Spin Transfer Torque Effect Research Articles

High-Speed, Low-Power, Magnetic Non-Volatile Flip-Flop With Voltage-Controlled, Magnetic Anisotropy Assistance

A magnetic non-volatile flip-flop (MNV-FF) design, with the assistance of voltage-controlled magnetic anisotropy (VCMA) effect, is proposed for low-power and high-speed applications. The MNV-FF employs perpendicular-anisotropy magnetic junnel junctions (MTJs) and exploits the VCMA effect for assisting data backup operations, in cooperation with either spin-transfer torque (STT) or spin-Hall effect (SHE). Thanks to the VCMA assistance, the required STT or SHE current density and pulse duration can be greatly reduced for MTJ switching, resulting in faster and more energy-efficient operations. Our simulation results show that the proposed VCMA-assisted MNV-FFs can achieve 98.4% and 74.6% improvement in data backup energy, and 89.5% and 19.0% improvement in data backup delay, compared to conventional STTand SHE-based MNV-FFs, respectively.

In-plane current induced domain wall nucleation and its stochasticity in perpendicular magnetic anisotropy Hall cross structures

Hall cross structures in magnetic nanowires are commonly used for electrical detection of magnetization reversal in which a domain wall (DW) is conventionally nucleated by a local Oersted field. In this letter, we demonstrate DW nucleation in Co/Ni perpendicular magnetic anisotropy nanowire at the magnetic Hall cross junction. The DWs are nucleated by applying an in-plane pulsed current through the nanowire without the need of a local Oersted field. The change in Hall resistance, detected using anomalous Hall effect, is governed by the magnetic volume switched at the Hall junction, which can be tuned by varying the magnitude of the applied current density and pulse width. The nucleated DWs are driven simultaneously under the spin transfer torque effect when the applied current density is above a threshold. The possibility of multiple DW generation and variation in magnetic volume switched makes nucleation process stochastic in nature. The in-plane current induced stochastic nature of DW generation may find applications in random number generation.

Electric Field Assisted Switching in Magnetic Random Access Memory

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 ( $H_{c}$ ) shows an increasing dependence with the thickness of the free layer ( $t_{\rm FL}$ ) at small $t_{\rm FL}$ 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 $H_{c}$ 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 $H_{c}$ 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.

Current-induced vortex domain wall depinning in ferromagnetic nanowires with artificial linear defect

Vortex domain wall depinning has been obtained in ferromagnetic nanowires with linear defect under the influence of spin transfer torque effect. It is demonstrated that vortex domain wall will be pinned at defect line in ferromagnetic nanowires under small applied current field. Moreover, the depinning of domain wall is strongly dependent on saturation magnetization and exchange coupling constant in linear defect region of magnetic nanowires. We have also discussed linear defect geometry modifying the depinning of domain wall, which is caused by the competition between demagnetization effect and exchange coupling effect.

Current-driven electromagnetic soliton collision in a ferromagnetic nanowire.

The propagation of an electromagnetic wave in a uniaxial ferromagnetic nanowire under the spin transfer torque effect is widely investigated in the soliton frame. The magnetization dynamics of the ferromagnetic nanowire is governed by the Landau-Lifshitz-Gilbert (LLG) equation coupled to the Maxwell equation for the electromagnetic wave propagation. A nonuniform multiscale analysis is invoked for the coupled LLG-Maxwell equations and obtains the extended derivative nonlinear Schrödinger (DNLS) equation for the magnetization and external magnetic field. The effect of electric current is explored by constructing multisoliton solutions to the extended DNLS equation and the possibility of the soliton collision is exploited using the Hirota bilinearization procedure.

Spin pumping through a topological insulator probed by x-ray detected ferromagnetic resonance

In the field of spintronics, the generation of a pure spin current (without macroscopic charge flow) through spin pumping of a ferromagnetic (FM) layer opens up the perspective of a new generation of dissipation-less devices. Microwave driven ferromagnetic resonance (FMR) can generate a pure spin current that enters adjacent layers, allowing for both magnetization reversal (through spin-transfer torque) and to probe spin coherence in non-magnetic materials. However, standard FMR is unable to probe multilayer dynamics directly, since the measurement averages over the contributions from the whole system. The synchrotron radiation-based technique of x-ray detected FMR (XFMR) offers an elegant solution to this drawback, giving access to element-, site-, and layer-specific dynamical measurements in heterostructures. In this work, we show how XFMR has provided unique information to understand spin pumping and spin transfer torque effects through a topological insulator (TI) layer in a pseudo-spin valve heterostructure. We demonstrate that TIs function as efficient spin sinks, while also allowing a limited dynamic coupling between ferromagnetic layers. These results shed new light on the spin dynamics of this novel class of materials, and suggest future directions for the development of room temperature TI-based spintronics.

