpendicular magnetic anisotropy has been the subject of intensive research because of its large potential for future spintronic devices. Recently, it has been shown that perpendicular anisotropy of thin films can be influenced by electric fields. Voltage-controlled magnetic switching has already been realized, which is envisioned to lead to low-power logic and memory devices. Here we demonstrate a radically new application of this effect, namely control of domain wall motion by electric fields. We show that an applied voltage perpendicular to a Co or CoB wire can significantly increase or decrease domain wall velocities. Velocity modification over an order of magnitude is demonstrated (from 0.4 to 4 μm s(-1)), providing a first step towards electrical control of domain wall devices. This opens up possibilities of real-time and local control of domain wall motion by electric fields at extremely low power cost.

title: Voltage-controlled domain wall traps in ferromagnetic nanowires.
abstract: Electrical control of magnetism has the potential to bring about revolutionary new spintronic devices, many of which rely on efficient manipulation of magnetic domain walls in ferromagnetic nanowires. Recently, it has been shown that voltage-induced charge accumulation at a metal-oxide interface can influence domain wall motion in ultrathin metallic ferromagnets, but the effects have been relatively modest and limited to the slow, thermally activated regime. Here we show that a voltage can generate non-volatile switching of magnetic properties at the nanoscale by modulating interfacial chemistry rather than charge density. Using a solid-state ionic conductor as a gate dielectric, we generate unprecedentedly strong voltage-controlled domain wall traps that function as non-volatile, electrically programmable and switchable pinning sites. Pinning strengths of at least 650 Oe can be readily achieved, enough to bring to a standstill domain walls travelling at speeds of at least ~20 m s(-1). We exploit this new magneto-ionic effect to demonstrate a prototype non-volatile memory device in which voltage-controlled domain wall traps facilitate electrical bit selection in a magnetic nanowire register.

title: The Skyrmion Switch: Turning Magnetic Skyrmion Bubbles on and off with an Electric Field.
abstract: Nanoscale magnetic skyrmions are considered as potential information carriers for future spintronics memory and logic devices. Such applications will require the control of their local creation and annihilation, which involves so far solutions that are either energy consuming or difficult to integrate. Here we demonstrate the control of skyrmion bubbles nucleation and annihilation using electric field gating, an easily integrable and potentially energetically efficient solution. We present a detailed stability diagram of the skyrmion bubbles in a Pt/Co/oxide trilayer and show that their stability can be controlled via an applied electric field. An analytical bubble model with the Dzyaloshinskii-Moriya interaction imbedded in the domain wall energy accounts for the observed electrical skyrmion switching effect. This allows us to unveil the origin of the electrical control of skyrmions stability and to show that both magnetic dipolar interaction and the Dzyaloshinskii-Moriya interaction play an important role in the skyrmion bubble stabilization.

title: Brownian motion of skyrmion bubbles and its control by voltage applications
abstract: Magnetic skyrmions are expected to be promising candidates for information carriers in spintronic devices. In previous work, precise position control of skyrmions has been the main focus of attention for memory and logic applications. Here, with the aim of employing the thermally activated random walk of skyrmion bubbles for logical operations, i.e., token-based Brownian computing, we investigated the dynamics of skyrmion bubbles in W/FeB/Ir/MgO structures. In addition to the observation of Brownian motion of skyrmion bubbles, we demonstrated the electrical control of the diffusion constant by voltage applications. The developed technique would be useful for various kinds of skyrmion-based spintronic devices as well as Brownian computing.Magnetic skyrmions are expected to be promising candidates for information carriers in spintronic devices. In previous work, precise position control of skyrmions has been the main focus of attention for memory and logic applications. Here, with the aim of employing the thermally activated random walk of skyrmion bubbles for logical operations, i.e., token-based Brownian computing, we investigated the dynamics of skyrmion bubbles in W/FeB/Ir/MgO structures. In addition to the observation of Brownian motion of skyrmion bubbles, we demonstrated the electrical control of the diffusion constant by voltage applications. The developed technique would be useful for various kinds of skyrmion-based spintronic devices as well as Brownian computing.

title: Voltage induction of interfacial Dzyaloshinskii–Moriya interaction in Au/Fe/MgO artificial multilayer
abstract: The Dzyaloshinskii–Moriya interaction (DMI) is an antisymmetric exchange interaction that plays a decisive role in the formation of chiral magnetic structures and in the determination of magnetoelectric properties. This study investigated the impact of an external voltage on the magneto-static surface waves in a Au/Fe/MgO multilayer. Spin waves were excited and detected using two coplanar waveguides and controlled by an external DC voltage. The DC bias voltage dependence of the resonant frequency in the spin waves revealed that the voltage effect has both directionally symmetric and asymmetric components, signifying voltage control of both interfacial magnetic anisotropy and interface-DMI.

title: Voltage-controlled interlayer coupling in perpendicularly magnetized magnetic tunnel junctions
abstract: Magnetic interlayer coupling is one of the central phenomena in spintronics. It has been predicted that the sign of interlayer coupling can be manipulated by electric fields, instead of electric currents, thereby offering a promising low energy magnetization switching mechanism. Here we present the experimental demonstration of voltage-controlled interlayer coupling in a new perpendicular magnetic tunnel junction system with a GdOx tunnel barrier, where a large perpendicular magnetic anisotropy and a sizable tunnelling magnetoresistance have been achieved at room temperature. Owing to the interfacial nature of the magnetism, the ability to move oxygen vacancies within the barrier, and a large proximity-induced magnetization of GdOx, both the magnitude and the sign of the interlayer coupling in these junctions can be directly controlled by voltage. These results pave a new path towards achieving energy-efficient magnetization switching by controlling interlayer coupling.

