Magnetic Logic Research Articles

Magnetic vortex transistor based tri-state buffer Switch

Magnetic analogue of electronic gates are advantageous in many ways. There is no electron leakage, higher switching speed and more energy saving in a magnetic logic device compared to a semiconductor one. Recently, we proposed a magnetic vortex transistor and fan-out devices based on carefully coupled magnetic vortices in isolated nanomagnetic disks. Here, we demonstrate a new type of magnetic logic gate based upon asymmetric vortex transistor by using micromagnetic simulation. Depending upon two main features (topology) of magnetic vortex, circulation and polarity, the network can behave like a tri-state buffer. Considering the asymmetric magnetic vortex transistor as a unit, the logic gate has been formed where two such transistors are placed parallel and another one is placed at the output. Magnetic energy given in the input transistors is transferred to the output transistor with giant amplification, due to the movement of antivortex solitons through the magnetic stray field. The loss and gain of energy at the output transistor can be controlled only by manipulating the polarities of the middle vortices in input transistors. Due to the asymmetric energy transfer of the antivortex solitons, we have shown successful fan-in operation in this topologically symmetric system. A tri-state buffer gate with fan-in of two transistors can be formed. This gate can be used as a ‘Switch’ to the logic circuit and it has technological importance for energy transfer to large scale vortex networks.

Enhancement of perpendicular magnetic anisotropy of ferromagnet/oxide heterointerface by an oxygen-dependent orbital modulation

Tailoring the magnetic anisotropy of a ferromagnet film is the critical issue to construct high-efficient magnetic memory and logic devices. In this paper, we demonstrated a controllable magnetic anisotropy of Pt/Co/HfO2 multilayers by tuning the oxygen-dependent orbital occupancy at the Co/HfO2 interface. Driven by the large difference in formation enthalpy between HfO2 with CoO, an effective O2− migration occurred at the Co/HfO2 interface by thermal activation, which increased the electronic occupation on the Co 3dz2 orbit and facilitated the out-of-plane Co-O orbital hybridization. Therefore, the interfacial magnetic anisotropy energy was enhanced by two times, leading to a transition from in-plane to out-of-plane anisotropy. Conversely, the O2− migration was suppressed when HfO2 was replaced by ZnO with the formation enthalpy close to CoO, which makes the orbital tunability effect disappear and the magnetic anisotropy of film unchangeable. These findings are crucial for exploiting the magnetoionic control of interfacial magnetism and advancing the functionalized design of memorizers based on orbital engineering.

A Novel Low Power and Reduced Transistor Count Magnetic Arithmetic Logic Unit Using Hybrid STT-MTJ/CMOS Circuit

One of the major concern for CMOS technology is the increase in power dissipation as the technology node lowers down to deep submicron region. Magnetic tunnel junction (MTJ) working on Spin transfer torque (STT) switching mechanism is recognized as one of the most promising spintronic device for post CMOS era due to its non-volatility, high speed, high endurance, CMOS compatibility and mainly the low power dissipation which can offer the solutions for the problems posed by existing CMOS technology. We have proposed a novel logic-in-memory (LIM) architecture of magnetic arithmetic logic unit (P-MALU) based on hybrid STT-MTJ/CMOS circuits. Simulation results reveal that there is significant reduction in the total power dissipation and transistor count of arithmetic unit by 28.44% and 29.16% compared to double pass transistor logic based clocked CMOS ALU design (DPTL-C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> MOS-ALU), while 58.87% and 45.16% to modified magnetic arithmetic logic unit (M-MALU) respectively. Reduction in average power dissipation for logical unit is 37.61% and 52.55% along with 47.22% and 42.42% fewer transistors than DPTL-C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> MOS-ALU and M-MALU design respectively. Monte-Carlo(MC) simulation is then performed by incorporating process and mismatch variations for CMOS and extracted parameters of MTJ, to study the behavior of DPTL-C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> MOS-ALU, M-MALU and P-MALU designs in terms of power dissipation. All the simulation results reveal that the P-MALU is superior than other two ALU designs in terms of power dissipation, delay and device count. Further, the P-MALU circuit is extended for 4-bits arithmetic operations. Electrical simulations are performed to verify the functionality of the design for higher bit operations which demonstrates the feasibility of the proposed design in VLSI circuits.

