Spin-transfer Nano-oscillators Research Articles

Néel Vector Auto-Oscillations and Reorientations Induced by Spin-Polarized Electric Currents in Antiferromagnetic Mn2Au Nanolayer

The electrical excitation of spin dynamics in antiferromagnets is important for the development of high-speed spintronic devices with a low power consumption. Here, we present a theoretical study of the spin dynamics generated in the Ni/Cu/Mn2Au/Cu/Ni nanostructure by a perpendicular-to-plane electric current. Our investigation includes atomistic simulations of spin reorientations in a few monolayer antiferromagnetic Mn2Au film and numerical calculations of nonequilibrium spin accumulation in the nanostructure comprising ferromagnetic Ni polarizers with antiparallel magnetizations. This combined approach enables us to quantify the dynamics of Mn magnetic moments induced by the spin-transfer torque created by the spin-polarized charge flow. The calculations show that the direct electric current with the density exceeding some temperature-dependent threshold value gives rise to steady-state auto-oscillations of the Néel vector in the [Formula: see text]-oriented Mn2Au nanolayer. As the precession of Mn magnetic moments occurs around an out-of-plane direction strongly deflected from their initial in-plane orientations, the emergence of auto-oscillations should be regarded as the realization of a dynamic spin reorientation transition in the antiferromagnetic crystal. Remarkably, the precession frequency rises with increasing current density J and exceeds 1[Formula: see text]THz at [Formula: see text][Formula: see text]A[Formula: see text]m[Formula: see text], which makes the described antiferromagnetic spin transfer nano-oscillator attractive for device applications. Furthermore, our simulations demonstrate that a picosecond current pulse injected into the Ni/Cu/Mn2Au/Cu/Ni nanostructure can induce a precessional switching of the Néel vector. Depending on the current density and pulse duration, the Néel vector rotates by either 90° or 180° and attains stable orientation along the corresponding [Formula: see text] easy axis in the plane of the Mn2Au nanolayer. This feature shows that the Ni/Cu/Mn2Au/Cu/Ni nanostructure could be used as a nonvolatile memory cell with the electrical writing and readout.

A New Stationary Mode of Coupled Oscillations in a Three-Layer Spin-Transfer Nano-Oscillator at High Currents

A New Stationary Mode of Coupled Oscillations in a Three-Layer Spin-Transfer Nano-Oscillator at High Currents

Magnetic vortex states manipulation in overlapping ferromagnetic disks

We present the study of vortex magnetic states in a system of two overlapping ferromagnetic disks by Lorenz microscopy (L-TEM) and micromagnetic modeling. It is shown that in the case of small overlaps in this system there are two magnetic configurations Vortex–Antivortex–Vortex (VAV) state and Vortex–Vortex (VV) state. They are promising for phase locking in vortex spin-transfer nanooscillators. However, our experiments show that the nucleation of VV and VAV states is random and, as a result, the arrays consisting of overlapping disks are inhomogeneous. To control the nucleation of vortices with a certain vorticity, we changed the shape of overlapping disks by cutting them off from different edges. Disks of one type have been cut at the same edge (SD), while disks of another type have been cut at different edges (AD). In situ L-TEM experiments showed that after the sample magnetizing in the magnetic field applied in the plane of disks, the VV states have been realized in both SD and AD arrays. On the other hand, when samples were magnetized in a perpendicular magnetic field, the VAV state was realized in SD array, while the VV state was still realized in the AD array. The peculiarities of vortex nucleation in SD and AD in an external magnetic field are analyzed by means of micromagnetic simulations. The proposed method for controlling the configuration of magnetic vortices can be applied to create large arrays of overlapping disks that are in the same magnetic state.

Nonlinear spin dynamics of a ferromagnetic ring in the vortex state and its application as a spin-transfer nano-oscillator

Nonlinear spin dynamics of a ferromagnetic ring in the vortex state and its application as a spin-transfer nano-oscillator

