Voltage-controlled Spin Research Articles

Highly tunable spin Hall magnetoresistance in room-temperature magnetoelectric multiferroic, Sr3Co2Fe24O41|Pt hybrids

We present spin transport studies on a low-field, room-temperature magnetoelectric multiferroic polycrystalline Sr3Co2Fe24O41|Pt heterostructure wherein a highly tunable transverse conical magnetic phase is responsible for static and dynamic magnetoelectric coupling. We measured angular dependence of spin Hall magnetoresistance (SMR) at constant magnetic fields (H) in the range; 50–100 kOe. Application of field below the critical value (Hc1 ≈ 2.5 kOe), yielded negative SMR and the H-evolution of normalized SMR ((ΔR∕R0) × 100 %) exhibited a negative gradient. Further, an increase in H resulted in the positive slope of (ΔR∕R0) × 100 % Vs. H and later at higher H around 14 kOe, a crossover from negative to positive SMR was observed. We employed a simple model for estimating the equilibrium magnetic configuration and computed the SMR modulation at various values of H. We argue that the tilting of the cone is dominant and in turn responsible for the observed nature of SMR below 2.5 kOe while, the closing of the cone-angle is pronounced at higher fields causing a reversal in sign of the SMR from negative to positive. Importantly, SMR experiments revealed that a change in the helicity with a reversal of the magnetic field has no influence on the observed SMR. Longitudinal spin Seebeck effect (LSSE) signal was measured to be ≈ 500 nV at 280 K, under application of thermal gradient, ΔT = 23 K and field, 60 kOe. The observed LSSE signal, originating from pure magnon spin current, showed a similar H-dependent behavior as that of the magnetization of Sr3Co2Fe24O41. Our detailed spin transport studies on polycrystalline Sr3Co2Fe24O41|Pt heterostructure demonstrate high tunability of the amplitude and the sign of the SMR, highlighting its potential for novel spintronic devices such as SMR-based spin valves and voltage-controlled spin transport devices.

Field-free switching model of spin–orbit torque (SOT)-MTJ device with thermal effect based on voltage-controlled magnetic anisotropy (VCMA)

Electrically driven magnetization switch has attracted much attention in the new spintronic memory, especially for spin–orbit torque (SOT)-based magnetic random access memory (MRAM). However, the published models are facing limitations with the continuous shrinkage of the feature size down to nanoscale. Also, the thermal effect caused by switching operation is non-negligible. Therefore, an effective model is needed to represent the switching dynamic of the device concerning the influences of the nanoscale and the thermal effect. In the paper, a compact model of three-terminal SOT-driven switching is established. The influence of the voltage-controlled magnetic anisotropy (VCMA) and spin transfer torque (STT) effect induced by bias voltage on the field-free SOT-driven switching is considered by numerically solving the LLG equations. Furthermore, a 3D model of the SOT-MTJ device is established by finite element method to trace the thermoelectric behavior inside the device. The thermoelectric behavior is integrated into the compact model to show the influence of the temperature on the switching behavior, highlighting the importance of the thermal effect for the realistic modelling of SOT-driven switching. Finally, a novel voltage pulse scheme is proposed, which can effectively shorten the switching time and improve the reliability of the device. The established model could provide strategies and guidelines for next-generation memory design and application.

Random Bitstream Generation Using Voltage-Controlled Magnetic Anisotropy and Spin Orbit Torque Magnetic Tunnel Junctions

Probabilistic computing using random number generators (RNGs) can leverage the inherent stochasticity of nanodevices for system-level benefits. Device candidates for this application need to produce highly random “coinflips” while also having tunable biasing of the coin. The magnetic tunnel junction (MTJ) has been studied as an RNG due to its thermally-driven magnetization dynamics, often using spin transfer torque (STT) current amplitude to control the random switching of the MTJ free layer magnetization, here called the stochastic write method. There are additional knobs to control the MTJ-RNG, including voltage-controlled magnetic anisotropy (VCMA) and spin orbit torque (SOT), and there is need to more systematically study and compare these methods. We build an analytical model of the MTJ to characterize using VCMA and SOT to generate random bit streams. The results show that both methods produce high quality, uniformly distributed bitstreams. Biasing the bitstreams using either STT current or an applied magnetic field shows a sigmoidal distribution vs. bias amplitude for both VCMA and SOT, compared to less sigmoidal for stochastic write. The energy consumption per sample is calculated to be 0.1 pJ (SOT), 1 pJ (stochastic write), and 20 pJ (VCMA), revealing the potential energy benefit of using SOT and showing using VCMA may require higher damping materials. The generated bitstreams are then applied to two tasks: generating an arbitrary probability distribution and using the MTJ-RNGs as stochastic neurons to perform simulated annealing, where both VCMA and SOT methods show the ability to effectively minimize the system energy with small delay and low energy. These results show the flexibility of the MTJ as a true RNG and elucidate design parameters for optimizing the device operation for applications.