title: Electric-field control of magnetic moment in Pd
abstract: Several magnetic properties have recently become tunable with an applied electric field. Particularly, electrically controlled magnetic phase transitions and/or magnetic moments have attracted attention because they are the most fundamental parameters in ferromagnetic materials. In this study, we showed that an electric field can be used to control the magnetic moment in films made of Pd, usually a non-magnetic element. Pd ultra-thin films were deposited on ferromagnetic Pt/Co layers. In the Pd layer, a ferromagnetically ordered magnetic moment was induced by the ferromagnetic proximity effect. By applying an electric field to the ferromagnetic surface of this Pd layer, a clear change was observed in the magnetic moment, which was measured directly using a superconducting quantum interference device magnetometer. The results indicate that magnetic moments extrinsically induced in non-magnetic elements by the proximity effect, as well as an intrinsically induced magnetic moments in ferromagnetic elements, as reported previously, are electrically tunable. The results of this study suggest a new avenue for answering the fundamental question of “can an electric field make naturally non-magnetic materials ferromagnetic?”

title: Peculiar temperature dependence of electric-field effect on magnetic anisotropy in Co/Pd/MgO system
abstract: We report on the temperature dependence of the magnetic anisotropy in Co/Pd/MgO system, in which magnetic moment in Pd is induced by the magnetic proximity effect. We demonstrate that the magnetic anisotropy is modulated by applying an electric field to the Pd surface. At temperatures below 100 K, we find the nonlinear electric-field dependence of the anisotropy with the sign reversal. We obtain a huge anisotropy modulation efficiency of ∼1600 fJ/V m at 10 K.

title: Electric-field-induced thermally assisted switching of monodomain magnetic bits
abstract: We present a study of the electric-field-induced switching of magnetic memory bits exhibiting interfacial voltage-controlled magnetic anisotropy (VCMA). Switching is analyzed in the single-domain approximation and in the thermally activated regime. The effects of external magnetic fields, magnitudes of the perpendicular anisotropy and VCMA effect, and voltage pulse width on the switching voltage are discussed. Both in-plane and perpendicular magnetic memory bits are considered. Experimental results are presented and compared to the theoretical model.

title: Electric Field Assisted Switching in Magnetic Random Access Memory
abstract: Electric field (EF)-assisted magnetization reversal is investigated in both top-pinned and bottom-pinned CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs). EF modulation in coercivity (Hc) shows an increasing dependence with the thickness of the free layer (tFL) at small tFL values. This abnormal variation was attributed to possible Ta diffusion through the free layer to the interface with MgO barrier. It is found that the bidirectional switching is not achievable using unipolar EF only in our MTJ devices, which is due to small spin-transfer torque (STT) effect and switching uncertainty due to unipolar feature of EF-induced Hc modulation. A bipolar external magnetic field should be applied to realize EF-controlled magnetic random access memory (MRAM). For quasistatic magnetization reversal, the required field can be as low as 10 Oe when Hc is effectively modulated to nearly zero. Switching field as a function of switching time shows that the magnetization switching is dominated by thermal activation. Simulation results show that switching time is mainly determined by the damping constant and the EF efficiency. Different from STT-MRAM, a large damping constant is desired to achieve fast switching. Using a sweeping field method, we show that MTJs with a thermal stability factor as high as 58 can be reliably switched using an EF-modulated anisotropy scheme with a bipolar magnetic field as low as 10 Oe.

title: Magnetization switching assisted by high-frequency-voltage-induced ferromagnetic resonance
abstract: Magnetization switching assisted by high-frequency voltage was demonstrated in magnetic tunnel junctions with perpendicular magnetization. Ferromagnetic resonance was excited by high-frequency voltage instead of by a high-frequency magnetic field. A frequency-dependent reduction in the switching field (coercive force) was clearly observed with a maximum reduction rate of more than 80% under a radio-frequency power application of 3 dBm. The developed technique can provide a novel approach to energy-assisted magnetization switching with low ohmic dissipation.

title: Electric-field-assisted switching in magnetic tunnel junctions.
abstract: The advent of spin transfer torque effect accommodates site-specific switching of magnetic nanostructures by current alone without magnetic field. However, the critical current density required for usual spin torque switching remains stubbornly high around 10(6)-10(7) A cm(-2). It would be fundamentally transformative if an electric field through a voltage could assist or accomplish the switching of ferromagnets. Here we report electric-field-assisted reversible switching in CoFeB/MgO/CoFeB magnetic tunnel junctions with interfacial perpendicular magnetic anisotropy, where the coercivity, the magnetic configuration and the tunnelling magnetoresistance can be manipulated by voltage pulses associated with much smaller current densities. These results represent a crucial step towards ultralow energy switching in magnetic tunnel junctions, and open a new avenue for exploring other voltage-controlled spintronic devices.

title: Magnetization switching in a CoFeB/MgO magnetic tunnel junction by combining spin-transfer torque and electric field-effect
abstract: We propose and demonstrate a scheme for magnetization switching in magnetic tunnel junctions, in which two successive voltage pulses are applied to utilize both spin-transfer torque and electric field effect. Under this switching scheme, a CoFeB/MgO magnetic tunnel junction with perpendicular magnetic easy axis is shown to switch faster than by spin-transfer torque alone and more reliably than that by electric fields alone.

title: Voltage-control spintronics memory (VoCSM) having potentials of ultra-low energy-consumption and high-density
abstract: We propose a new spintronics-based memory employing the voltage-control-magnetic-anisotropy effect as a bit selecting principle and the spin-orbit-torque effect as a writing principle. We have fabricated the prototype structure, and successfully demonstrated the writing scheme specific to this memory architecture.

title: Pulse voltage-induced dynamic magnetization switching in magnetic tunneling junctions with high resistance-area product
abstract: We investigated pulse voltage-induced dynamic magnetization switchings in magnetic tunneling junctions with a high resistance-area product of 2 kΩ μm2. We found that bistable switching and the oscillatory behavior of switching probability as a function of voltage pulse duration are realized at a lower current density (−1.1 × 105 A/cm2) than in conventional spin-transfer-torque-induced magnetization switching. In addition, the switching probability at different voltage pulse strengths confirmed the existence of a voltage torque induced by a change in perpendicular magnetic anisotropy. This voltage-induced magnetization switching can be a useful technique in future spintronics devices with fast and highly reliable writing processes.

