V. Sverdlov

TU Wien | TU Wien · Institute for Microelectronics

We generalize the description of transport in ferromagnetic systems using a coupled spin and charge drift-diffusion approach to account for the spin-orbit torques acting on the magnetization. We consider both bulk and interfacial spin-orbit coupling and treat the transverse spin transport in the bulk of ferromagnets. We compare our approach with th...

In advancing the study of magnetization dynamics in STT-MRAM devices, we employ the spin drift–diffusion model to address the back-hopping effect. This issue manifests as unwanted switching either in the composite free layer or in the reference layer in synthetic antiferromagnets—a challenge that becomes more pronounced with device miniaturization....

Although the miniaturization of metal–oxide–semiconductor field effect transistors (MOSFETs)—the main driver behind an outstanding increase in the speed, performance, density, and complexity of modern integrated circuits—is continuing, numerous outstanding technological challenges in complimentary metal–oxide–semiconductor (CMOS) device miniaturiza...

Emerging spin-transfer torque magnetoresistive random access memory (STT-MRAM) is attractive for stand-alone and embedded applications. STT-MRAM is nonvolatile, operates fast, and possesses excellent endurance. STT-MRAM is compatible with CMOS technology and can be integrated during the back-end-of-line process. This makes STT-MRAM promising for au...

We employ a fully three-dimensional model coupling magnetization, charge, spin, and temperature dynamics to study temperature effects in spin-orbit torque (SOT) magnetoresistive random access memory (MRAM). SOTs are included by considering spin currents generated through the spin Hall effect. We scale the magnetization parameters with the temperatu...

We compute the spin torque acting on elongated magnetic layers inrecently proposed ultra-scaled STT-MRAM devices. For thispurpose we evaluate the non-equilibrium spin accumulation bysolving the coupled spin and charge transport equations. This goesbeyond the Slonczewski torque approximation which describestorques localized at the interface of the t...

Because of their nonvolatile nature and simple structure, the interest in MRAM devices has been steadily growing in recent years. Reliable simulation tools, capable of handling complex geometries composed of multiple materials, provide valuable help in improving the design of MRAM cells. In this work, we describe a solver based on the finite elemen...

The development of advanced magnetic tunnel junctions with a single-digit nanometer footprint can be achieved using an elongated multilayered ferromagnetic free layer structure. In this work, we demonstrate the switching of a composite free layer consisting of two ferromagnets separated by an MgO layer and an additional capping MgO layer to boost p...

We present a generalization of the coupled spin-charge drift-diffusion formalism capable of accurately describing the spin and charge transport properties through magnetic tunnel junctions. Correction terms enable reproducing oscillations of the spin current in ferromagnets typical for quasi-ballistic transport. Our approach proves necessary to acc...

The development of reliable simulation tools provides a valuable help in the design of modern MRAMdevices. Thanks to its versatility in the choice of meshes and discretization, the finite element method is a useful framework for the numerical solution of the magnetization dynamics. We review a finite element implementation of both the Landau-Lifshi...

Designing advanced single-digit shape-anisotropy MRAM cells requires an accurate evaluation of spin currents and torques in magnetic tunnel junctions (MTJs) with elongated free and reference layers. For this purpose, we extended the analysis approach successfully used in nanoscale metallic spin valves to MTJs by introducing proper boundary conditio...

We employ the stochastic Landau-Lifschitz-Gilbert (sLLG) equation to explore thermal effects on switching in the spin-transfer torque magnetoresistive random access memory (STT-MRAM). The distribution of the switching times depends on the meshes used for the discretization within the finite element method (FEM) implementation and we introduce an ef...

A reinforcement learning strategy is applied to find a reliable switching scheme for deterministic switching of a perpendicularly magnetized spin-orbit torque magnetoresistive memory cell. Current pulses sent along orthogonal metal wires allow the field-free reversal of the magnetization. The current pulses are optimized such that reliable switchin...

As the scaling of CMOS-based technology displays signs of an imminent saturation, employing the second intrinsic electron characteristics – the electron spin – is attractive to further boost the performance of integrated circuits and to introduce new computational paradigms. A single electron spin forms a qubit and is suitable for quantum applicati...

Emerging spin transfer torque magnetoresistive random access memories (STT MRAM) are nonvolatile and offer high speed and endurance. MRAM cells include a fixed reference magnetic layer and a free-to-switch ferromagnetic layer (FL), separated by a tunnel barrier. The FL usually consists of several sub-layers separated by nonmagnetic buffer layers. T...

Designing advanced single-digit shape-anisotropy MRAM cells requires an accurate evaluation of spin currents and torques in magnetic tunnel junctions (MTJs) with elongated free and reference layers. For this purpose, we extended the analysis approach successfully used in nanoscale metallic spin valves to MTJs by introducing proper boundary conditio...

MRAM structures employing two ferromagnetic layers with fixed magnetization were recently introduced in experimental works, allowing the reduction of both switching currents and cell sizes. We verify by means of simulation their improved switching characteristics over structures with a single fixed magnetization layer.

The development of advanced magnetic tunnel junctions with a single-digit nanometer footprint can be achieved using an elongated multilayered ferromagnetic free layer structure. We demonstrate the switching of a composite free layer consisting of two ferromagnets separated by an MgO layer and an additional capping MgO layer to boost perpendicular a...

