Spin-transfer Nano-oscillators Research Articles

Dynamic response for Dzyaloshinskii–Moriya interaction on bubble-like magnetic solitons driven by spin-polarized current

By using micromagnetic simulations, we studied the dynamic response for different bubble-like magnetic solitons in the [CoPt-CoNi]/Cu/CoNi magnetic multilayer with perpendicular magnetic anisotropy. It is found that a localized spin-polarized current can not only nucleate a dissipative magnetic droplet but also excite the in-plane domain wall (DW) oscillation at the edge of bubble-like magnetic solitons. The dependence of oscillation frequency on current for the dissipative magnetic droplet is hysteretic in the absence of the Dzyaloshinskii–Moriya interactions (DMI). In the presence of DMI, three different bubble-like magnetic solitons are excited: (1) singular magnetic droplet, (2) pseudonormal magnetic droplet, (3) dynamical skyrmion. Meanwhile, the oscillation frequencies of these magnetic solitons have different response as current density varies. These results open up new possibilities for the applications of magnetic soliton-based spin transfer nano-oscillators.

Enhanced modulation rates via field modulation in spin torque nano-oscillators

Spin Transfer Nano-Oscillators (STNOs) are promising candidates for telecommunications applications due to their frequency tuning capabilities via either a dc current or an applied field. This frequency tuning is of interest for Frequency Shift Keying concepts to be used in wireless communication schemes or in read head applications. For these technological applications, one important parameter is the characterization of the maximum achievable rate at which an STNO can respond to a modulating signal, such as current or field. Previous studies of in-plane magnetized STNOs on frequency modulation via an rf current revealed that the maximum achievable rate is limited by the amplitude relaxation rate Γp, which gives the time scale over which amplitude fluctuations are damped out. This might be a limitation for applications. Here, we demonstrate via numerical simulation that application of an additional rf field is an alternative way for modulation of the in-plane magnetized STNO configuration, which has the advantage that frequency modulation is not limited by the amplitude relaxation rate, so that higher modulation rates above GHz are achievable. This occurs when the modulating rf field is oriented along the easy axis (longitudinal rf field). Tilting the direction of the modulating rf field in-plane and perpendicularly with respect to the easy axis (transverse rf field), the modulation is again limited by the amplitude relaxation rate similar to the response observed in current modulation.

Zero-Magnetic-Field Oscillation of Spin Transfer Nano-Oscillator with a Second-Order-Perpendicular-Anisotropy Free Layer**Supported by the National Natural Science Foundation of China under Grant Nos 11204203 and 61274089, and the International Technology Collaboration Program of Shanxi Province under Grant No 201481029-2.

The zero-magnetic-field oscillation behavior of spin torque nano-oscillator (STNO) with a perpendicularly magnetized free layer with second-order uniaxial anisotropy is studied theoretically based on the Landau–Lifshitz–Gilbert–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.

Efficient Synchronization of Dipolarly Coupled Vortex-Based Spin Transfer Nano-Oscillators.

Due to their nonlinear properties, spin transfer nano-oscillators can easily adapt their frequency to external stimuli. This makes them interesting model systems to study the effects of synchronization and brings some opportunities to improve their microwave characteristics in view of their applications in information and communication technologies and/or to design innovative computing architectures. So far, mutual synchronization of spin transfer nano-oscillators through propagating spinwaves and exchange coupling in a common magnetic layer has been demonstrated. Here we show that the dipolar interaction is also an efficient mechanism to synchronize neighbouring oscillators. We experimentally study a pair of vortex-based spin transfer nano-oscillators, in which mutual synchronization can be achieved despite a significant frequency mismatch between oscillators. Importantly, the coupling efficiency is controlled by the magnetic configuration of the vortices, as confirmed by an analytical model and micromagnetic simulations highlighting the physics at play in the synchronization process.