title: Electric field-induced magnetization reversal in a perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction
abstract: The electric field-induced ∼180° magnetization reversal is realized for a sputtered CoFeB/MgO-based magnetic tunnel junction with perpendicular magnetic easy axis in a static external magnetic field. Application of bias voltage with nanoseconds duration results in a temporal change of magnetic easy axis in the free layer CoFeB to in-plane, which induces precessional motion of magnetization in the free layer. The magnetization reversal takes place when the bias voltage pulse duration is adjusted to a half period of the precession. We show that the back and forth magnetization reversal can be observed by using successive application of half-period voltage pulses.

title: In-plane magnetic field effect on switching voltage and thermal stability in electric-field-controlled perpendicular magnetic tunnel junctions
abstract: The effect of in-plane magnetic field on switching voltage (Vsw) and thermal stability factor (Δ) are investigated in electric-field-controlled perpendicular magnetic tunnel junctions (p-MTJs). Dwell time measurements are used to determine the voltage dependence of the energy barrier height for various in-plane magnetic fields (Hin), and gain insight into the Hin dependent energy landscape. We find that both Vsw and Δ decrease with increasing Hin, with a dominant linear dependence. The results are reproduced by calculations based on a macrospin model while accounting for the modified magnetization configuration in the presence of an external magnetic field.

title: Evaluation of write error rate for voltage-driven dynamic magnetization switching in magnetic tunnel junctions with perpendicular magnetization
abstract: We investigated the write error rate (WER) for voltage-driven dynamic switching in magnetic tunnel junctions with perpendicular magnetization. We observed a clear oscillatory behavior of the switching probability with respect to the duration of pulse voltage, which reveals the precessional motion of magnetization during voltage application. We experimentally demonstrated WER as low as 4 × 10−3 at the pulse duration corresponding to a half precession period (~1 ns). The comparison between the results of the experiment and simulation based on a macrospin model shows a possibility of ultralow WER (<10−15) under optimum conditions. This study provides a guideline for developing practical voltage-driven spintronic devices.

title: Reduction in write error rate of voltage-driven dynamic magnetization switching by improving thermal stability factor
abstract: In this study, we demonstrate voltage-driven dynamic magnetization switching for the write error rate (WER) of the order of 10−5. The largest voltage effect on the perpendicular magnetic anisotropy in Ta/(CoxFe100–x)80B20/MgO structure (x = 0, 10, 31, 51) is obtained for x = 31 after annealing at 250 °C. Based on investigations using perpendicularly magnetized magnetic tunnel junctions that have different (Co31Fe69)80B20 free layer thicknesses, we demonstrate that the improvement in the thermal stability factor is important to reduce the WER. Our results will facilitate the design of highly reliable, voltage–torque, magnetoresistive random access memory.

title: Tight-binding approach to the orbital magnetic moment and magnetocrystalline anisotropy of transition-metal monolayers.
abstract: A perturbative theory of magnetocrystalline anisotropy and orbital moment in itinerant ferromagnets is presented that clearly outlines the close connection between these two quantities. The theory is used to study the magnetocrystalline anisotropy in transition-metal monolayers. The importance of the crystal-field energy and of the filling of the valence band is emphasized. For the first time the orbital contribution to the magnetization in monolayers is estimated; it is shown that it may produce an anisotropy in the magnetization of the order of 0.1\ensuremath{\mu}B per atom.

title: Electric-field-induced changes of magnetic moments and magnetocrystalline anisotropy in ultrathin cobalt films
abstract: In this study, the microscopic origins of the voltage-controlled magnetic anisotropy (VCMA) in 3d-ferromagnetic metals are revealed. Using in-situ X-ray fluorescence spectroscopy that provides a high quantum efficiency, electric-field-induced changes in orbital magnetic moment and magnetic dipole Tz terms in ultrathin Co films are demonstrated. An orbital magnetic moment difference of 0.013{\mu}B. was generated in the presence of electric fields of +(-)0.2 V/nm. The VCMA of Co was properly estimated by the induced change in orbital magnetic moment, according to the perturbation theory model. The induced change in magnetic dipole Tz term only slightly contributed to the VCMA in 3d-ferromagnetic metals.

title: Magnetic anisotropy of ferromagnetic metals in low-symmetry systems
abstract: We have constructed an analytic formula to treat the perpendicular magnetic anisotropy energy in ferromagnetic metals with low symmetry, such as C 4 V and C 3 V . We find that the anisotropy energy is proportional to a part of the expectation values of the orbital angular momentum and magnetic dipole operator. Although the result is similar to the model proposed by Laan (1998)  , we have derived a concrete expression for the spin-flip virtual excitation process term, which can be dominant in atoms with small magnetic moments and/or small exchange splitting. Pt monatomic layer with proximity-induced spin polarization grown on Fe is an example of this. Other multilayer systems such as Co/Pd and Co/Ni, and bilayer systems such as Fe(CoB)/MgO can be discussed similarly. Moreover, the relation between perpendicular magnetic anisotropy energy and measurable physical parameters is discussed based on X-ray magnetic circular dichroism spectroscopy.

title: Microscopic origin of magnetocrystalline anisotropy in transition metal thin films
abstract: We investigate the relation between the magnetocrystalline anisotropy energy (MAE) and the electronic structure for transition metal thin films and surfaces which can display enhanced orbital magnetic moments. When the spin-orbit interaction is treated in second order, the MAE is proportional to the expectation value of the orbital magnetic moment as given by Bruno's model. However, there are additional terms which are related to the spin-subband orbital moment and to the magnetic dipole operator due to the anisotropy of the field of the spin. The latter term accounts for the spin-flip excitations between the exchange split majority and minority spin bands. A conjecture is proposed which relates the MAE to the expectation values of the orbital moments and the magnetic dipole term. It is shown how the different terms can be obtained experimentally with (transverse) magnetic circular x-ray dichroism. The model explains the experimentally observed perpendicular magnetic anisotropy in Co and Fe based multilayers and thin films.

