Spin torque nano-oscillators with a perpendicular spin polarizer

We present an overview in the understanding of spin-transfer torque(STT) induced magnetization dynamics in spintorque nano-oscillator(STNO) devices. The STNO contains an in-plane(IP) magnetized free layer and an out-of-plane(OP) magnetized spin polarizing layer. After a brief introduction, we first use mesoscopic micromagnetic simulations,which are based on the Landau–Lifshitz–Gilbert equation including the STT effect, to specify how a spin-torque term may tune the magnetization precession orbits of the free layer, showing that the oscillator frequency is proportional to the current density and the z-component of the free layer magnetization. Next, we propose a pendulum-like model within the macrospin approximation to describe the dynamic properties in such type of STNOs. After that, we further show the procession dynamics of the STNOs excited by IP and OP dual spin-polarizers. Both the numerical simulations and analytical theory indicate that the precession frequency is linearly proportional to the spin-torque of the OP polarizer only and is irrelevant to the spin-torque of the IP polarizer. Finally, a promising approach of coordinate transformation from the laboratory frame to the rotation frame is introduced, by which the nonstationary OP magnetization precession process is therefore transformed into the stationary process in the rotation frame. Through this method, a promising digital frequency shift-key modulation technique is presented, in which the magnetization precession can be well controlled at a given orbit as well as its precession frequency can be tuned with the co-action of spin polarized current and magnetic field(or electric field) pulses.

Synchronization of nanowire-based spin Hall nano-oscillators

The synchronization of the spin Hall nano-oscillator(SHNO)device driven by the pure spin current has been investigated with micromagnetic simulations.It was found that the power spectra of nanowire-based SHNO devices can be synchronized by varying the current flowing in the heavy metal(HM)layer.The synchronized signals have relatively high power and narrow linewidth,favoring the potential applications.We also found that the synchronized spectra are strongly dependent on both the number and length of nanowires.Moreover,a periodic modulation of power spectra can be obtained by introducing interfacial Dzyaloshinskii–Moriya interaction(iDMI).Our findings could enrich the current understanding of spin dynamics driven by the pure spin current.Further,it could help to design novel spintronic devices.

Recent progress on excitation and manipulation of spin-waves in spin Hall nano-oscillators

Spin Hall nano oscillator(SHNO),a new type spintronic nano-device,can electrically excite and control spin waves in both nanoscale magnetic metals and insulators with low damping by the spin current due to spin Hall effect and interfacial Rashba effect.Several spin-wave modes have been excited successfully and investigated substantially in SHNOs based on dozens of different ferromagnetic/nonmagnetic(FM/NM)bilayer systems(e.g.,FM=Py,[Co/Ni],Fe,CoFeB,Y3Fe5O12;NM=Pt,Ta,W).Here,we will review recent progress about spin-wave excitation and experimental parameters dependent dynamics in SHNOs.The nanogap SHNOs with in-plane magnetization exhibit a nonlinear self-localized bullet soliton localized at the center of the gap between the electrodes and a secondary high-frequency mode which coexists with the primary bullet mode at higher currents.While in the nanogap SHNOs with out of plane magnetization,besides both nonlinear bullet soliton and propagating spin-wave mode are achieved and controlled by varying the external magnetic field and current,the magnetic bubble skyrmion mode also can be excited at a low in-plane magnetic field.These spin-wave modes show thermal-induced mode hopping behavior at high temperature due to the coupling between the modes mediated by thermal magnon mediated scattering.Moreover,thanks to the perpendicular magnetic anisotropy induced effective field,the single coherent mode also can be achieved without applying an external magnetic field.The strong nonlinear effect of spin waves makes SHNOs easy to achieve synchronization with external microwave signals or mutual synchronization between multiple oscillators which improve the coherence and power of oscillation modes significantly.Spin waves in SHNOs with an external free magnetic layer have a wide range of applications from as a nanoscale signal source of low power consumption magnonic devices to spin-based neuromorphic computing systems in the field of artificial intelligence.