Self-supported vortex texture in 3D curved magnets

Magnetic textures in the self-supported nanostructure, such as vortex, are promising for magnetic hyperthermia therapy and spintronics due to their low remanent state and topological protection. These configurations emerge from energy minimization in confined systems, such as nanodisk or nanoparticles. There are many techniques used to confine these magnetic textures. However, the most robust, cheap, and reproducible is always sought. This work applies colloidal lithography to produce self-supported nanocaps with a vortex as the ground state. Firstly, we perform micromagnetic simulations to determine which diameters and thicknesses stabilize the vortex as a ground state on nanocaps. Secondly, we simulate the magnetization curves to find the conditions with the smallest remanent state and largest loop hysteresis curves area. Finally, we experimentally corroborate the vortex configuration ground state using electron holography and vibrating the sample magnetometer. In addition, we performed a dynamic simulation to investigate the gyrotropic modes of the vortex core. We present a concise route to the fabrication of scalable vortex magnetic nanocaps. Our results show that the magnetic nanocaps produced have a great potential for application in medicine, such as magnetic hyperthermia, and in spintronics, for spin-transfer torque nano-oscillators.

Dynamical Neural Network Based on Spin Transfer Nano-Oscillators

Dynamical Neural Network Based on Spin Transfer Nano-Oscillators

Influence of the thickness of a nonmagnetic layer on the coupled dynamics of magnetic vortices in a spin-transfer nanooscillator

A spin torque nano-oscillator in the form of a three-layer magnetic tunneling junction of small diameter (120 nm), where the magnetizations in both magnetic layers are in vortex state, is considered. The effect of the thickness of a nonmagnetic layer on the coupled dynamics of two magnetic vortices in a spin torque nano-oscillator has been studied. The thick permalloy magnetic layer has a thickness of 15 nm, the middle non-magnetic layer has a thickness in the first case of 12.5 and in the second 15 nm, and the thin permalloy magnetic layer has a thickness of 4 nm. Numerical calculation of the dynamics of magnetostatically coupled vortices was carried out using the software package SpinPM for micromagnetic modeling. The features of the vortex motion dynamic are studied for different thicknesses of the nonmagnetic interlayer. It is shown that in all cases of thickness of the nonmagnetic interlayer, three regimes of vortex dynamics are observed: the oscillations of magnetic vortices damped over time, the mode of stationary coupled oscillations of magnetic vortices, and regime, when vortices “leave” the edge of the disk. It is found that increasing in the thickness of the nonmagnetic layer leads to decreasing in the values of the first, second, and third critical currents.

Multifunction spin transfer nano-oscillator based on elliptical skyrmion

As microwave signal generators, the spin transfer nano-oscillator (STNO) based on magnetic skyrmion has attracted huge interest recently. However, the circular magnetic skyrmion-based oscillator only produces a microwave signal with a single oscillation frequency, and detecting the precession of skyrmion in the entire nanodisk is also a challenge. In the paper, we propose an STNO based on elliptical skyrmion, which can simultaneously generate a single frequency or multiple frequency oscillation signal, and the oscillation frequency also can be increased to some extent. Moreover, to reduce the difficulty of detection, the method to restrict elliptical skyrmion moving in a specific orbit is suggested. Our results provide a possible candidate to design the spintronic devices.

Spin transfer nano-oscillator based on synthetic antiferromagnetic skyrmion pair assisted by perpendicular fixed magnetic field

As a microwave generator, spin transfer nano-oscillator (STNO) based on skyrmion promises to become one of the next-generation spintronic devices. However, there still exist a few limitations to the practical applications. In this paper, we propose a new STNO based on synthetic antiferromagnetic (SAF) skyrmion pair assisted by a perpendicular fixed magnetic field. It is found that the oscillation frequency of this kind of STNO can reach up to 5.0 GHz, and the multiple oscillation peak with higher frequency can be realized under a fixed out-of-plane magnetic field. Further investigation shows that the skyrmion stability is improved by bilayer antiferromagnetic coupling, which guarantees the stability process of skyrmion under higher spin-polarized current density. Our results provide the alternative possibilities for designing new skyrmion-based STNO to further improve the oscillation frequency, and realize the output of multiple frequency microwave signal.

Frequency of the Spin-Transfer Nano-Oscillator Based on a Perpendicular Tunnel Nanoheterostructure with Nonzero Ellipticity

Frequency of the Spin-Transfer Nano-Oscillator Based on a Perpendicular Tunnel Nanoheterostructure with Nonzero Ellipticity

Joint Effect of a Magnetic Field and a Spin-Polarized Current on Polarity Switching of Magnetic Vortices in a Spin-Transfer Nanooscillator

The combined influence of a spin polarized current and an external magnetic field for switching the polarity of vortices in spin-transfer nanooscillators 400 nm in diameter is studied. A diagram is constructed of the dependence of the magnitude of the spin-polarized current on the magnitude of the magnetic field, which separately switches the polarity of the vortex in the magnetic layers of the spin-transfer nanooscillator. Keywords: magnetic vortice, spin-transfer nanooscillator, magnetic nanodisk.