title: Voltage controlled interfacial magnetism through platinum orbits
abstract: Electric fields at interfaces exhibit useful phenomena, such as switching functions in transistors, through electron accumulations and/or electric dipole inductions. We find one potentially unique situation in a metal–dielectric interface in which the electric field is atomically inhomogeneous because of the strong electrostatic screening effect in metals. Such electric fields enable us to access electric quadrupoles of the electron shell. Here we show, by synchrotron X-ray absorption spectroscopy, electric field induction of magnetic dipole moments in a platinum monatomic layer placed on ferromagnetic iron. Our theoretical analysis indicates that electric quadrupole induction produces magnetic dipole moments and provides a large magnetic anisotropy change. In contrast with the inability of current designs to offer ultrahigh-density memory devices using electric-field-induced spin control, our findings enable a material design showing more than ten times larger anisotropy energy change for such a use and highlight a path in electric-field control of condensed matter.

title: Giant tunnelling magnetoresistance at room temperature with MgO (100) tunnel barriers
abstract: Magnetically engineered magnetic tunnel junctions (MTJs) show promise as non-volatile storage cells in high-performance solid-state magnetic random access memories (MRAM)1. The performance of these devices is currently limited by the modest (<∼70%) room-temperature tunnelling magnetoresistance (TMR) of technologically relevant MTJs. Much higher TMR values have been theoretically predicted for perfectly ordered (100) oriented single-crystalline Fe/MgO/Fe MTJs. Here we show that sputter-deposited polycrystalline MTJs grown on an amorphous underlayer, but with highly oriented (100) MgO tunnel barriers and CoFe electrodes, exhibit TMR values of up to ∼220% at room temperature and ∼300% at low temperatures. Consistent with these high TMR values, superconducting tunnelling spectroscopy experiments indicate that the tunnelling current has a very high spin polarization of ∼85%, which rivals that previously observed only using half-metallic ferromagnets2. Such high values of spin polarization and TMR in readily manufactureable and highly thermally stable devices (up to 400 °C) will accelerate the development of new families of spintronic devices.

title: X-ray circular dichroism as a probe of orbital magnetization.
abstract: A new magneto-optical sum rule is derived for circular magnetic dichroism in the x-ray region (CMXD). The integral of the CMXD signal over a given edge allows one to determine the ground-state expectation value of the orbital angular momentum. Applications are discussed to transition-metal and rare-earth magnetic systems.

title: X-ray circular dichroism and local magnetic fields.
abstract: Sum rules are derived for the circular dichroic response of a core line (CMXD). They relate the intensity of the CMXD signal to the ground-state expectation value of the magnetic field operators (orbital, spin, and magnetic dipole) of the valence electrons. The results obtained are discussed and tested for transition metals and rare earths.

title: Reversible change in the oxidation state and magnetic circular dichroism of Fe driven by an electric field at the FeCo/MgO interface
abstract: The influence of an electric field on an ultrathin FeCo film was investigated by x-ray absorption spectroscopy and magnetic circular dichroism. Measurements were done on sub-millimeter sized pillars, with partial fluorescence yield detection. Fe L2,3 absorption spectra revealed that partial oxidation of Fe occurred during the microfabrication. The oxidation state could be reversibly controlled by an electric field, which also induced variations of the dichroic signal. These results show that electrochemical phenomena may influence the magnetism at a ferromagnet/insulator interface.

title: Voltage-controlled magnetic anisotropy in Fe|MgO tunnel junctions studied by x-ray absorption spectroscopy
abstract: In this study, voltage-controlled magnetic anisotropy (VCMA) in Fe|MgO tunnel junctions was investigated via the magneto-optical Kerr effect, soft x-ray absorption spectroscopy, and magnetic circular dichroism spectroscopy. The Fe|MgO tunnel junctions showed enhanced perpendicular magnetic anisotropy under external negative voltage, which induced charge depletion at the Fe|MgO interface. Despite the application of voltages of opposite polarity, no trace of chemical reaction such as a redox reaction attributed to O2− migration was detected in the x-ray absorption spectra of the Fe. The VCMA reported in the Fe|MgO-based magnetic tunnel junctions must therefore originate from phenomena associated with the purely electric effect, that is, surface electron doping and/or redistribution induced by an external electric field.

title: Extended X-ray absorption fine structure analysis of voltage-induced effects in the interfacial atomic structure of Fe/Pt/MgO
abstract: The voltage-controlled magnetic anisotropy of ferromagnetic metals may offer potential applications of nonvolatile memories with ultralow power consumption. For achieving ultrafast recording and long-time endurance, voltage-induced effects without undesirable lattice distortions should be ensured. In this study, in-situ extended X-ray absorption fine structure analysis of an Fe/Pt/MgO junction demonstrated the unaltered interfacial atomic structure, in which the radial distances between the Pt and the neighboring Fe, Pt, O, and Mg atoms changed by less than ±0.01 A under electric fields of ±0.18 V/nm. Therefore, the anisotropy change is driven by a purely electronic mechanism without lattice deformation or atomic relaxation.

title: Reversible control of Co magnetism by voltage-induced oxidation.
abstract: We demonstrate that magnetic properties of ultrathin Co films adjacent to Gd2O3 gate oxides can be directly manipulated by voltage. The Co films can be reversibly changed from an optimally oxidized state with a strong perpendicular magnetic anisotropy to a metallic state with an in-plane magnetic anisotropy or to an oxidized state with nearly zero magnetization, depending on the polarity and time duration of the applied electric fields. Consequently, an unprecedentedly large change of magnetic anisotropy energy up to 0.73 erg/cm(2) has been realized in a nonvolatile manner using gate voltages of only a few volts. These results open a new route to achieve ultralow energy magnetization manipulation in spintronic devices.