Vortex Dynamics in Magnetic Nanodisks With a Ring of Magnetic Defects

Recently, nanoscale ferromagnetic disk-shaped elements have aroused intensive investigation for their potential applications in numerous magnetoelectronic devices [1-2]. In a magnetic nanodisk, the equilibrium between the exchange interaction and the magnetostatic energy leads to one special magnetization configuration, called magnetic vortex. Many theoretical and experimental researches have verified that the gyrotropic mode of the vortex core motion can be induced by the external field or spin polarized current [2], and then some works focused on investigating the precession frequency. It is well known that the oscillation frequency is one most important parameter for spin-transfer nano-oscillators (STNOs) based on vortex precession [3]. Nowadays, great efforts have been devoted to study the effect of pointlike magnetic defects in the eigenfrequency of vortex core [2], however, the effect of ring-shaped magnetic defects in the precession frequency has not yet been detailed studied. In this paper, we inserted a ring of magnetic defects into magnetic nano-disk, and studied the dynamics of magnetic vortex by micromagnetic simulation (Fig. 1).

Impact of MgO Thickness on the Performance of Spin-Transfer Torque Nano-Oscillators

A magnetic tunnel junction (MTJ) stack incorporating an MgO wedge was deposited over a 200 mm wafer and nanofabricated into circular nanopillars with a diameter of 200 nm. Due to the variable MgO thickness, we could obtain MTJ nanopillars with resistance $\times $ area (RA) ranging from less than $1~\Omega \mu \text{m}^{2}$ up to $15~\Omega \mu \text{m}^{2}$ and tunnel magnetoresistance ratios up to $\sim 100$ %. It was observed that the ferromagnetic coupling ( $H_{F})$ displayed by the transfer curves increases for smaller values of RA. This variation is attributed to the orange-peel coupling, which is caused by the MgO roughness, with a larger impact for thinner barriers. The RF output of the nanopillars caused by the spin-transfer torque-induced magnetic precession of the free layer was measured as a function of bias current and external magnetic fields for nanopillars with different RA values. Results concerning typical devices in the thin MgO region ( $1.3~\Omega \mu \text{m}^{2})$ and in a thicker MgO region ( $2.7~\Omega \mu \text{m}^{2})$ are shown to illustrate the dependence found. It was observed that the higher output power ( $\sim 300$ nW) and the smaller linewidth ( $\sim 90$ MHz) could be obtained with the higher RA region. These results suggest that there must be an optimum RA value for the production of nano-oscillator devices, which depends on the tradeoffs between the need for large breakdown voltages and strong spin polarization of the bias current (favored for larger RA values) and between the need of endurance under large current densities (favored for smaller RA values). Provided the breakdown voltage of the MgO barrrier can be made large enough, nanopillars based on MTJ stacks with an intermediate RA can provide a solution for the production of large output power spin-transfer nano-oscillators.

Amplitude and phase noises of a spin-transfer nano-oscillator synchronized by a phase-lock loop

We have studied the amplitude and phase noises of a spin-transfer nano-oscillator (STNO) with a phase synchronization system (phase-lock loop, PLL). Spectral characteristics of the amplitude and phase noises of the isochronous and nonisochronous STNO are obtained and compared to the analogous characteristics of an autonomous (nonsynchronized) oscillator. The PLL bandwidth is determined.

Improved Spectral Stability in Spin-Transfer Nano-Oscillators: Single Vortex Versus Coupled Vortices Dynamics

We perform a comparative study of spin transfer induced excitation of the gyrotropic motion of a vortex core with either uniform or vortex spin polarizers. The microwave output voltage associated with the vortex dynamics, detected in both cases, displays a strong reduction of phase fluctuations in the case of the vortex polarizer, with a decrease of the peak linewidth by one order of magnitude down to 200kHz at zero field. A thorough study of rf emission features for the different accessible vortex configurations shows that this improvement is related to the excitation of coupled vortex dynamics by spin transfer torques.

Understanding of Phase Noise Squeezing Under Fractional Synchronization of a Nonlinear Spin Transfer Vortex Oscillator.