Helical magnetic ordering in thin FeGe membranes

A detailed magnetic phase diagram of FeGe membranes with fields applied out-of-plane is determined using small-angle soft x-ray elastic scattering. In addition to the well-established skyrmion and helical phase, we identify an additional ordered phase which partly coexists with the helical phase in a pocket beneath the skyrmion phase. Furthermore, an evolution of the modulation wave vector, $\mathbf{q}$, is observed when traversing the magnetic phase pocket. For the ordinary and the additional helical phase, $\mathbf{q}$ only varies with normalized field but not with temperature---in sharp contrast to the skyrmion phase, for which $\mathbf{q}$ is both field and temperature dependent. This study of the helical phases in thin FeGe membranes lays the foundation for their use in future magnetic storage and logic devices.

AFM-Based Spin-Exchange Microscopy Using Chiral Molecules.

Local magnetic imaging at nanoscale resolution is desirable for basic studies of magnetic materials and for magnetic logic and memories. However, such local imaging is hard to achieve by means of standard magnetic force microscopy. Other techniques require low temperatures, high vacuum, or strict limitations on the sample conditions. A simple and robust method is presented for locally resolved magnetic imaging based on short-range spin-exchange interactions that can be scaled down to atomic resolution. The presented method requires a conventional AFM tip functionalized with a chiral molecule. In proximity to the measured magnetic sample, charge redistribution in the chiral molecule leads to a transient spin state, caused by the chiral-induced spin-selectivity effect, followed by the exchange interaction with the imaged sample. While magnetic force microscopy imaging strongly depends on a large working distance, an accurate image is achieved using the molecular tip in proximity to the sample. The chiral molecules' spin-exchange interaction is found to be 150 meV. Using the tip with the adsorbed chiral molecules, two oppositely magnetized samples are characterized, and a magnetic imaging is performed. This method is simple to perform at room temperature and does not require high-vacuum conditions.

Investigating the influence of the depinning energy barrier on domain wall motion in perpendicularly magnetized Pt/Co/Cr/Ta multilayers

Abstract Domain wall (DW) motion via current induced spin-orbit torque (SOT) and/or a magnetic field in ferromagnet/heavy metal heterostructure has attracted abundant interests due to its potential applications in magnetic memories and logic devices. In this work, we study the influence of the depinning energy barrier on DW motion in the creep regime using the polar magneto-optical Kerr microscopy in the Cr layer decorated Pt/Co/Ta structure. We find that although the SOT efficiency shows a significant enhancement in the Pt/Co/Cr/Ta system, but the current-induced DW motion efficiency still shows an attenuation. We confirm that this unexpected result is caused by the enhanced pinning energy barrier, extracted from the relationship between the cumulative depinning probability (Pd) and the depinning time (t). Moreover, the relationship between Pd and t exhibits preferred exponential distribution, indicating that a single energy barrier governs the depinning behavior in our systems. We also reveal that the energy barrier depends on the interface between the ferromagnet and heavy metal. Our study suggests that the current-induced DW motion efficiency is not only determined by the SOT efficiency, but also the depinning energy barrier.

Magnetic logic inverter from crossed structures of defect-free graphene with large unsaturated room temperature negative magnetoresistance

Introducing defects into graphene has been widely utilized to realize the negative magnetoresistance (MR) effect in graphene. However, the reported graphene negative MR exhibits only ∼ 10% under 10 T at room temperature to date, which extremely limits the resolution of future spintronics devices. Moreover, intentional defect introduction can also cause unintentional degradation in graphene’s intrinsic properties. In this paper, we report a magnetic logic inverter based on a crossed structure of defect-free graphene, resulting in a substantial gain of 4.81 mV/T while exhibiting room temperature operation. This crossed structure of graphene shows large unsaturated room temperature negative MR with an enhancement of up to 1,000% at 9 T. A transition behavior between negative and positive MR is observed in this crossed structure and the transition temperature can be tuned by a ratio of the conductivity between in-plane and out-of-plane transport. Our results open an intriguing path for future two-dimensional spintronics device applications.

Voltage pulse-induced fast and repeated switching in a uniaxial nanomagnet at room temperature

Abstract Fast 180° magnetization switching is one of the main challenges for straintronics to be used in magnetic storage and logic. By deflecting the electrodes pair axis by a small angle from the long axis of a uniaxial multiferroic nanomagnet, the voltage pulse can directly induce the nanomagnet to complete repeatable 180° switchings without requiring a start-up time. The dynamic magnetization of the repeatable 180° switching in the nanomagnet and the optimal voltage pulse waveform is simulated through micromagnetic method. Is obtained by simulation calculation, when the electrodes pair axis tilting angle is about 5°, the required stress is small, the switching frequency is high, and the initial switching time is short, which is about 1/5 of the existing research scheme. The switching at room temperature is also calculated by a mathematical model. The switching time is longer under the influence of thermal fluctuations. These findings may provide guidance for the application of straintronic devices.

