All-Electrical Skyrmionic Bits in a Chiral Magnetic Tunnel Junction
Shaohai Chen, Pin Ho, James Lourembam, Alexander K. J. Toh, Jifei Huang, Xiaoye Chen, Hang Khume Tan, Sherry K. L. Yap, Royston J. J. Lim, Hui Ru Tan, T. S. Sur… 展开作者
Shaohai Chen, Pin Ho, James Lourembam, Alexander K. J. Toh, Jifei Huang, Xiaoye Chen, Hang Khume Tan, Sherry K. L. Yap, Royston J. J. Lim, Hui Ru Tan, T. S. Suraj, Yeow Teck Toh, Idayu Lim, Jing Zhou, Hong Jing Chung, Sze Ter Lim, Anjan Soumyanarayanan
Topological spin textures such as magnetic skyrmions hold considerable promise as robust, nanometre-scale, mobile bits for sustainable computing. A longstanding roadblock to unleashing their potential is the absence of a device enabling deterministic electrical readout of individual spin textures. Here we present the wafer-scale realization of a nanoscale chiral magnetic tunnel junction (MTJ) hosting a single, ambient skyrmion. Using a suite of electrical and multi-modal imaging techniques, we show that the MTJ nucleates skyrmions of fixed polarity, whose large readout signal - 20-70% relative to uniform states - corresponds directly to skyrmion size. Further, the MTJ exploits complementary mechanisms to stabilize distinctly sized skyrmions at zero field, thereby realizing three nonvolatile electrical states. Crucially, it can write and delete skyrmions using current densities 1,000 times lower than state-of-the-art. These results provide a platform to incorporate readout and manipulation of skyrmionic bits across myriad device architectures, and a springboard to harness chiral spin textures for multi-bit memory and unconventional computing.
Magnetic skyrmions are promising for building next-generation magnetic memories and spintronic devices due to their stability, small size and the extremely low currents needed to move them. In particular, skyrmion-based racetrack memory is attractive for information technology, where skyrmions are used to store information as data bits instead of traditional domain walls. Here we numerically demonstrate the impacts of skyrmion-skyrmion and skyrmion-edge repulsions on the feasibility of skyrmion-based racetrack memory. The reliable and practicable spacing between consecutive skyrmionic bits on …
The correct identification of topological magnetic objects in experiments is an important issue. In the present work we report on the characterization of metastable skyrmionic spin structures with various topological charges ($Q=-3,-2,-1,0,+1,+2$) in the (Pt$_{1-x}$Ir$_{x}$)Fe/Pd(111) ultrathin magnetic film by performing spin-polarized scanning tunneling microscopy (SP-STM) calculations. We find that an out-of-plane magnetized tip already results in distinguished SP-STM contrasts for the different skyrmionic structures corresponding to their symmetries. Our work also establishes an understand…
High-resolution tunneling electron spin transport properties (longitudinal spin current (LSC) and spin transfer torque (STT) maps) of topologically distinct real-space magnetic skyrmionic textures are reported by employing a 3D-WKB combined scalar charge and vector spin transport theory in the framework of spin-polarized scanning tunneling microscopy (SP-STM). For our theoretical investigation metastable skyrmionic spin structures with various topological charges ($Q=-3,-2,-1,0,1,2$) in the (Pt$_{0.95}$Ir$_{0.05}$)/Fe/Pd(111) ultrathin magnetic film are considered. Using an out-of-plane magnet…
When magnetic skyrmions are moved via currents, they do not strictly travel along the path of the current, instead their motion also gains a transverse component. This so-called skyrmion Hall effect can be detrimental in potential skyrmion devices because it drives skyrmions towards the edge of their hosting material where they face potential annihilation. Here we experimentally modify a skyrmion model system - an atomic Pd/Fe bilayer on Ir(111) - by decorating the film edge with ferromagnetic Co/Fe patches. Employing spin-polarized scanning tunneling microscopy, we demonstrate that this ferro…
We theoretically investigated a method to detect a single skyrmion in a magnetic tunneling junction (MTJ) geometry. Using the tunneling Hamiltonian approach, we calculated the tunneling magnetoresistance (TMR) ratio of the skyrmion-ferromagnet bilayer system. We show the TMR ratio is determined sorely by the spin profile of the skyrmion and geometrical factor of the device, if only the system is reasonably clean such that the spectral broadening is smaller than the exchange coupling between the local and the itinerant magnetic moment. The TMR ratio in that case can amount to $30\%$ or higher w…
Probabilistic computing using random number generators (RNGs) can leverage the inherent stochasticity of nanodevices for system-level benefits. 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 systematically stu…
Magnetic skyrmions are promising candidates as elementary nanoscale bits in logic-in-memory devices, intrinsically merging high density memory and computing capabilities. Here we exploit the dynamics of skyrmions interacting with anisotropy energy barriers patterned by ion irradiation to design programmable logic gates. Using micromagnetic simulations with experimental parameters, we show that a fine tuning of the barrier height and width allows the selective tunneling of skyrmions between parallel nanotracks triggered by skyrmion-skyrmion interaction. This can be leveraged to design skyrmion …