Voltage assisted control of spin-transfer nano-oscillators

The spin-transfer nano-oscillator （STNO） has recently acquired a huge amount of research interest, due to its promising easy tunability along with the miniature size. The output frequency control of an STNO through magnetic field and current has been examined almost to its full extent; however, there are issues that still need to be addressed. Here, we propose a novel way of voltage control of the output frequency of an STNO, and alongside reducing its power requirement.

Zero-Magnetic-Field Oscillation of Spin Transfer Nano-Oscillator with a Second-Order-Perpendicular-Anisotropy Free Layer

The zero-magnetic-field oscillation behavior of spin torque nano-oscillator （STNO） with a perpendicularly mag- netized free layer with second-order uniaxial anisotropy is studied theoretically based on the Landau-Lifshitz- Cilbert-Slonczewski equation. It is demonstrated numerically that the second-order uniaxial anisotropy plays a significant role in the occurrence of a zero-magnetic-field steady-state precession, which can be understood in terms of the energy balance between the energy accumulation due to the spin torque and the energy dissipation due to the Gilbert damping. In particular, a relatively large zero-magnetic-field-oscillation current region, in which the corresponding microwave frequency is increased while the threshold current still maintains an almost constant value, can be obtained by modulating the second-order uniaxial anisotropy of the free layer. These results suggest a tunable zero-magnetic-field STNO, and it may be a promising configuration for STNO＇s applications in future wireless communications.

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 skyrmionbased STNO to further improve the oscillation frequency, and realize the output of multiple frequency microwave signal.

Experiments and SPICE simulations of double MgO-based perpendicular magnetic tunnel junction

We investigate properties of perpendicular anisotropy magnetic tunnel junctions(pMTJs) with a stack structure MgO/CoFeB/Ta/CoFeB/MgO as the free layer(or recording layer),and obtain the necessary device parameters from the tunneling magnetoresistance(TMR) vs.field loops and current-driven magnetization switching experiments.Based on the experimental results and device parameters,we further estimate current-driven switching performance of pMTJ including switching time and power,and their dependence on perpendicular magnetic anisotropy and damping constant of the free layer by SPICE-based circuit simulations.Our results show that the pMTJ cells exhibit a less than 1 ns switching time and write energies <1.4 pJ;meanwhile the lower perpendicular magnetic anisotropy(PMA) and damping constant can further reduce the switching time at the studied range of damping constant α <0.1.Additionally,our results demonstrate that the pMTJs with the thermal stability factor■73 can be easily transformed into spin-torque nano-oscillators from magnetic memory as microwave sources or detectors for telecommunication devices.

Multi-pulse homoclinic orbits and chaotic dynamics of a parametrically excited nonlinear nano-oscillator with coupled cubic nonlinearities

In this paper,the complicated dynamics and multi-pulse homoclinic orbits of a two-degree-of-freedom parametrically excited nonlinear nano-oscillator with coupled cubic nonlinearities are studied.The damping,parametrical excitation and the nonlinearities are regarded as weak.The averaged equation depicting the fast and slow dynamics is derived through the method of multiple scales.The dynamics near the resonance band is revealed by doing a singular perturbation analysis and combining the extended Melnikov method.We are able to determine the criterion for the existence of the multi-pulse homoclinic orbits which can form the Shilnikov orbits and give rise to chaos.At last,numerical results are also given to illustrate the nonlinear behaviors and chaotic motions in the nonlinear nano-oscillator.

Observation of magnetic droplets in magnetic tunnel junctions

Magnetic droplets,a class of highly nonlinear magnetodynamic solitons,can be nucleated and stabilized in nanocontact spintorque nano-oscillators.Here we experimentally demonstrate magnetic droplets in magnetic tunnel junctions(MTJs).The droplet nucleation is accompanied by power enhancement compared with its ferromagnetic resonance modes.The nucleation and stabilization of droplets are ascribed to the double-Co Fe B free-layer structure in the all-perpendicular MTJ,which provides a low Zhang-Li torque and a high pinning field.Our results enable better electrical sensitivity in fundamental studies of droplets and show that the droplets can be utilized in MTJ-based applications and materials science.