Control of vortex chirality in bi-component magnetic nanodisks

In the last decade, special attention has been paid to the nucleation and control of the vortex state in magnetic nanostructures, for possible applications in nonvolatile magnetic random access memories and logic networks. Here, we study bicomponent magnetic nanodisks, in both single and 2D array arrangements, as potential storage systems where the information unit is represented by vortex chirality (magnetization rotational direction). We mainly focus on 300 nm diameter disks made of a permalloy lens and an iron crescent, with variable locations of the permalloy-iron interface. Through an extensive micromagnetic modeling analysis, we demonstrate the possibility of tuning, via the application of a magnetic field parallel to the interface, the nucleation of the vortex (always occurring in the permalloy region), its motion up to the expulsion site, and its chirality. Moreover, we find that the vortex stability is preserved over a wide field range also for strongly packed disks, due to the reduced effects of interdisk magnetostatic coupling. The main features of the magnetization reversal mechanism are preserved by reducing the disk diameter up to about 60 nm, making possible their exploitation in concept devices for magnetic encoding and logic computation.

Lowering critical current density for spin-orbit torque induced magnetization switching by ion irradiation

In ferromagnet/heavy metal heterostructures, critical current density (JC) refers to the minimum current density required to generate spin–orbit torque (SOT) for effective magnetization manipulation, including switching of magnetization and moving of domain walls and magnetic skyrmions. This critical current density is a key factor for next-generation SOT-based magnetic random access memory, racetrack memory, and logic devices. In this work, the critical current density for magnetization switching and the thermal stability of Pt/Co/Ta heterostructures in response to ion irradiation are studied. It is found that ion irradiation represents a promising approach for the wide tuning of the magnetic properties, such as the coercive force and the perpendicular magnetic anisotropy constant. It is also found that JC is significantly reduced after 500 eV Ar+ irradiation. Meanwhile, the ratio between the thermal stability factor E/kBT and JC increases with an increase in the irradiation dose, although E/kBT decreases slightly with the irradiation dose. This work demonstrates that JC can be significantly reduced by an appropriate ion irradiation process and thereby demonstrates a promising approach for effective reduction of the power consumption in SOT-based spintronic devices.

Spin-Orbit-Torque Switching Mediated by an Antiferromagnetic Insulator

Spin-orbit torque is widely studied as a switching mechanism for magnetic memory and spin logic devices. In typical device configurations, a lot of current flows through the shunting path of the ferromagnetic metal, significantly increasing energy consumption. This work demonstrates that spin-torque transfer can be realized across an antiferromagnetic insulator: By applying current in the heavy-metal layer, one can achieve magnetic switching in the magnetic free layer, despite the antiferromagnetic-insulator layer in between. Thus electrically isolated reading and writing paths in spin-orbit-torque devices, for energy-thrifty favorable magnetic control, are possible.

Two-dimensional skyrmion bags in liquid crystals and ferromagnets

Reconfigurable, ordered matter offers great potential for future low-power computer memory by storing information in energetically stable configurations. Among these, skyrmions—which are topologically protected, robust excitations that have been demonstrated in chiral magnets1–4 and in liquid crystals5–7—are driving much excitement about potential spintronic applications8. These information-encoding structures topologically resemble field configurations in many other branches of physics and have a rich history9, although chiral condensed-matter systems so far have yielded realizations only of elementary full and fractional skyrmions. Here we describe stable, high-degree multi-skyrmion configurations where an arbitrary number of antiskyrmions are contained within a larger skyrmion. We call these structures skyrmion bags. We demonstrate them experimentally and numerically in liquid crystals and numerically in micromagnetic simulations either without or with magnetostatic effects. We find that skyrmion bags act like single skyrmions in pairwise interaction and under the influence of current in magnetic materials, and are thus an exciting proposition for topological magnetic storage and logic devices. Structures containing multiple skyrmions inside a larger skyrmion—called skyrmion bags—are experimentally created in liquid crystals and theoretically predicted in magnetic materials. These may have applications in information storage technology.

Spin orbit torque driven magnetization switching with sputtered Bi2Se3 spin current source

Current induced spin orbit torques (SOTs) offer an efficient pathway to manipulate the magnetization of a ferromagnet for future magnetic memories and logic devices. Among the various non-magnets utilized, the topological insulators, such as Bi2Se3, have proven to be one of most efficient spin current generators for SOTs. So far, the preferred growth technique for such materials is the molecular beam epitaxy technique which is not compatible with the magnetic memory industry. In this study, we utilize a sputter deposited Bi2Se3 to demonstrate highly efficient SOT generation and subsequently magnetization switching.