Review of voltage-controlled magnetic anisotropy and magnetic insulator

This Festschrift is dedicated to Chia-Ling Chien on the occasion of his eightieth birthday. Prof. Chien is one of the pioneering physicists in the field of spintronics with groundbreaking achievements in giant magnetoresistance, proximity effects in superconductor/ferromagnet multilayers, exchange bias, spin-transfer torque effect, voltage-controlled spin mechanisms, spin-caloritronics and so on. We will focus and give a brief review on the two aspects, the voltage-controlled magnetic anisotropy (VCMA), the magnetic insulator (MI), and related works.

Energy-Efficient Advanced Data Encryption System Using Spin-Based Computing-in-Memory Architecture

Spintronic-based computing-in-memory (CiM) architecture has emerged as one of the efficient solutions to eliminate the latency/bandwidth bottleneck of conventional von-Neumann architecture. Voltage-controlled spin–orbit torque (SOT) memory offers ultralow power and high-speed operation among the various spintronic memories. In this article, advanced encryption standard (AES) system within CiM architecture using voltage-controlled SOT device has been presented. The entire encryption process is performed within the high-density spintronic-based memory array to achieve low power and high processing speed. The reconfigurable logic operations and random key generation for AES are achieved by using a single voltage-controlled SOT device within the memory array. The results show that the proposed architecture is 96%, 52.1%, and 14% more efficient in terms of energy consumption, throughput, and area, respectively, when compared with one of the most efficient SOT-based AES systems.

Spintronic Computing-in-Memory Architecture Based on Voltage-Controlled Spin–Orbit Torque Devices for Binary Neural Networks

Binary neural networks (BNNs) are promising for resource-constrained Internet of Things (IoT) devices owing to the lightweight memory and computation requirements. Moreover, BNNs based on computing-in-memory (CIM) architectures have attracted much attention in both algorithm and hardware designs. Recently, a variety of CIM-based BNN hardware designs has been proposed, particularly based on emerging nonvolatile memories (NVMs), which have merits in terms of nonvolatility and intrinsic resistance-based computing capabilities. However, mainstream NVMs utilize the one transistor plus one memory device (1T1M) cell structure, limiting the computing efficiency and throughput. In this article, we propose a high-throughput CIM architecture for BNN hardware based on a voltage-controlled spin–orbit torque (VC-SOT) memory device, which enables parallel programming and computing operations thanks to its specific cell structure. In VC-SOT devices, multiple magnetic tunnel junctions (MTJs) are stacked on a heavy metal and share the same SOT write current. Furthermore, computing can be achieved based on the normal memory-like write and read operations. Based on a physics-based VC-SOT MTJ model, we designed and evaluated the proposed CIM-based key BNN hardware in the 40-nm technology node. Our simulation results validated the parallel programming/computing functionality and illustrated the performance in terms of power consumption (~4 fJ/bit) and speed (~2 ns/write, 0.36–1.5 ns/read).

Voltage-controlled spin–orbit torque switching in W/CoFeB/MgO

Voltage control of magnetism and spintronics have been highly desirable but rarely realized. In this work, we show voltage-controlled spin–orbit torque (SOT) switching in W/CoFeB/MgO films with perpendicular magnetic anisotropy (PMA) with voltage administered through SrTiO3 with a high dielectric constant. We show that a DC voltage can significantly lower PMA by 45%, reduce switching current by 23%, and increase the damping-like torque as revealed by the first- and second-harmonic measurements. These are characteristics that are prerequisites for voltage-controlled and voltage-select SOT switching spintronic devices.