Naturally random devices that exploit ambient thermal noise have recently attracted attention as hardware primitives for accelerating probabilistic computing applications. One such approach is to use a low barrier nanomagnet as the free layer of a magnetic tunnel junction (MTJ) whose magnetic fluctuations are converted to resistance fluctuations in the presence of a stable fixed layer. Here, we propose and theoretically analyze a magnetic tunnel junction with no fixed layers but two free layers that are circularly shaped disk magnets. We use an experimentally benchmarked model that accounts fo…
Superparamagnetic tunnel junctions (SMTJs) have emerged as a competitive, realistic nanotechnology to support novel forms of stochastic computation in CMOS-compatible platforms. One of their applications is to generate random bitstreams suitable for use in stochastic computing implementations. We describe a method for digitally programmable bitstream generation based on pre-charge sense amplifiers. This generator is significantly more energy efficient than SMTJ-based bitstream generators that tune probabilities with spin currents and a factor of two more efficient than related CMOS-based imple…
When a foreign atom is placed on a surface of a metal, the surrounding sea of electrons responds screening the additional charge leading to oscillations or ripples. On surfaces, those electrons are sometimes confined to two-dimensional surface states, whose spin-degeneracy is lifted due to the Rashba effect arising from the spin-orbit interaction of electrons and the inversion asymmetric environment. It is believed that at least for a single adatom scanning tunneling microscopy measurements are insensitive to the Rashba splitting i.e. no signatures in the charge oscillations will be observed. …
This work investigates nanosecond superparamagnetic switching in 50 nm diameter in-plane magnetized magnetic tunnel junctions (MTJs). Due to the small in-plane uniaxial anisotropy, dwell times below 10 ns and auto-correlation times down to 5 ns are measured for circular superparamagnetic tunnel junctions (SMTJs). SMTJs exhibit probabilistic switching of the magnetic free layer, which can be used for the generation of true random numbers. The quality of random bitstreams, generated by our SMTJ, is evaluated with a statistical test suite (NIST STS, sp 800-22) and shows true randomness after thre…
We propose an improved scheme for low-power writing of binary bits in non-volatile (multiferroic) magnetic memory with electrically generated mechanical stress. Compared to an earlier idea [Tiercelin, et al., J. Appl. Phys., 109, 07D726 (2011)], our scheme improves distinguishability between the stored bits when the latter are read with magneto-tunneling junctions. More importantly, the write energy dissipation and write error rate are reduced significantly if the writing speed is kept the same. Such a scheme could be one of the most energy-efficient approaches to writing bits in magnetic non-…
Research on magnetic systems with broken inversion symmetry has been stimulated by the experimental proof of particle-like configurations known as skyrmions, whose non-trivial topological properties make them ideal candidates for spintronic technology. This class of materials enables Dzyaloshinskii-Moriya interactions (DMI) which favor the stabilization of chiral configurations. Recent advances in material engineering have shown that in confined geometries it is possible to stabilize skyrmionic configurations at zero field. Moreover, it has been shown that in systems based on Pd/Fe bilayers on…
Critical current fluctuations with a 1/ f spectral density (f is frequency) are potentially a limiting source of intrinsic decoherence in superconducting quantum bits (qubits) based on Josephson tunnel junctions. Prior measurements of this noise were made at nonzero voltages whereas qubits are operated in the zero voltage state. We report measurements of 1/ f noise in a dc superconducting quantum interference device first, coupled to a resonant tank circuit and operated in a dispersive mode at zero voltage, and, second, operated conventionally with a current bias in the voltage regime. Both me…
Gate-tunable quantum-mechanical tunnelling of particles between a quantum confined state and a nearby Fermi reservoir of delocalized states has underpinned many advances in spintronics and solid-state quantum optics. The prototypical example is a semiconductor quantum dot separated from a gated contact by a tunnel barrier. This enables Coulomb blockade, the phenomenon whereby electrons or holes can be loaded one-by-one into a quantum dot. Depending on the tunnel-coupling strength, this capability facilitates single spin quantum bits or coherent many-body interactions between the confined spin …
The highest-density magnetic storage media will code data in single-atom bits. To date, the smallest individually addressable bistable magnetic bits on surfaces consist of 5-12 atoms. Long magnetic relaxation times were demonstrated in molecular magnets containing one lanthanide atom, and recently in ensembles of single holmium (Ho) atoms supported on magnesium oxide (MgO). Those experiments indicated the possibility for data storage at the fundamental limit, but it remained unclear how to access the individual magnetic centers. Here we demonstrate the reading and writing of individual Ho atom…