Spin-transfer torque effects in the dynamic forced response of the magnetization of nanoscale ferromagnets in superimposed ac and dc bias fields in the presence of thermal agitation

Spin-transfer torque (STT) effects on the stationary forced response of nanoscale ferromagnets subject to thermal fluctuations and driven by an ac magnetic field of arbitrary strength and direction are investigated via a generic nanopillar model of a spin-torque device comprising two ferromagnetic strata representing the free and fixed layers and a nonmagnetic conducting spacer all sandwiched between two ohmic contacts. The STT effects are treated via the Brown magnetic Langevin equation generalized to include the Slonczewski STT term thereby extending the statistical moment method [Y. P. Kalmykov et al., Phys. Rev. B 88, 144406 (2013)] to the forced response of the most general version of the nanopillar model. The dynamic susceptibility, nonlinear frequency-dependent dc magnetization, dynamic magnetic hysteresis loops, etc. are then evaluated highlighting STT effects on both the low-frequency thermal relaxation processes and the high-frequency ferromagnetic resonance, etc., demonstrating a pronounced dependence of these on the spin polarization current and facilitating interpretation of STT experiments.

Design of a Fast and Low-Power Sense Amplifier and Writing Circuit for High-Speed MRAM

A high-speed and low-power prepared and write sense amplifier (PWSA) is presented for magnetoresistive RAM (MRAM). The sense amplifier incorporates a writing circuit for MRAM bits switched via timing of precessional dynamics (~GHz speed) in a magnetic tunnel junction (MTJ). By combining read and write functions in a single power-efficient circuit, the PWSA allows for fast read and write operations while minimizing the bit error rate after data programming. The PWSA circuit is designed based on a 65 nm CMOS technology, and the magnetic dynamics are captured by a Verilog-A compact model based on macrospin behavior for MTJs. Using the preread and comparison steps in the data program operation, we are able to reduce write power consumption by up to 50% under random data input conditions. Furthermore, using the voltage-controlled magnetic anisotropy effect for precessional switching, more than 10× reduction of write power and transistor size both in the memory cell and the write circuit is achieved, compared with using the spin transfer torque effect. The circuit achieves 2 ns read time, 1.8 ns write time, and 8 ns total data program operation time (consisting of two read steps, one write step, and a pass/fail check step) using this PWSA concept, and a 2× larger sensing margin through the current feedback circuit.

Microwave detectors based on the spin-torque diode effect

The spin-transfer torque (STT) effect provides a new method of manipulation of magnetization in nanoscale objects. The STT effect manifests itself as a transfer of spin angular momentum between the parallel magnetic layers separated by a nonmagnetic spacer and traversed by a dc electric current. The transfer of the spin angular momentum from one layer to another could result in the excitation of the microwave-frequency magnetization dynamics in one of the magnetic layers. On the other hand, when a magnetization dynamics is excited in a magnetic layered structure by an external microwave signal both the structure electrical resistance and current through the structure will acquire microwave components resulting in the appearance of a rectified dc voltage on the magnetic structure. This “spin-torque diode effect” can be used for the development of ultra-sensitive spin-torque microwave detectors (STMD). Below we present a brief review of our recent work on the general properties of STMDs, analyze the performance of the “resonance-type” and “threshold-type STMD” and consider the possible applications for such microwave detectors.

Perspectives of Electric Field Controlled Switching in Perpendicular Magnetic Random Access

Based on requirements for spin-transfer torque (STT)-magnetic random access memory (MRAM), fundamental challenges in current CoFeB- and MgO-based STT-MRAM with perpendicular magnetic anisotropy (PMA) are addressed when scaling down to sub-20 nm. Electric-field (EF)-induced PMA modulation has been proposed to significantly reduce the switching current. As EF alone is unable to induce the magnetization reversal but PMA reduction, additional mechanisms should be applied to realize EF switching. We discuss difficulties in the implementation of currently reported two switching approaches and propose a new approach using Oersted field guided EF switching in MRAM arrays. Our latest experimental results on the EF effect show that the spin reorientation transition can be realized at an EF of 0.8 V/nm for the top-pin CoFeB/MgO magnetic tunnel junctions (MTJs) with the thermal stability factor of as high as 57 in the absence of EF. However, the bidirectional switching of the free layer cannot be achieved at a fixed biasing field, which is attributed to small STT effect and switching uncertainty due to unipolar feature of EF-induced PMA modulation. Perspective of EF-MRAM is analyzed based on currently published data on EF efficiency. We show that the EF switching is very promising for future MRAM applications in spite of challenges in materials and MTJ stack engineering.