Magnetic field-driven spintronic logic gates in one-dimensional manganese phthalocyanine nanoribbons based molecular spintronic devices

Abstract Along with the development of molecular electronics and spintronics, how to realize logic operation in nanoscale molecular devices has been a key issue. Thus it is essential to achieve basic logic gates in a small scale, which are the fundamental elements of digital circuits. By using nonequilibrium Green's function method and density functional theory, we propose an ideal candidate: one-dimensional (1D) manganese phthalocyanine nanoribbons (MnPcNRs), with half-metallic characteristics for making spintronic logic gates. Firstly, we calculate the spin transport properties of 1D MnPcNRs, in which the spin orientation of different areas can be changed by applying magnetic fields. Nearly 100% spin-polarized current can be generated and tuned by a proper magnetic configuration in these two-terminal MnPcNRs devices. Moreover, giant magnetoresistance, negative different resistance and dual-orientation spin rectification effects can be also detected in these devices. These unique transport properties are attributed to the intrinsic transmission selection rule of the wave function of spin subbands near the Fermi level in 1D MnPcNRs. More interestingly, the current-voltage relationships under the control of magnetic configuration of different areas in MnPcNR devices can be used to design spin logic devices. It is worth mentioning that the design proposal we suggested realizes to implement multiple logic operations in a single device. Our result suggests the application potential of 1D MnPcNRs in future nanoelectronics, and also provides a feasible solution to design spintronic integrated circuit in atomic scale.

Design of NAND/NOR Logic Gate Clocked by Voltage-generated Stress

By using the preferred magnetization of clock-tilted nanomagnet, we propose a design of NAND/NOR magnetic logic gates clocked by voltage-generated stress. Unlike designs based on slanted nanomagnet, our design uses regular-shaped nanomagnets, allowing high ratio (2:1) of the nanomagnets to be used. It can eliminate C-shaped and eddy current clock errors, and reduce the complexities of fabrication process. In addition, our design consumes only one-tenth the energy of general designs using a spintronics clock.

Modeling of stress-regulated AND (OR) logic gate based on flipping preference of tilted nanomagnet

Nano-magnetic logic device (NMLD) is a novel nanoelectronic device that stores, processes, and transfers information by dipole-coupled magneto-static interactions between nanomagnets. In the NMLD, long axis tilted nanomagnet attracts the attention of researchers due to its flexibility in magnetic logic design. Edge-slanted nanomagnet is wildly used, whose long axis is tilted due to its asymmetric shape. However, there are three defects in edge-slanted nanomagnets. 1) This type of nanomagnet requires a larger size, thus increasing the nano-magnetic logic (NML) space and introducing the C-shape and vortex clock errors that are often found in large-sized nanomagnets. 2) The irregular shape of nanomagnet increases the requirements for fabrication. 3) Complex calculations caused by the irregular shape are inevitable.&lt;/br&gt;In this paper, the tilt of the long axis of the nanomagnet is realized by placing the regular-shaped (elliptical cylinder) nanomagnet (50 nm×100 nm×20 nm) obliquely. According to the flipping preference of tilted nanomagnet, the authors design a two-input AND (OR) logic gate clocked by stress. The authors choose PMN-PT (Pb (Mg&lt;sub&gt;1/3&lt;/sub&gt;Nb&lt;sub&gt;2/3&lt;/sub&gt;) O&lt;sub&gt;3&lt;/sub&gt;-PbTiO&lt;sub&gt;3&lt;/sub&gt;) as the piezoelectric layer material to use its high piezoelectric coefficient. For magnetic materials, the authors choose Terfenol-D (Tb&lt;sub&gt;0.7&lt;/sub&gt;Dy&lt;sub&gt;0.3&lt;/sub&gt;Fe&lt;sub&gt;2&lt;/sub&gt;), whose magnetic crystal anisotropy is smaller. The material of the subatrate is not discussed in this paper, which will be further studied in future experimental work. The mathematical model is established, and the dynamic magnetization of the gate is calculated. A stress of 90 MPa is applied to the output nanomagent for 3 ns. The nanomagnet is flipped to “NULL” at 1.8 ns and is then flipped to the final stable state after the stress has been removed for 0.9 ns. The output will become logic “0” (“1”) only if the input is logic “00” (“11”), otherwise the output will be logic “1” (“0”), thus successfully implementing OR (AND) logic. In addition, the gate is simulated by using the micromagnetic method. The results are basically consistent with our model. Unlike the designs based on edge-slanted nanomagnets, the basic logic gate based on tilted nanomagnets has three advantages. 1) This design allows high-aspect-ratio (2:1) nanomagnets to be used in logic functions. Therefore, less vortex and C-shaped error will be generated. 2) The regular shape can reduce the fabrication requirements and computational complexities. 3) Using stress as a clock, the energy consumption is greatly reduced, which can be only one-tenth of the general designs clocked by spin electronics.&lt;/br&gt;This model provides a greater energy efficiency and reliable basic logic unit for NML design. In the experimental preparation, there may be a large preparation error tilting the nanomagnet. As a solution, the stress electrodes can be tilted instead. So the stress will also make an angle with respect to the long axis of the nanomagnet.