Bayesian reasoning machine on a magneto-tunneling junction network

The recent trend in adapting ultra-energy-efficient (but error-prone) nanomagnetic devices to non-Boolean computing and information processing (e.g. stochastic/probabilistic computing, neuromorphic, belief networks, etc) has resulted in rapid strides in new computing modalities. Of particular interest are Bayesian networks (BN) which may see revolutionary advances when adapted to a specific type of nanomagnetic devices. Here, we develop a novel nanomagnet-based computing substrate for BN that allows high-speed sampling from an arbitrary Bayesian graph. We show that magneto-tunneling junctions (MTJs) can be used for electrically programmable ‘sub-nanosecond’ probability sample generation by co-optimizing voltage-controlled magnetic anisotropy and spin transfer torque. We also discuss that just by engineering local magnetostriction in the soft layers of MTJs, one can stochastically couple them for programmable conditional sample generation as well. This obviates the need for extensive energy-inefficient hardware like OP-AMPS, gates, shift-registers, etc to generate the correlations. Based on the above findings, we present an architectural design and computation flow of the MTJ network to map an arbitrary Bayesian graph where we develop circuits to program and induce switching and interactions among MTJs. Our discussed framework can lead to a new generation of stochastic computing hardware for various other computing models, such as stochastic programming and Bayesian deep learning. This can spawn a novel genre of ultra-energy-efficient, extremely powerful computing paradigms, which is a transformational advance.

Quantitative Study of the Energy Changes in Voltage-Controlled Spin Crossover Molecular Thin Films.

Voltage-controlled nonvolatile isothermal spin state switching of a [Fe{H2B(pz)2}2(bipy)] (pz = tris(pyrazol-1-1y)-borohydride, bipy = 2,2'-bipyridine) film, more than 40 to 50 molecular layers thick, is possible when it is adsorbed onto a molecular ferroelectric substrate. Accompanying this high-spin and low-spin state switching, at room temperature, we observe a remarkable change in conductance, thereby allowing not only nonvolatile voltage control of the spin state ("write") but also current sensing of the molecular spin state ("read"). Monte Carlo Ising model simulations of the high-spin state occupancy, extracted from X-ray absorption spectroscopy, indicate that the energy difference between the low-spin and high-spin state is modified by 110 meV. Transport measurements demonstrate that four terminal voltage-controlled devices can be realized using this system.

Modeling of Voltage-Controlled Spin–Orbit Torque MRAM for Multilevel Switching Application

Magnetic tunnel junction (MTJ) has emerged as a viable candidate for next-generation memory and logic applications. Manipulation of the magnetic and electric field can control the spin and charge state variables that are of utmost importance in spintronics devices. In this article, the modeling of voltage-controlled spin–orbit torque (VCSOT)-based magnetic random access memory (MRAM) is demonstrated. VCSOT MRAM is an ultrafast device owing to a magnetic switching time of 0.7 ns. It is more energy-efficient as compared to spin–orbit torque (SOT) MRAM. VCSOT MRAM has been exemplified for multilevel cell (MLC) application to enhance the memory integration density. Two-bit serial MLC (sMLC) and parallel MLC (pMLC) designs are proposed using VCSOT MRAM. These MLCs are more energy-efficient as compared to SOT-MLCs. Moreover, 2-bit sMLC and pMLC VCSOT-MRAM significantly reduce average write energy by 91.5% and 49.6%, respectively, as compared to SOT-based MLC. Performance metrics for these devices have been illustrated, which shows the negligible read disturbance and write error rates.

Nonvolatile voltage controlled molecular spin state switching

Voltage-controlled room temperature isothermal reversible spin crossover switching of [Fe{H2B(pz)2}2(bipy)] thin films is demonstrated. This isothermal switching is evident in thin film bilayer structures where the molecular spin crossover film is adjacent to a molecular ferroelectric. The adjacent molecular ferroelectric, either polyvinylidene fluoride hexafluoropropylene or croconic acid (C5H2O5), appears to lock the spin crossover [Fe{H2B(pz)2}2(bipy)] molecular complex largely in the low or high spin state depending on the direction of ferroelectric polarization. In both a planar two terminal diode structure and a transistor structure, the voltage controlled isothermal reversible spin crossover switching of [Fe{H2B(pz)2}2(bipy)] is accompanied by a resistance change and is seen to be nonvolatile, i.e., retained in the absence of an applied electric field. The result appears general, as the voltage controlled nonvolatile switching can be made to work with two different molecular ferroelectrics: croconic acid and polyvinylidene fluoride hexafluoropropylene.