title: Characterization of the magnetic moments of ultrathin Fe film in an external electric field via high-precision X-ray magnetic circular dichroism spectroscopy
abstract: The magnetic moments of a Pt|Fe(0.5 nm)|MgO film at an external electric field of in a 0.35 V/nm were characterized by analyzing the X-ray magnetic circular dichroism (XMCD) at the L-edges of Fe using a partial fluorescence yield method with a precision 40 times greater than that in our previous study. The XMCD induced by the electric field was negligible (<0.2%). Furthermore, although an induced orbital magnetic moment seemingly existed, it was within the precision error (0.3%). This paper demonstrates that slight electron doping and/or redistribution without any electrochemical reaction causes voltage-controlled magnetic anisotropy at Fe|MgO interfaces.

title: Microscopic Investigation into the Electric Field Effect on Proximity-Induced Magnetism in Pt.
abstract: Electric field effects on magnetism in metals have attracted widespread attention, but the microscopic mechanism is still controversial. We experimentally show the relevancy between the electric field effect on magnetism and on the electronic structure in Pt in a ferromagnetic state using element-specific measurements: x-ray magnetic circular dichroism (XMCD) and x-ray absorption spectroscopy (XAS). Electric fields are applied to the surface of ultrathin metallic Pt, in which a magnetic moment is induced by the ferromagnetic proximity effect resulting from a Co underlayer. XMCD and XAS measurements performed under the application of electric fields reveal that both the spin and orbital magnetic moments of Pt atoms are electrically modulated, which can be explained not only by the electric-field-induced shift of the Fermi level but also by the change in the orbital hybridizations.

title: Magneto-ionic control of interfacial magnetism.
abstract: In metal/oxide heterostructures, rich chemical, electronic, magnetic and mechanical properties can emerge from interfacial chemistry and structure. The possibility to dynamically control interface characteristics with an electric field paves the way towards voltage control of these properties in solid-state devices. Here, we show that electrical switching of the interfacial oxidation state allows for voltage control of magnetic properties to an extent never before achieved through conventional magneto-electric coupling mechanisms. We directly observe in situ voltage-driven O(2-) migration in a Co/metal-oxide bilayer, which we use to toggle the interfacial magnetic anisotropy energy by >0.75 erg cm(-2) at just 2 V. We exploit the thermally activated nature of ion migration to markedly increase the switching efficiency and to demonstrate reversible patterning of magnetic properties through local activation of ionic migration. These results suggest a path towards voltage-programmable materials based on solid-state switching of interface oxygen chemistry.

title: Structural and magnetic depth profiles of magneto-ionic heterostructures beyond the interface limit
abstract: Electric field control of magnetism provides a promising route towards ultralow power information storage and sensor technologies. The effects of magneto-ionic motion have been prominently featured in the modification of interface characteristics. Here, we demonstrate magnetoelectric coupling moderated by voltage-driven oxygen migration beyond the interface in relatively thick AlOx/GdOx/Co(15 nm) films. Oxygen migration and Co magnetization are quantitatively mapped with polarized neutron reflectometry under electro-thermal conditioning. The depth-resolved profiles uniquely identify interfacial and bulk behaviours and a semi-reversible control of the magnetization. Magnetometry measurements suggest changes in the microstructure which disrupt long-range ferromagnetic ordering, resulting in an additional magnetically soft phase. X-ray spectroscopy confirms changes in the Co oxidation state, but not in the Gd, suggesting that the GdOx transmits oxygen but does not source or sink it. These results together provide crucial insight into controlling magnetism via magneto-ionic motion, both at interfaces and throughout the bulk of the films. Mechanisms allowing electrical manipulation of magnetic material possess potential applications in low power memory and sensor technologies. Here, the authors demonstrate the control of magnetic characteristics via voltage-driven migration of oxygen across a GdOx/Co interface, well into the bulk of the cobalt.

title: Observation of an electric field-induced interface redox reaction and magnetic modification in GdOx/Co thin film by means of depth-resolved X-ray absorption spectroscopy.
abstract: We study an electric field-induced redox reaction at the interface of GdOx/Co thin film, by means of soft X-ray absorption spectroscopy (XAS). The fluorescence-yield depth-resolved XAS analysis reveals that the interfacial Co layer at GdOx/Co is composed of ∼50% Co oxide when the negative field is applied, while metallic Co is dominant in the case of the positive field. We suppose that the interfacial layer is oxidized by oxygen migration from the GdOx layer with the negative field while the interfacial layer shows the metallic state with the positive field, which means that the redox reaction is induced by the electric field. In addition, it is found from the X-ray magnetic circular dichroism (XMCD) measurement that the orbital magnetic moment of Co is larger when the negative electric field is applied to the film, as compared to the positive field. Moreover, the depth-resolved XMCD analysis reveals that the interfacial Co layer shows no or little magnetization regardless of the electric field, while for the inner layer, an increase of the orbital magnetic moment is suggested with the negative field. The field-induced magnetic modification could be attributed to the change of the orbital moment in the inner Co layer due to interfacial modifications. We thus succeed in the direct observation of the redox reaction-induced change in the interface magnetic state.

title: Magnetoelectric charge trap memory.
abstract: It is demonstrated that a charge-trapping layer placed in proximity to a ferromagnetic metal enables efficient electrical and optical control of the metal's magnetic properties. Retention of charge trapped inside the charge-trapping layer provides nonvolatility to the magnetoelectric effect and enhances its efficiency by an order of magnitude. As such, an engineered charge-trapping layer can be used to realize the magnetoelectric equivalent to today's pervasive charge trap flash memory technology. Moreover, by supplying trapped charges optically instead of electrically, a focused laser beam can be used to imprint the magnetic state into a continuous metal film.

title: Magnetic anisotropy modified by electric field in V/Fe/MgO(001)/Fe epitaxial magnetic tunnel junction
abstract: Single-crystalline V/Fe(0.7 nm)/MgO(1.2nm)/Fe(20 nm) magnetic tunnel junctions are studied to quantify the influence of an electric field on the Fe/MgO interface magnetic anisotropy. The thinnest Fe soft layer has a perpendicular magnetic anisotropy (PMA), whereas the thickest Fe layer acts as sensor for magnetic anisotropy changes. When electrons are added at the PMA Fe/MgO interface (negative voltage), no anisotropy changes are observed. For positive voltage, the anisotropy constant decreases with increasing bias voltage. A huge 1150 fJ V−1 m−1 anisotropy variation with field is observed and the magnetization is found to turn from out-of-plane to in-plane of the sample with the applied voltage.