Nitrogen Tuned Charge Redistribution and Orbital Reconfiguration in Fe/MgO Interface for Significant Interfacial Magnetism Tunability

AbstractModulating the orbital configuration of ferromagnetic metal (FM)/metal‐oxide (MO) interfaces is crucial for obtaining a controllable interfacial magnetism for constructing energy‐efficient magnetic memory and logic devices. The traditional works of orbital regulation depend on external fields, such as electric field, temperature field, and stress field. This work proposes a novel orbital modulation strategy by modifying the coordination environment of FM/MO interface with nitrogen (N) incorporation. By preparing a Fe/MgO bilayer at a N2 atmosphere, N atoms occupy the interstitial sites of the Fe lattice, which induces a charge redistribution at the Fe/MgO interface and toggles a prominent orbital reconstruction of Fe with an increment of out‐of‐plane orbital occupancy. Therefore, the orbital magnetism is tuned effectively, which remarkably strengthens the interfacial magnetic anisotropy energy by 0.6 erg cm−2 and enables a broad magnetic anisotropy tunability from in‐plane to perpendicular direction. Besides, the Fe thickness for maintaining perpendicular magnetic anisotropy extends from less than 1 to 3 nm, which is favorable for improving the signal‐to‐noise ratio and stability of devices in nanoscale. These findings provide an external‐field‐independent strategy of orbital engineering for tailoring obit‐controlled performance at FM/MO heterointerfaces, which practically advances the magnetic storage and logic devices.

Hybrid Magnetodynamical Modes in a Single Magnetostrictive Nanomagnet on a Piezoelectric Substrate Arising from Magnetoelastic Modulation of Precessional Dynamics.

Magnetoelastic (or "straintronic") switching has emerged as an extremely energy-efficient mechanism for switching the magnetization of magnetostrictive nanomagnets in magnetic memory and logic, and non-Boolean circuits. Here, we investigate the ultrafast magnetodynamics associated with straintronic switching in a single quasielliptical magnetostrictive Co nanomagnet deposited on a piezoelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 substrate using time-resolved magneto-optical Kerr effect (TR-MOKE) measurements. The pulsed laser pump beam in the TR-MOKE plays a dual role: it causes precession of the nanomagnet's magnetization about an applied bias magnetic field and it also generates surface acoustic waves in the piezoelectric substrate that produce periodic strains in the magnetostrictive nanomagnet and modulate the precessional dynamics. This modulation gives rise to intriguing hybrid magnetodynamical modes in the nanomagnet, with a rich spin-wave texture. The characteristic frequencies of these modes are 5-15 GHz, indicating that strain can affect magnetization in a magnetostrictive nanomagnet in time scales much smaller than 1 ns (∼100 ps). This can enable ∼10 GHz range magnetoelastic nano-oscillators that are actuated by strain instead of a spin-polarized current, as well as ultrafast magnetoelectric generation of spin waves for magnonic logic circuits, holograms, etc.

Tailoring the Current-Driven Domain Wall Motion by Varying the Relative Thickness of Two Heavy Metal Underlayers

Current-driven domain wall motion (DWM) in heavy metal (HM)/ferromagnet (FM)/oxide heterogeneous structures has attracted strong research interests due to its potential applications in novel magnetic memory and logic devices. It has been reported that current-driven DWM can be either along or against the current direction depending on the selection of HM and/or FM materials. Here, we study the current-driven DWM in a perpendicularly magnetized system with two HM underlayers (Pt and W) that are of opposite spin Hall angles. Instead of changing the material, it is observed that domain walls can move along opposite directions by varying the relative thickness of the two underlayers. We attribute such phenomenon to the competition of two spin currents with opposite signs of spin polarization injected into the ferromagnetic layer. More evidence is shown by the spin Hall effect-induced switching under a fixed in-plane field, where opposite switching behaviors are observed for devices with different relative thicknesses of the underlayers.

Effect of Annealing on the Structural and FMR Properties of Epitaxial YIG Thin Films Grown by RF Magnetron Sputtering

Effect of Annealing on the Structural and FMR Properties of Epitaxial YIG Thin Films Grown by RF Magnetron Sputtering