Using a spin drift-diffusion model for coupled spin and charge transport, including the spin Hall effect and the inverse spin Hall effect, spin-orbit torques acting on the magnetization in heavy metal/ferromagnet bilayers are evaluated employing a finite element approach. The behavior of the resulting spin accumulation and torques is analyzed for d...

As scaling of semiconductor devices displays signs of saturation, the focus of research in microelectronics shifts towards finding new computing paradigms based on novel physical principles [...].

The writing process in spin transfer torque magnetoresistive random access memories (STT–MRAM) is facilitated by elevated temperatures. In this work we investigate the temperature distribution and development in the free layer (FL) of an STT-MRAM during switching. With our fully three-dimensional (3D) finite element method simulation approach, we n...

Employing novel 2D materials with topologically protected current-carrying edge states is promising to boost the on-current in electronic devices. Using nanoribbons is essential to reduce the contribution of the 2D bulk states to the current. Making the nanoribbon widths narrower allows one to put more current-carrying edge states under the gate of...

Finding and optimizing robust schemes for field-free switching remains a challenging problem in spin-orbit torque magnetoresistive random access memories. In this work reinforcement learning is employed for the optimization of switching schemes for such memory cells. A cell is switched purely electrically by applying pulses to two orthogonal metal...

We demonstrate by means of numerical simulations the switching of a perpendicularly magnetized free layer by spin-orbit torques based on a two-pulse switching scheme with improved writing power efficiency. In this scheme, the first pulse selects the cell, while the second pulse completes the switching deterministically. It is shown that the magnitu...

A drift-diffusion approach to coupled spin and charge transport has been commonly applied to determine the spin-transfer torque acting on the magnetization in metallic valves. This approach, however, is not suitable to describe the predominant tunnel transport in magnetic tunnel junctions. In this work we present a coupled Finite Element solution t...

Continuous miniaturization has brought the feature size of silicon technology down into the nanometer scale, where performance enhancement cannot easily be achieved by further size reduction. The use of new materials with advanced properties has become mandatory to meet the needs for higher performance at reduced power. Topological insulators posse...

Spin-orbit torque memory is a suitable candidate for next generation nonvolatile magnetoresistive random access memory. It combines high-speed operation with excellent endurance, being particularly promising for application in caches. In this work, a two-current pulse magnetic field-free spin-orbit torque switching scheme is combined with reinforce...

As scaling of the feature size -the main driving force behind an outstanding increase of the performance of modern electronic circuits -displays signs of saturation, the main focus of engineering research in microelectronics shifts towards finding new paradigms. Any future solution must be scalable and energy efficient while delivering high perform...

We propose a magnetic field-free spin-orbit torque switching scheme based on two orthogonal current pulses, for which deterministic switching is demonstrated via numerical simulations. The first current pulse selects the cell, while the second current pulse ensures deterministic switching of the selected cell. 100% switching probability has been ob...

The use of new materials with advanced properties has become mandatory to meet the needs for higher electronics performance at reduced power. Topological insulators (TIs) possess highly conductive topologically protected edge states which are insensitive to scattering and thus suitable for energy-efficient highspeed devices. Here, we evaluate the s...

In spin-transfer torque magnetoresistive random access memory, the magnetization dynamics of a free layer is usually assumed to be determined by the torque created via a position-independent current density. In circuits, however, it is the voltage, not the current density, which stays fixed during switching. Therefore, the approximate evaluation of...

Simulation of switching in spin-transfer torque magnetoresistive random access memory is usually performed by assuming that the torque is created by a position- and time-independent current density. However, in real circuits the voltage is fixed, not the current density. The assumption of a fixed current density, especially in modern devices with a...

Monte Carlo methods are convenient to model the electron transport due to single electron hopping. The algorithm allows to incorporate a restriction that due to the Coulomb repulsion each trap can only be occupied by a single electron. With electron spin gaining increasing attention, the trap-assisted electron transport has to be generalized to inc...

We investigate the robustness of a purely electrical field-free switching of a perpendicularly magnetized free layer based on SOT. The effective magnetic field which leads to deterministic switching of a rectangular as well as of a square free layer is created dynamically by a two-current pulse scheme. It is demonstrated that the switching is very...

A magnetic field-free switching of a symmetric square free layer with perpendicular magnetization by spin–orbit torque is demonstrated based on micromagnetic modeling and numerical simulations. The field-free switching is accomplished by using a two-pulse switching scheme. An appropriate design of the cell structure yields a deterministic and fast...

The evaluation of the spin lifetime in an ultra-thin silicon film is a massive computational challenge because of the necessity of performing appropriate double integration of the strongly scattering momentum-dependent spin relaxation rates. We have tackled the problem by dividing the whole computation range into two levels. Our scheme in each leve...

The steady increase in performance and speed of modern integrated circuits is continuously supported by constant miniaturization of complementary metal-oxide semiconductor (CMOS) devices. However, a rapid growth of the dynamic and stand-by power due to transistor leakages becomes a pressing issue. A promising way to slow down this trend is to intro...

Upcoming mass production of energy efficient spin-transfer torque magnetoresistive random access memory will revolutionize microelectronics by introducing non-volatility not only in memory but also in logic. The pressing issue is to boost the sensing margin by improving the tunneling magnetoresistance ratio. We demonstrate that spin-dependent trap-...