title: Giant voltage modulation of magnetic anisotropy in strained heavy metal/magnet/insulator heterostructures
abstract: Ab initio electronic structure calculations reveal that epitaxial strain has a dramatic effect on the voltage-controlled magnetic anisotropy (VCMA) in Ta/FeCo/MgO junctions. Strain can give rise to a wide range of novel VCMA behaviors where the MA can change from a $\ensuremath{\vee}$- to a $\ensuremath{\wedge}$-shape electric-field dependence with giant VCMA coefficients which are asymmetric under voltage reversal. The underlying mechanism is the interplay of the strain- and electric-field-induced changes in the spin-orbit coupled $d$ states at the interfaces and the strain-induced modification of the dielectric constant of MgO. These findings demonstrate the feasibility of highly sensitive VCMA through strain engineering, which may provide a viable avenue for tailoring magnetoelectric properties for spintronic applications.

title: Giant strain control of magnetoelectric effect in Ta|Fe|MgO
abstract: The exploration of electric field controlled magnetism has come under scrutiny for its intriguing magnetoelectric phenomenon as well as technological advances in spintronics. Herein, the tremendous effect of an epitaxial strain on voltage-controlled perpendicular magnetic anisotropy (VPMA) is demonstrated in a transition-metal|ferromagnet|MgO (TM|FM|MgO) heterostructure from first-principles electronic structure computation. By tuning the epitaxial strain in Ta|Fe|MgO as a model system of TM|FM|MgO, we find distinctly different behaviours of VPMA from V- to Λ-shape trends with a substantially large magnetoelectric coefficient, up to an order of 103 fJV−1m−1. We further reveal that the VPMA modulation under strain is mainly governed by the inherently large spin-orbit coupling of Ta 5d–Fe 3d hybridized orbitals at the TM|FM interface, although the Fe 3d–O 2p hybridization at the FM|MgO interface is partly responsible in determining the PMA of Ta|Fe|MgO. These results suggest that the control of epitaxial strain enables the engineering of VPMA, and provides physical insights for the divergent behaviors of VPMA and magnetoelectric coefficients found in TM|FM|MgO experiments.

title: Giant voltage-controlled magnetic anisotropy effect in a crystallographically strained CoFe system
abstract: We experimentally demonstrate a giant voltage-controlled magnetic anisotropy (VCMA) coefficient in a crystallographically strained CoFe layer (~15 monolayers in thickness) in a MgO/CoFe/Ir system. We observed a strong applied voltage dependence of saturation field and an asymmetric concave behavior with giant VCMA coefficients of −758 and 1043 fJ V−1 m−1. The result of structural analysis reveals epitaxial growth in MgO/CoFe/Ir layers and the orientation relationship MgO(001)[110] ∥ CoFe(001)[100] ∥ Ir(001)[110]. The CoFe layer has a bcc structure and a tetragonal distortion due to the lattice mismatch; therefore, the CoFe layer has a large perpendicular magnetic anisotropy.

title: Large Voltage-Induced Changes in the Perpendicular Magnetic Anisotropy of an MgO-Based Tunnel Junction with an Ultrathin Fe Layer
abstract: Voltage control of magnetic anisotropy (VCMA) in an ultrathin ferromagnet metal layer is a promising approach for ultralow-power spin manipulation, but interface effects spoil its efficiency for applications such as memory devices. The authors study VCMA in the fully epitaxial ultrathin Fe layer of a magnetic tunnel junction, and achieve an efficiency of almost 300 fJ V${}^{-1}$ m${}^{-1}$ with high perpendicular anisotropy, which satisfies the requirements for gigabit-class spintronic memory.

title: Large perpendicular magnetic anisotropy at Fe/MgO interface
abstract: A large perpendicular magnetic anisotropy (PMA) of 1.4 MJ/m3 was observed from ultrathin Fe/MgO(001) bilayers grown on Cr-buffered MgO(001). The PMA strongly depends on the surface state of Fe prior to the MgO deposition. A large PMA energy density of 1.4 MJ/m3 was achieved for a 0.7 nm thick Fe layer having adsorbate-induced surface reconstruction, which is likely to originate from oxygen atoms floating up from the Cr buffer layer. This large magnitude of PMA satisfies the criterion that is required for thermal stability of magnetization in a few tens nanometer-sized magnetic memory elements.

title: Quantifying perpendicular magnetic anisotropy at the Fe-MgO(001) interface
abstract: We show that Fe-MgO interfaces possess strong perpendicular magnetic anisotropy of 1.0 ± 0.1 erg/cm2 in fully epitaxial MgO/V/Fe/MgO(001) and MgO/Cr/Fe/MgO(001) heterostructures. The sign and amplitude of the total anisotropy are quantified as a function of Fe thickness using magnetometry and ferromagnetic resonance. There is a transition from out-of-plane to in-plane anisotropy for 6 Fe monolayers in V/Fe/MgO and only 4 monolayers in Cr/Fe/MgO. A detailed study of the Fe magnetization and effective anisotropy in both systems explains this difference and quantifies the Fe-MgO interface anisotropy.

title: Perpendicular magnetic anisotropy at the interface between ultrathin Fe film and MgO studied by angular-dependent x-ray magnetic circular dichroism
abstract: Interface perpendicular magnetic anisotropy (PMA) in ultrathin Fe/MgO (001) has been investigated using angular-dependent x-ray magnetic circular dichroism (XMCD). We found that anisotropic orbital magnetic moments deduced from the analysis of XMCD contribute to the large PMA energies, whose values depend on the annealing temperature. The large PMA energies determined from magnetization measurements are related to those estimated from the XMCD and the anisotropic orbital magnetic moments through the spin-orbit interaction. The enhancement of anisotropic orbital magnetic moments can be explained mainly by the hybridization between the Fe 3dz2 and O 2pz states.