We investigate experimentally the synchronization of vortex based spin transfer nano-oscillators to an external rf current whose frequency is at multiple integers, as well as at an integer fraction, of the oscillator frequency. Through a theoretical study of the locking mechanism, we highlight the crucial role of both the symmetries of the spin torques and the nonlinear properties of the oscillator in understanding the phase locking mechanism. In the locking regime, we report a phase noise reduction down to -90 dBc/Hz at 1kHz offset frequency. Our demonstration that the phase noise of these nanoscale nonlinear oscillators can be tuned and eventually lessened, represents a key achievement for targeted radio frequency applications using spin torque devices.

Alternating spin-polarized current induces parametric resonance in spin valves

Ferromagnetic systems under the influence of spin-polarized currents exhibit rich spatiotemporal dynamics at nanoscales. We study spin-transfer nano-oscillators driven by the combination of alternating and direct spin-polarized electric currents. We show here that the alternating current induces parametric instabilities on spin valves, that is, the magnetization responses at half the forcing frequency. A spatial self-organization emerges as a result of the oscillatory current, which includes dissipative solitons and Faraday-type waves. The parametric regime is described analytically by means of the Landau-Lifshitz-Gilbert-Slonczewski equation, in good agreement with micromagnetic simulations including the full dipolar field.

Phase-Lock Requirements in a Serial Array of Spin Transfer Nano-Oscillators.

The most promising approach to attain a narrow linewidth and a large output power simultaneously in spin torque oscillators is self-phase-locking of an array of oscillators. Two long range coupling mechanisms, magnetostatic interaction and self-induced current, are explored. Synchronization occurs with MR ratio ~14% and volume ~2.1 × 10−5 μm3 at room temperature for an experimental frequency dispersion, when only the self-induced microwave current is present. The dipole interaction decreases the MR ratio requirement when the elements are properly spaced.

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.

Nonlinear dynamics of spin transfer nano-oscillators

The evolution equation of a ferromagnetic spin system described by Heisenberg nearest-neighbour interaction is given by Landau–Lifshitz–Gilbert (LLG) equation, which is a fascinating nonlinear dynamical system. For a nanomagnetic trilayer structure (spin valve or pillar) an additional torque term due to spin-polarized current has been suggested by Slonczewski, which gives rise to a rich variety of dynamics in the free layer. Under appropriate conditions the spin-polarized current gives a time-varying resistance to the magnetic structure thereby inducing magnetization oscillations of frequency which lies in the microwave region. Such a device is called a spin transfer nanooscillator (STNO). However, this interesting nanoscale level source of microwaves lacks efficiency due to its low emitting power typically of the order of nWs. To overcome this difficulty, one has to consider the collective dynamics of synchronized arrays/networks of STNOs as suggested by Fert and coworkers so that the power can be enhanced N 2 times that of a single STNO. We show that this goal can be achieved by applying a common microwave magnetic field to an array of STNOs. In order to make the system technically more feasible to practical level integration with CMOS circuits, we establish suitable electrical connections between the oscillators. Although the electrical connection makes the system more complex, the applied microwave magnetic field drives the system to synchronization in large regions of parameter space.

Current-induced magnetic skyrmions oscillator

Spin transfer nano-oscillators (STNOs) are nanoscale devices which are promising candidates for on-chip microwave signal sources. For application purposes, they are expected to be nano-sized, to have broad working frequency, narrow spectral linewidth, high output power and low power consumption. In this paper, we demonstrate by micromagnetic simulation that magnetic skyrmions, topologically stable nanoscale magnetization configurations, can be excited into oscillation by a spin-polarized current. Thus, we propose a new kind of STNO using magnetic skyrmions. It is found that the working frequency of this oscillator can range from nearly 0 Hz to gigahertz. The linewidth can be smaller than 1 MHz. Furthermore, this device can work at a current density magnitude as small as 108 A m−2, and it is also expected to improve the output power. Our studies may contribute to the development of skyrmion-based microwave generators.