Frequency selection for magnetization switching in spin torque magnetic memory

The change of magnetization states by spin transfer torque brought momentum to research on magnetic random access memory (MRAM), however, there is still a need for improvement of memory performances. The conventional multi-bit per cell (MBPC) scheme has the potential of increasing the storage capacity of MRAM but the overwritability issue remains the major drawback of this scheme. In fact, for systems with more than one free layer, the low anisotropy layer can have its magnetization reversed during the writing on the higher anisotropy one. To access each free layer independently, a spin torque oscillator with an optimal frequency is proposed to assist the magnetization switching. This study reveals that the free layer magnetization can be reversed through a selection of a frequency value which depends on its intrinsic magnetic properties. This resonance phenomenon based on frequency selection and spin transfer torque effect can be used for writing in the MBPC scheme without undesirable overwriting. A spin torque oscillator with an optimal frequency integrated with a conventional magnetic tunnel junction could be the platform of future magnetic memory.

Interfacial spin-filter assisted spin transfer torque effect in Co/BeO/Co magnetic tunnel junction

The first-principles calculation is employed to demonstrate the spin-selective transport properties and the non-collinear spin-transfer torque (STT) effect in the newly proposed Co/BeO/Co magnetic tunnel junction. The subtle spin-polarized charge transfer solely at O/Co interface gives rise to the interfacial spin-filter (ISF) effect, which can be simulated within the tight binding model to verify the general expression of STT. This allows us to predict the asymmetric bias behavior of non-collinear STT directly via the interplay between the first-principles calculated spin current densities in collinear magnetic configurations. We believe that the ISF effect, introduced by the combination between wurtzite-BeO barrier and the fcc-Co electrode, may open a new and promising route in semiconductor-based spintronics applications.

Proposal for a graphene-based all-spin logic gate

In this work, we present a graphene-based all-spin logic gate (G-ASLG) that integrates the functionalities of perpendicular anisotropy magnetic tunnel junctions (p-MTJs) with spin transport in graphene-channel. It provides an ideal integration of logic and memory. The input and output states are defined as the relative magnetization between free layer and fixed layer of p-MTJs. They can be probed by the tunnel magnetoresistance and controlled by spin transfer torque effect. Using lateral non-local spin valve, the spin information is transmitted by the spin-current interaction through graphene channels. By using a physics-based spin current compact model, the operation of G-ASLG is demonstrated and its performance is analyzed. It allows us to evaluate the influence of parameters, such as spin injection efficiency, spin diffusion length, contact area, the device length, and their interdependence, and to optimize the energy and dynamic performance. Compared to other beyond-CMOS solutions, longer spin information transport length (∼μm), higher data throughput, faster computing speed (∼ns), and lower power consumption (∼μA) can be expected from the G-ASLG.

Microwave Signal Sources Based on Spin-Torque Nano-Oscillators

The spin-transfer torque (STT) effect provides a new method of manipulating the magnetization in nano-scale objects. According to the STT effect, a bias dc current traversing magnetic multilayers can transfer angular magnetic moments from one layer to another, which can give rise to the microwave dynamics of magnetization in the layer. This phenomenon can be used to develop novel microwave signal sources (MSSs). In this work, we review the main research results on MSSs based on the STT effect obtained at Taras Shevchenko National University of Kyiv and its foreign collaborating institutions in the recent years.

Controlling of magnetic vortex chirality and polarity by spin-polarized current

For a nanodisk, magnetic vortex characterized by a curling magnetization is an energetically stable state. The magnetization in the center of the magnetic vortex is directed upward or downward, namely, the vortex core polarity p=+1 or p=-1 refers to up or down, respectively. The curling direction of magnetization, namely, the vortex chirality, is either counter-clockwise or clockwise. Thus, different combinations of chirality and polarity in a vortex structure demonstrate four stable magnetic states, which can be used to design a multibit memory cell. Such a multibit memory application requires the independent controlling of both the vortex chirality and vortex polarity, which has received considerable attention recently. Switching the vortex polarity has been achieved by using either a magnetic field or a current. The vortex chirality can be controlled by introducing asymmetric geometry of nanodisks. In this article, by using micromagnetic simulations, we present an effective method to simultaneously control the vortex chirality and polarity in a spin valve structure, in which the fixed spin polarizer layer is magnetized in the film plane when the free layer has a magnetic vortex configuration. The free layer is designed into a ladder shape with the right part being thicker than the left part. Our simulations indicate that a combination of desirable vortex chirality and polarity can be easily controlled by a Gaussian current pulse with proper strength and pulse duration through the spin-transfer torque effect. The insight into physical mechanism of the controllable vortex is demonstrated by a series of snapshots. If the magnetic moment of the free layer is saturated in the direction of 0θ θ is the angle between the magnetization and+x axis, the vortex with the counter-clockwise chirality will be generated after the pulse. In contrast, if the free layer magnetization is saturated along the direction πθ <2π, after the pulse, the vortex will have the clockwise chirality. The core polarity of the remanent vortex state is determined by the sign of the magnetic charges which are formed in the step-side of nanodisk during the current pulse.