Совместное влияние магнитного поля и спин-поляризованного тока на переключение полярности магнитных вихрей в спин-трансферном наноосцилляторе

The combined influence of a spin polarized current and an external magnetic field for switching the polarity of vortices in spin-transfer nanooscillators 400 nm in diameter is studied. A diagram is constructed of the dependence of the magnitude of the spin-polarized current on the magnitude of the magnetic field, which separately switches the polarity of the vortex in the magnetic layers of the spin-transfer nanooscillator.

Joint Effect of a Magnetic Field and a Spin-Polarized Current on the Coupled Dynamics of Magnetic Vortices in a Spin-Transfer Nano-Oscillator

Joint Effect of a Magnetic Field and a Spin-Polarized Current on the Coupled Dynamics of Magnetic Vortices in a Spin-Transfer Nano-Oscillator

Strain-modulated magnetization precession in skyrmion-based spin transfer nano-oscillator

The magnetic skyrmion-based spin transfer nano-oscillators have been set as microwave signal emitters and gaining in popularity due to their performance and integrability in the existing CMOS process flow. In this work, we report our investigations on the strain-modulated magnetization precession in the breathing mode skyrmion-based spin transfer nano-oscillator, which strains from the juxtaposed piezoelectric materials. The strain-modulated skyrmion breathing frequency can be strongly controlled by biaxial-isotropy in-plane strain, in which the tunable range varies from 500 MHz to 7 GHz under the fixed current density −4 ×1012 A/m2 and varying strain from −2000 ppm to 4000 ppm. Such tunability is external magnetic free and even does not reduce the dynamic resistance variation. Through this method, a promising digital binary frequency shift-key and amplitude shift-key modulation technique are presented, in which the magnetization precession frequency and amplitude can be tuned with the co-action of current and strain pulses. These findings are providing an optional route to design future tunable spin transfer nano-oscillators based on skyrmion with better performance in the wireless communication.

The Effect of the Spin-Polarized Current on the Dynamics and Structural Changes of Magnetic Vortices in a Large-Diameter Three-Layer Conducting Nanocylinder

The structure and magnetization dynamics are investigated in a vortex spin-transfer nanooscillator, which is a three-layer spin-valve magnetic nanocolumn with a large diameter of 400 nm, under flowing spin-polarized current through it. The dynamic variation in the structure of vortices and their trajectory of motion are studied, using micromagnetic modeling, as a function of the value of the spin-polarized current. It is demonstrated that different modes of motion of vortices may exist: decaying oscillations of vortices, stationary oscillations of vortices, and the mode of switching polarity of one of vortices. The time for which different dynamic modes are settled is determined. The dependence of the oscillation frequency on the value of spin-polarized current is determined for the case of stationary dynamics of coupled vortices. It is shown that, at large values of current, switching of vortex polarity is only possible in a thick layer with the dynamic switching mechanism accompanied by generation of a vortex–antivortex pair. The software package for micromagnetic simulation, SpinPM, is used for numerical calculations of the dynamics of magnetic vortices.

Спиновые вентили в микроэлектронике. Обзор

The base element of micromagnetic devices are the layered spin-valve structures. Small sizes, compatibility with the CMOS technology, scaling ability and various work conditions make the spin-valve structures a universal component of modern microelectronics. The purpose of present work is the analysis, systematization and generalization of the data of the work theoretical bases, experimental data and the application of spin valves. In the review, the hard disc drives, random-access magnetoresistive memory, the spin-transfer nano-oscillators, the magnetic biosensors, as well as various computing systems, operating on the principles of stochastic and deterministic logic, have been considered. The key theoretical works devoted to giant magnetoresistance and spin transfer have been used. The data on various types of the hard-disc readheads have been systematized, their architecture and parameters have been compared, and it has been shown how modern scientific research of nanomagnetic phenomena accelerates the growth rate of the recording density. The analysis of modern research devoted to magnetoresistive random access memory has been carried out. The problems of energy efficiency and increasing the degree of the integration for these devices have been discussed. The latest achievements in the field of materials, geometry and the properties of the spin-transfer nano-oscillators, as well as the problems and prospects for the development of this technology have been considered. The analysis of theoretical and experimental works, in which the spin-gate structures have been used to perform the logical operations of Boolean and non-Boolean logic, has been carried out. It has been shown how the probabilistic nature of the unstable switching of spin gates is used in the op-eration of the unconventional computing systems, namely, neuromorphic or Bayesian networks. The principles of operation of the spin valves as magnetic biosensors have been considered and the advantages of their application have been discussed.