Axisymmetric solitonic states (chiral skyrmions) have been predicted theoretically more than two decades ago. However, until recently they have been observed in a form of skyrmionic condensates (hexagonal lattices and other mesophases). In this paper we report experimental and theoretical investigations of isolated chiral skyrmions discovered in PdFe/Ir(111) bilayers two years ago (Science 341 , 636 (2013)). The results of spin-polarized scanning tunneling microscopy analyzed within the continuum and discrete models provide a consistent description of isolated skyrmions in thin layers. The exi…
Dynamical control of random telegraph noise in magnetic tunnel junctions
Tohoku University（东北大学）；Advanced Institute for Materials Research, Tohoku University（东北大学材料科学高等研究所）；Center for Science and Innovation in Spintronics, Tohoku University（东北大学自旋电子学科学与创新中心）；Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University（东北大学电气通信研究所纳米电子与自旋电子学实验室）；PRESTO, Japan Science and Technology Agency (JST)（日本科学技术振兴机构战略创造事业部PRESTO项目）；Division for the Establishment of Frontier Sciences of Organization for Advanced Studies at Tohoku University（东北大学高级研究组织前沿科学设立分部）；National Institutes for Quantum Science and Technology（国立量子科学技术研究院）；Center for Innovative Integrated Electronic Systems, Tohoku University（东北大学创新集成电子系统中心）
Digital magnetic recording is based on the storage of a bit of information in the orientation of a magnetic system with two stable ground states. Here we address two fundamental problems that arise when this is done on a quantized spin: quantum spin tunneling and back-action of the readout process. We show that fundamental differences exist between integer and semi-integer spins when it comes to both, read and record classical information in a quantized spin. Our findings imply fundamental limits to the miniaturization of magnetic bits and are relevant to recent experiments where spin polarize…
With the rise in in-memory computing architectures to reduce the compute-memory bottleneck, a new bottleneck is present between analog and digital conversion. Analog content-addressable memories (ACAM) are being recently studied for in-memory computing to efficiently convert between analog and digital signals. Magnetic memory elements such as magnetic tunnel junctions (MTJs) could be useful for ACAM due to their low read/write energy and high endurance, but MTJs are usually restricted to digital values. The spin orbit torque-driven domain wall-magnetic tunnel junction (DW-MTJ) has been recentl…
We demonstrate single-shot readout of quantum states of the Josephson charge qubit. The quantum bits are transformed into and stored as classical bits (charge quanta) in a dynamic memory cell - a superconducting island. The transformation of state |1> (differing form state |0> by an extra Cooper pair) is a result of a controllable quasiparticle tunneling to the island. The charge is then detected by a conventional single-electron transistor, electrostatically decoupled from the qubit. We study relaxation dynamics in the system and obtain the readout efficiency of 87% and 93% for |1> and |0> st…
We have employed spin-polarized scanning tunneling microscopy and Monte-Carlo simulations to investigate the effect of lateral confinement onto the nanoskyrmion lattice in Fe/Ir(111). We find a strong coupling of one diagonal of the square magnetic unit cell to the close-packed edges of Fe nanostructures. In triangular islands this coupling in combination with the mismatching symmetries of the islands and of the square nanoskyrmion lattice leads to frustration and triple-domain states. In direct vicinity to ferromagnetic NiFe islands, the surrounding skyrmion lattice forms additional domains. …
Temperature-Resilient True Random Number Generation with Stochastic Actuated Magnetic Tunnel Junction Devices
New York University（纽约大学）；University of Virginia（弗吉尼亚大学）；Sandia National Laboratories（桑迪亚国家实验室）；University of Illinois at Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校）；Spin Memory Inc.（Spin Memory公司）
Due to underlying symmetries the ground states of magnetic adatoms may be highly stable, which opens perspectives for application as single-atom memory. A specific example is a single holmium atom (with $J=8$) on a platinum (111) surface for which exceptionally long lifetimes were observed in recent scanning tunneling microscopy studies. For control and read-out the atom must be coupled to electronic contacts. Hence the spin dynamics of the system is governed by a quantum master equation. Our analysis shows that in general it cannot be reduced to a classical master equation in the basis of the…
True random number generators are of great interest in many computing applications such as cryptography, neuromorphic systems and Monte Carlo simulations. Here we investigate perpendicular magnetic tunnel junction nanopillars (pMTJs) activated by short duration (ns) pulses in the ballistic limit for such applications. In this limit, a pulse can transform the Boltzmann distribution of initial free layer magnetization states into randomly magnetized down or up states, i.e. a bit that is 0 or 1, easily determined by measurement of the junction's tunnel resistance. It is demonstrated that bitstrea…