Spin-torque oscillators with thermal noise: A constant energy orbit approach

We consider a biaxial macrospin with an easy and hard axis, and study its dynamical evolution under the combined effects of thermal noise and spin transfer torque. The spin-torque is associated with both a perpendicularly magnetized polarizer and an in-plane magnetized reference layer, leading to an effective tilt between the easy and spin polarization axes. Using techniques based on energy averaging over the relevant dynamical trajectories, we analyze the effects of tilt on the dynamics and derive the conditions for the occurrence of stable out-of-plane precessionary states. The presence of these states and their predicted stability boundaries can be tested in experiments on orthogonal spin-transfer devices, and may also serve as a test of the applicability of the macrospin model to real devices, which have internal magnetic degrees of freedom.

Adaptive Compact Magnetic Tunnel Junction Model

Electrical control of magnetic tunnel junctions (MTJs) provides opportunities to introduce MTJs into high-performance applications requiring low power consumption. The magnetic state of an MTJ can be electrically controlled through: 1) the spin transfer torque (STT) effect; 2) the voltage controlled magnetic anisotropy (VCMA) effect; and 3) the fusion of STT and VCMA. Several compact models have been published for MTJs. All of these models consider an MTJ whose magnetic state is controlled through the STT effect. In this paper, a model of an MTJ comprising a free layer, an analysis layer, and a spin polarizing layer is described. The MTJ compact model, adaptive compact MTJ (ACM) model, includes the effects of asymmetry on the MTJ behavior, and models a device controlled through the STT, VCMA, or a fused STT-VCMA mechanism. The ACM model includes the dynamics of the junction temperature. The proposed model can be adapted to experimental configurations including in-plane MTJ (IMTJ), IMTJ with a perpendicular-to-the-plane polarizer, perpendicular-to-the-plane MTJ (PMTJ), and PMTJ with an additional easy axis. The ACM model is validated with published experimental data, showing reasonably accurate results with an average error of less than 6%.

Self-oscillations of spins in a nanodimensional spin waveguide with localized sources of spin-polarized current

Spin oscillations and their phase synchronization are discovered in a spin 1D waveguide (nanowire) after the local injection of a spin-polarized current in the vertical (perpendicular) geometry of magnetization. The mode composition of nonlinear spin self-oscillations is analyzed for a single nano-oscillator using the effect of spin transfer torque with regard to spin wave runaway over the 1D waveguide and synchronized spin self-oscillations at current pumping by two nano-oscillators. It is shown specifically that, along with simple (“nontopological”) oscillation modes, in which singular points in the oscillation amplitude spatial distribution are absent, the 1D waveguide may support modes with pole-type singular points inside the current pumping domain, which are characteristic of the geometry of a precessing 2π-domain boundary. A diagram for frequency- and current-detuning-synchronized spin self-oscillations that are excited by two spin nano-oscillators in the 1D spin waveguide is constructed.

Magnetic anisotropies in epitaxial Fe3O4/GaAs(100) patterned structures

Previous studies on epitaxial Fe3O4 rings in the context of spin-transfer torque effect have revealed complicated and undesirable domain structures, attributed to the intrinsic fourfold magnetocrystalline anisotropy in the ferrite. In this Letter, we report a viable solution to this problem, utilizing a 6-nm-thick epitaxial Fe3O4 thin film on GaAs(100), where the fourfold magnetocrystalline anisotropy is negligible. We demonstrate that in the Fe3O4 planar wires patterned from our thin film, such a unique magnetic anisotropy system has been preserved, and relatively simple magnetic domain configurations compared to those previous reports can be obtained.

Magnetization reversal driven by a spin torque oscillator

Magnetization reversal of a magnetic free layer under spin transfer torque (STT) effect from a magnetic hard layer with a fixed magnetization direction and an oscillating layer is investigated. By including STT from the oscillating layer with in-plane anisotropy and orthogonal polarizer, magnetization-time dependence of free layer is determined. The results show that the frequency and amplitude of oscillations can be varied by adjusting the current density and magnetic properties. For an optimal oscillation frequency (fopt), a reduction of the switching time (t0) of the free layer is observed. Both fopt and t0 increase with the anisotropy field of the free layer.