title: Investigation of Au and Ag segregation on Fe(001) with soft X-ray absorption
abstract: We investigated the X-ray absorption at the Fe L3 edge of Fe grown on Cr(001), Au(001) and Ag(001). Systematic variations in the absorption were observed depending on the nature of the buffer. They are explained by segregation of Au and Ag during Fe deposition at room temperature. By measuring wedge shaped samples, the segregated amounts could be measured. Taking into account the possible influence of roughness, we find that the segregated amounts of Au and Ag on a 1 nm-thick Fe film are 0.8 ± 0.3 ML and 2.0 ± 0.3 ML, respectively.

title: A Comprehensive Study of Sign Change in Electric Field Control Perpendicular Magnetic Anisotropy Energy at Fe/MgO Interface: First Principles Calculation
abstract: Electric field (EF) control on magnetic anisotropy energy (MAE) in the system which contains Fe/MgO interface with Cr underlayer was performed by means of first-principles electronic structure calculations. An opposite sign in EF coefficient <inline-formula> <tex-math notation="LaTeX">$(\gamma)$ </tex-math></inline-formula> was observed by introducing Fe/Cr intermixing. This intermixing leads to a substantial rearrangement in the electronic structure due to proximity effects of Fe and Cr. The origin of such an opposite sign in <inline-formula> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula> was discussed along with the modulation of occupied and unoccupied <inline-formula> <tex-math notation="LaTeX">$d$ </tex-math></inline-formula>-band near the Fermi level. By introducing a strain effect, the enhancements of MAE and <inline-formula> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula> were achieved with the maximum absolute values, 2.43 mJ/m<sup>2</sup> and 170 fJ/Vm, respectively. The present result provides a new possible degree of freedom for the EF control in MAE as the underlayer configuration.

title: Model of orbital populations for voltage-controlled magnetic anisotropy in transition-metal thin films
abstract: Voltage-controlled magnetic anisotropy (VCMA) is an efficient way to manipulate the magnetization states in nanomagnets and is promising for low-power spintronic applications. The underlying physical mechanism for VCMA is known to involve a change in the $d$ orbital occupation on the transition-metal interface atoms with an applied electric field. However, a simple qualitative picture of how this occupation controls the magnetocrystalline anisotropy (MCA) and even why in certain cases the MCA has the opposite sign remains elusive. In this paper, we exploit a simple model of orbital populations to elucidate a number of features typical for the interface MCA, and the effect of the electric field on it, for $3d$ transition-metal thin films used in magnetic tunnel junctions. We find that in all considered cases, including the Fe(001) surface, clean $\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{x}(001)/\mathrm{MgO}$ interface, and oxidized Fe(001)/MgO interface, the effects of alloying and the electric field enhance the MCA energy with electron depletion, which is largely explained by the occupancy of the minority-spin ${d}_{xz,yz}$ orbitals. However, the hole-doped Fe(001) exhibits an inverse VCMA in which the MCA enhancement is achieved when electrons are accumulated at the Fe (001)/MgO interface with the applied electric field. In this regime, we predict a significantly enhanced VCMA that exceeds 1 pJ/Vm. Realizing this regime experimentally may be favorable for the practical purpose of voltage-driven magnetization reversal.

title: Effect of heavy-metal insertions at Fe/MgO interfaces on electric-field-induced modification of magnetocrystalline anisotropy
abstract: Magnetocrystalline anisotropy (MCA) at Fe/MgO interfaces with insertions of 3d (Co, Ni), 4d (Ru, Rh, Pd), and 5d (Os, Ir, Pt) elements in external electric fields was investigated from first-principles calculations. The MCA energy and the electric-field-induced MCA modification dramatically depend on the inserted elements. Large MCA modification may be achieved by heavy-metal insertions, in which the strength of spin-orbit coupling of inserted elements and the position of the Fermi level relative to d band level play key roles.

title: Enhancement of electric-field-induced change of magnetic anisotropy by interface engineering of MgO magnetic tunnel junctions
abstract: Electric-field-induced modification of magnetic anisotropy is studied using tunnel magnetoresistance of the Co40Fe40B20/ MgO/ Co40Fe40B20 and Co40Fe40B20/ Hf (0.08 nm)/ MgO/ Co40Fe40B20 magnetic tunnel junctions. In both systems, the interfacial perpendicular magnetic anisotropy is increased with increasing electron density at the MgO interface. A quantitative comparison between the two systems reveals that the change of magnetic anisotropy energy with electric field is significantly enhanced in Co40Fe40B20/ Hf/ MgO/ Co40Fe40B20 compared to Co40Fe40B20/ MgO/ Co40Fe40B20. The sub-monolayer Hf insertion at the Co40Fe40B20/MgO interface turns out to be critical to the enhanced electric field control of the magnetic anisotropy, indicating the interface sensitive nature of the effect.

title: Enhancement in the interfacial perpendicular magnetic anisotropy and the voltage-controlled magnetic anisotropy by heavy metal doping at the Fe/MgO interface
abstract: We investigated the influence of heavy metal doping at the Fe/MgO interface on the interfacial perpendicular magnetic anisotropy (PMA) and the voltage-controlled magnetic anisotropy (VCMA) in magnetic tunnel junctions prepared by sputtering-based deposition. The interfacial PMA was increased by tungsten doping and a maximum intrinsic interfacial PMA energy, Ki,0 of 2.0 mJ/m2 was obtained. Ir doping led to a large increase in the VCMA coefficient by a factor of 4.7 compared with that for the standard Fe/MgO interface. The developed technique provides an effective approach to enhancing the interfacial PMA and VCMA properties in the development of voltage-controlled spintronic devices.