Bias-field-free high frequency microwave emission of spin-transfer nano-oscillator with magnetizations all in-plane

We reported microwave measurements on a nanoscale MgO-based magnetic tunnel junction having an elliptical shape with large aspect ratios to obtain enough in-plane shape anisotropy to ensure free layer magnetization along the long axis. Combined with the magnetization of a synthetic antiferromagnet pinned layer along the short axis, this results in the perpendicular configuration between the magnetizations of free and pinned layers. A steady high frequency oscillation up to 5 GHz was achieved in such devices at zero magnetic field. Meanwhile, a large frequency tunability of 0.11 Hz·cm2/mA (2.67 GHz/mA) was obtained. The micromagnetic simulations confirm the origin of the high oscillation frequency of our spin transfer nano-oscillators (STNOs). These results suggest the great possibility of improved integration and potential application of STNOs for developing the next-generation of on-chip oscillators.

Совместное влияние магнитного поля и спин-поляризованного тока на связанную динамику магнитных вихрей в спин-трансферном наноосцилляторе

The joint effect of the spin polarized current and an external magnetic field on the dynamics of magnetization in vortex spin-transfer nano-oscillators with a diameter of 400 nm is investigated. For the numerical calculation of the coupled dynamics of magnetic vortices, the SpinPM software package for micromagnetic modeling was used. The dependence of the frequency of stationary coupled oscillations of vortices on the magnitude of the magnetic field, which determines the operating frequency range of a tunable vortex spin-transfer nano-oscillator.

Auto-oscillations for the coupling between breathing mode and chiral switching in magnetic skyrmions

Magnetic skyrmions are topological protected nanometric swirling spin structures, and usually possess a fixed chiral feature. Herein, we theoretically demonstrate a dynamic switching for the chirality in magnetic skyrmions by optimizing the spin-transfer torque (STT). Such a switching process is coupled with the breathing mode, presented as an auto-oscillation phenomenon. In this case, the STT plays a role as the compensation of the intrinsic magnetic damping over a period of oscillation. The oscillation frequency could be tuned by the applied current density and type, and the material parameters such as the anisotropy constant and ferromagnetic exchange coefficient. Our results would be significant for designing nanoscale spin-transfer nano-oscillators.

Noise Properties of Two Mutually Coupled Spin-Transfer Nanooscillators in the Phase Locking Regime

Introduction. Today, many research endeavors are devoted to the miniaturization of microwave sources. One of the promising approaches is the use of magnetic nanostructures (spintronics elements), providing a wide range of frequency tuning and low power consumption. The main disadvantage of spintronics generators (spintransfer nanoscillators ‒ STNO) is a low output power of generated oscillations (tens of nanowatts and less). A possible solution is to sum up the power of many STNOs in a mutual synchronization mode.Aim. The investigation of noise properties of two connected STNOs with identical and non-identical parameters in a phase synchronization mode.Materials and methods. A model was developed of two STNOs interconnected by spin waves taking into account thermal noises. Spectral power densities of the amplitude and phase noise were obtained by the method of effective linearization.Results. Dependencies were obtained in a general form for attenuation coefficients of the amplitude and phase fluctuations of noise sources for each STNO. Three cases of synchronization were considered: completely identical STNOs, two identical STNOs but with different oscillation frequencies, and two non-identical STNOs, differing in an allowance of self-excitation by frequencies and amplitudes of the oscillations. It was possible to obtain a gain in the amplitude and phase noise for two identical STNOs. In this case, an increase in the allowance of self-excitation led to a decrease in the level of phase and amplitude noise.Conclusion. This analysis of the attenuation coefficients for non-identical STNOs demonstrates the possibility of improving the noise properties of each of the generators. In this case, the best noise value is obtained for an STNO with greater stability in a stand-alone mode.