Josephson tunnel junctions are widely used as nonlinear elements in superconducting circuits such as low noise amplifiers and quantum bits. However, microscopic defects in the oxide tunnel barrier can produce low and high frequency noise which can potentially limit the coherence times and quality factors of resonant circuits. Weak link Josephson junctions are an attractive alternative provided that sufficient nonlinearity can be engineered. We compute the current phase relation for superconducting weak links, with dimensions comparable to the zero temperature coherence length, connected to two…
Graphical probabilistic circuit models of stochastic computing are more powerful than the predominant deep learning models, but also have more demanding requirements. For example, they require "programmable stochasticity", e.g. generating two random binary bit streams with tunable amount of correlation between the corresponding bits in the two streams. Electronic implementation of such a system would call for several components leaving a large footprint on a chip and dissipating excessive amount of energy. Here, we show an elegant implementation with just two dipole-coupled magneto-tunneling j…
We developed a method to detect the quantum nature of high spin molecules using muon spin rotation, and a three-step field cycle ending always with the same field. We use this method to demonstrate that the Fe8 molecule can remember 6 (possibly 8) different histories (bits). A wide range of fields can be used to write a particular bit, and the information is stored in discrete states. Therefore, Fe8 can be used as a model compound for Multi-bit Magnetic Memory. Our experiment also paves the way for magnetic quantum tunneling detection in films.
With the recent proposition of skyrmion utilization in racetrack memories at room temperature, skyrmionics has become a very attractive field. However, for the stability of skyrmions, it is essential to incorporate the Dzyaloshinskii-Moriya interaction (DMI) and the out-of-plane magnetic field into the system. In this work, we explore a system without these interactions. First, we propose a controlled way for the creation of magnetic skyrmions and skyrmioniums imprinted on a ferromagnetic nanotrack via a nanopatterned nanodisk with the magnetic vortex state. Then we investigate the detachment …
Single-molecule memory device based on a single-molecule magnet (SMM) is one of the ultimate goals of semiconductor nanofabrication technologies. Here, we study how to manipulate and readout the SMM's two spin-state of stored information that characterized by the maximum and minimum average value of the $Z$-component of the total spin of the SMM and the conduction-electron, which are recognized as the information bits "$1$" and "$0$". We demonstrate that the switching time depends on both the sequential tunneling gap $\varepsilon_{se}$ and the spin-selection-rule allowed transition-energy $\va…
Several physical realizations of quantum bits have been proposed. Of those, nano-electronic devices appear most suitable for large-scale integration and potential applications. We suggest to use low-capacitance Josephson junctions, exploiting the coherence of tunneling in the superconducting state combined with the possibility to control individual charges by Coulomb blockade effects (cond-mat/9706016,cond-mat/9808067). These systems constitute quantum bits, with logical states differing by one Cooper-pair charge. Single- and two-bit operations can be performed by applying a sequence of gate v…
A cluster composed of a few magnetic atoms assembled on the surface of a nonmagnetic substrate is one suitable realization of a bit for future concepts of spin-based information technology. The prevalent approach to achieve magnetic stability of the bit is decoupling the cluster spin from substrate conduction electrons in order to suppress spin-flips destabilizing the bit. However, this route entails less flexibility in tailoring the coupling between the bits which is ultimately needed for spin-processing. Here, we show using a spin-resolved scanning tunneling microscope, that we can write, re…
Magnetic skyrmions (MS) are particle-like spin structures with whirling configuration, which are promising candidates for spin-based memory. MS contains alluring features including remarkably high stability, ultra low driving current density, and compact size. Due to their higher stability and lower drive current requirement for movement, skyrmions have great potential in energy efficient spintronic device applications. We propose a skyrmion-based cache memory where data can be stored in a long nanotrack as multiple bits. Write operation (formation of skyrmion) can be achieved by injecting spi…
Spin-orbit torque (SOT) represents an energy efficient method to control magnetization in magnetic memory devices. However, deterministically switching perpendicular memory bits usually requires the application of an additional bias field for breaking lateral symmetry. Here we present a new approach of field-free deterministic perpendicular switching using a strain-mediated SOT switching method. The strain-induced magnetoelastic anisotropy breaks the lateral symmetry, and the resulting symmetry-breaking is controllable. A finite element model and a macrospin model are used to numerically simul…