title: Current-driven spin torque induced by the Rashba effect in a ferromagnetic metal layer.
abstract: Methods to manipulate the magnetization of ferromagnets by means of local electric fields or current-induced spin transfer torque allow the design of integrated spintronic devices with reduced dimensions and energy consumption compared with conventional magnetic field actuation. An alternative way to induce a spin torque using an electric current has been proposed based on intrinsic spin-orbit magnetic fields and recently realized in a strained low-temperature ferromagnetic semiconductor. Here we demonstrate that strong magnetic fields can be induced in ferromagnetic metal films lacking structure inversion symmetry through the Rashba effect. Owing to the combination of spin-orbit and exchange interactions, we show that an electric current flowing in the plane of a Co layer with asymmetric Pt and AlO(x) interfaces produces an effective transverse magnetic field of 1 T per 10(8) A cm(-2). Besides its fundamental significance, the high efficiency of this process makes it a realistic candidate for room-temperature spintronic applications.

title: Reduction in the write error rate of voltage-induced dynamic magnetization switching using the reverse bias method
abstract: Using macro-spin modeling, we studied the reduction in the write error rate (WER) of voltage-induced dynamic magnetization switching by enhancing the effective thermal stability of the free layer using a voltage-controlled magnetic anisotropy change. Marked reductions in WER can be achieved by introducing reverse bias voltage pulses both before and after the write pulse. This procedure suppresses the thermal fluctuations of magnetization in the initial and final states. The proposed reverse bias method can offer a new way of improving the writing stability of voltage-driven spintronic devices.

title: Voltage-Induced Precessional Switching at Zero-Bias Magnetic Field in a Conically Magnetized Free Layer
abstract: Voltage-induced magnetization dynamics in a conically magnetized free layer with an elliptic cylinder shape is theoretically studied on the basis of the macrospin model. It is found that an application of voltage pulse can induce the precessional switching of magnetization even at zero-bias magnetic field, which is of substantial importance for device applications such as voltage-controlled nonvolatile memory. Analytical expressions of the conditions for precessional switching are derived.

title: Spin-reorientation transitions in ultrathin Co films on Pt(111) and Pd(111) single-crystal substrates
abstract: Spin-reorientation transition (SRT) behaviors in ultrathin Co films grown on both Pt(111) and Pd(111) single-crystal substrates are investigated with in situ magneto-optical Kerr effects. At the region of SRT from the perpendicular to in-plane magnetization with the increasing of Co thickness, canted out-of-plane orientations are observed, from which we can determine not only the effective second- ${(K}_{2})$ but also fourth- ${(K}_{4})$ order anisotropy constants. Both the constants, which can be separated into bulk ${K}_{2b(4b)}$ and interface anisotropy terms ${K}_{2s(4s)}$ are essential to understand the stable canted out-of-plane magnetization. We found that the value of ${K}_{2s}$ for Co/Pt(111) is larger than that for Co/Pd (111), resulting in the later onset thickness and wider range of the observed SRT, while the negative small values of ${K}_{4s}$ in the both systems yield the stable canted phases. In addition, we observed different polarization effects of Pd and Pt near the interfaces which might also contribute the contrasting SRT behaviors.

title: Anisotropies, cone states, and stripe domains in Co/Pt multilayers
abstract: We present results for perpendicular uniaxial anisotropies obtained using torque magnetometry and magnetic force microscopy on epitaxially grown Co/Pt multilayers. A strong dependence of the two lowest order uniaxial anisotropies on Pt film thickness and temperature were observed. The possibility of an interfacial strain origin for the anisotropies due to lattice mismatch between the Co and Pt is discussed. A surprising result is the appearance of magnetic orientations at oblique angles with respect to the film plane and the surface normal. These orientations are identified as “cone states” and found to exist in limited ranges of Pt film thickness and temperatures. The temperature dependence is very unusual and controlled through the behavior of the lowest order perpendicular uniaxial anisotropies. The magnetization process was also examined in terms of saturation fields and domain formation. Narrow stripe domains with periods less than 100 nm were observed in the demagnetized state using magnetic force m...

title: Magnetic anisotropy and magnetization reversal processes in Pt/Co/Pt films
abstract: The dependence of the magnetic anisotropy, magnetization reversal processes and magnetic domain patterns in Pt/Co(h)/Pt(1 1 1) ultra-thin ferromagnetic films have been studied, as a function of the Co layer thickness d (0.5⩽d⩽3 nm), by magneto-optical polar Kerr magnetometry and microscopy. The anisotropy constants have been determined from the field-induced coherent rotation magnetization curves measured for different sets of in-plane, H‖, and perpendicular, H⊥, applied fields. The particular variation of Kerr rotation and anisotropy at small thickness (d

title: Temperature dependence of perpendicular magnetic anisotropy in CoFeB thin films
abstract: We study perpendicular magnetic anisotropy in thin films of Ta/Co20Fe60B20/MgO by ferromagnetic resonance and find a linear temperature dependence for the first and second order uniaxial terms from 5 to 300 K. Our data suggest the possible hybridization of Fe-O orbitals at the CoFeB/MgO interface for the origin of the first order anisotropy. However, we also find that non-interfacial contributions to the anisotropy are present. An easy-cone anisotropy is found for the entire temperature range in the narrow region of film thicknesses around the spin reorientation transition 1.2–1.35 nm.

title: Electric field control of magnetic anisotropy in the easy cone state of Ta/Pt/CoFeB/MgO structures
abstract: The electric-field control of magnetic anisotropy is of particular interest because it allows the manipulation of the magnetization direction in spintronic devices with high performance and low power consumption. In this work, we investigate the effect of an electric field on the magnetic anisotropy in Ta/Pt/CoFeB/MgO structures, whose easy axis of magnetization is canted from the z-axis, forming a cone state. When an electric field is applied to the sample, its anisotropy constants change, thus modulating the cone state. It is demonstrated that the cone angle is controlled between 22° and 32° by a bias field of 4 MV/cm and that it can persist even after removing the bias. Moreover, it fully recovers to the original value when a bias voltage with an opposite polarity is applied. The non-volatile and reversible control of the cone state paves the way towards the utilization of the magnetic cone state in spintronic devices.