Nonlinear Behavior and Mode Coupling in Spin-Transfer Nano-Oscillators

Spin-transfer nano-oscillators (STNOs) are promising candidates for integrated spintronic microwave devices but improving their spectral coherence remains a key issue for applications. The authors have constructed multilayered nanopillars for harnessing the nonlinearity of the collective oscillation modes associated with two dipole-coupled vortices that in turn permits the properties of the collective modes (i.e. frequency and linewidth) to be tuned precisely. This is a crucial issue for applications as it can greatly improve the quality factor of these new types of nanoscale oscillators.

Specific features of the phase locking of two nonidentical spin-transfer nanooscillators

The methods of the nonlinear oscillation theory are used to study a system of coupled nonidentical spin-transfer nanooscillators that interact due to spin waves in a common ferromagnetic layer. The effect of nonidentity with respect to parameters on the phase locking of such a system is analyzed in the presence of variations in the distance between nanocontacts. The effect of broadband locking under significant nonidentity of the control parameters related to the domination of one oscillator is studied.

Large amplitude spin torque vortex oscillations at zero external field using a perpendicular spin polarizer

We investigate the microwave response of a spin transfer vortex based oscillator in a magnetic tunnel junction with an in-plane reference layer combined with a spin valve with an out-of-plane magnetization spin polarizing layer. The main advantage of this perpendicular spin polarizer is to induce a large spin transfer force even at zero magnetic field, thus leading to a record emitted power (up to 0.6 μW) associated to a very narrow spectral linewidth of a few hundreds of kHz. The characteristics of this hybrid vortex based spin transfer nano-oscillator obtained at zero field and room temperature are of great importance for applications based on rf spintronic devices as integrated and tunable microwave source and/or microwave detector.

自旋纳米振荡器

Spin transfer nano-oscillator (STO), a new class of nano-sized microwave device based on giant magnetoresistance and spin transfer torque effect, has aroused tremendous and continuous research interest due to its potential applications in wireless communications system as either microwave source or detectors. The key features of STOs are simple structure, nanoscale size, frequency tunability, broad working temperature, low operating voltage and easy integration with standard silicon technology. In this review paper, we give an overview of the working principle of STOs, recent developments and breakthroughs. Challenges and perspectives of the STOs that might be productive topics for future research are also briefly discussed.

A system of phase synchronization of a spin-transfer nano-oscillator

We have studied the dynamics of a filterless phase-synchronization system (PSS) of a spin-transfer nano-oscillator (STNO) generating microwave oscillations in a broad range of frequencies under the effect of direct current and external magnetic field. Bifurcations in the system caused by a change in the frequency detuning of synchronized oscillations are considered. Bands of phase locking and synchronism are determined. The existence of a phase-locking band in the filterless PSS of STNOs basically distinguishes these oscillators from other types of microwave generators.

Origin of spectral purity and tuning sensitivity in a spin transfer vortex nano-oscillator.

We investigate the microwave characteristics of a spin transfer nano-oscillator (STNO) based on coupled vortices as a function of the perpendicular magnetic field H(⊥). Interestingly, we find that our vortex-based oscillator is quasi-isochronous independently of H(⊥) and for a dc current ranging between 18 and 25 mA. It means that the severe nonlinear broadening usually observed in STNOs can be suppressed on a broad range of bias. Still, the generation linewidth displays strong variations on H(⊥) (from 40 kHz to 1 MHz), while the frequency tunability in current remains almost constant (7 MHz/mA). This demonstrates that isochronicity does not necessarily imply a loss of frequency tunability, which is here governed by the current induced Oersted field. It is not sufficient either to achieve the highest spectral purity in the full range of H(⊥). We show that the observed linewidth broadenings are due to the excited mode interacting with a lower energy overdamped mode, which occurs at the successive crossings between harmonics of these two modes. These findings open new possibilities for the design of STNOs and the optimization of their performance.