Tunnel-Field-Effect Spin Filter from Two-Dimensional Antiferromagnetic Stanene

We propose a device concept, based on monolayer stanene, able to provide highly polarized spin currents (up to a $98\%$) with voltage-controlled spin polarization operating at room temperature and with small operating voltage ($0.3$ V). The concept exploits the presence of spin-polarized edge states in a stanene nanoribbon. The spin polarization of the total current can be modulated by a differential tuning of the transmission properties, and of the occupation of edge states of different spin, via the application of an in-plane electric field. We demonstrate device operation using ab-initio and quantum transport simulations.

Voltage-controlled spin injection with an endohedral fullerene Co@C60 dimer

Spin-dependent transport through an endohedral fullerene Co@C60 dimer with gold electrodes is explored theoretically using density functional and extended Hückel theory. Density of states spin polarizations up to 95%, due to spin-splitting of Co 3d orbitals, are found by varying the gate and/or bias voltage. The current-voltage characteristics and strong (up to 100%) spin polarization of the current indicate that the device can be utilized for highly efficient spin injection into nonmagnetic conductors. This finding opens the way to the realization of electrostatically tuned spintronic nano devices less than 2 nm in size, without ferromagnetic electrodes.

Electrostatic Spin Control in InAs/InP Nanowire Quantum Dots

Very robust voltage-controlled spin transitions in few-electron quantum dots are demonstrated. Two lateral-gate electrodes patterned on opposite sides of an InAs/InP nanowire are used to apply a transverse electric field and tune orbital energy separation down to level-pair degeneracy. Transport measurements in this regime allow us to demonstrate the breakdown of the standard alternate up/down spin filling scheme and unambiguously show singlet-triplet spin transitions. The strong confinement of the present devices leads to a large energy gain for the observed anomalous spin configurations that exceeds 4 meV. As a consequence, this behavior is well visible even at temperatures exceeding T = 20 K.

Voltage-Induced Ferromagnetic Resonance in Magnetic Tunnel Junctions

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.

Voltage-controlled spin precession in InAs quantum wells

In this work we investigate spin diffusion in InAs quantum wells with the Rashba spin–orbit coupling modulated by a gate voltage. The gate-voltage dependence of spin diffusion under different temperatures is studied with all the scattering explicitly included. Our result partially supports the claim of the realization of the Datta–Das spin-injected field effect-transistor by Koo et al (2009 Science 325 1515). We also show that the scattering plays an important role in spin diffusion in such a system.

Voltage-controlled spin mechanics

Voltage-controlled spin mechanics

Spin transport across a quantum dot doped with a magnetic ion

A quantum theory of spin-dependent electronic transport through a single Mn2+-ion-doped II-VI quantum dot weakly coupled to ferromagnetic (FM) leads is presented. The electron-Mn-ion exchange interaction in the quantum dot strongly enhances the spin polarization of the current and the tunneling magnetoresistance (TMR) and yields a highly spin-polarized current even when the FM source is not 100% polarized. Both the spin polarization and the TMR depend strongly on the bias voltage VSD. In the presence of an external magnetic field, the TMR oscillates with VSD. The results open the way to voltage-controlled spin filters and magnetic sensors.

Voltage-controlled hole spin injection in nonmagneticGaAs∕AlAsresonant tunneling structures

We have investigated polarization-resolved photoluminescence under applied voltage in $p\text{\ensuremath{-}}i\text{\ensuremath{-}}p$ $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ double-barrier diodes. We have observed oscillations in the degree of polarization up to 36% at $B=15\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ with sign reversals occurring near to the hole subband resonances. At high voltages a polarization saturation up to 25% at $B=15\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ is observed. The data are interpreted by using simulations based on a simple theoretical model that considers spin conservation for tunneling and the relaxation processes for carriers at Zeeman states in the quantum well. Our work offers the prospect for the development of voltage-controlled spin filtering systems using standard nonmagnetic semiconductor heterostructures.

Rashba spin precession in quantum-Hall edge channels

Quasi-one-dimensional edge channels are formed at the boundary of a two-dimensional electron system subject to a strong perpendicular magnetic field. We consider the effect of Rashba spin-orbit coupling, induced by structural inversion asymmetry, on their electronic and transport properties. Both our analytical and numerical results show that spin-split quantum-Hall edge channels exhibit properties analogous to that of Rashba-split quantum wires. Suppressed backscattering and a long spin lifetime make these edge channels an ideal system for observing voltage-controlled spin precession. Based on the latter, we propose a magnetless spin-dependent electron interferometer.