Spin Transfer Torque Nano-oscillators Research Articles

The Role of Damping Rate Amplitude in the Synchronization of Two Coupled Oscillators

In this paper, we investigate the synchronization phenomenon between two Spin-transfer Torque Nanooscillators (STNOs) of different frequencies in two pillar systems under vortex configuration detunings or driving frequencies. The oscillators’ mutual synchronization occurs through magnetic dipolar interaction. Our micromagnetic simulations show that an amplitude fluctuation referred to as [Formula: see text] has a significant impact on determining the synchronization frequency. The evolution of frequency and amplitude fluctuation rate in two different oscillator sizes versus external perpendicular field will be compared and discussed. Our results reveal that the oscillator with lower [Formula: see text], referred to as the “Leader” oscillator, leads the synchronization process. As such, the “follower” oscillator adjusts its frequency as to that of the “Leader”, thus achieving synchronization. We believe that taking [Formula: see text] into consideration can help in controlling synchronization frequencies in future building blocks of any network multi-array spintronics’ devices.

Time- and space-resolved nonlinear magnetoacoustic dynamics

Nonlinear magnetization dynamics are of great interest, being, for instance, leveraged for neuromorphic computing in spin-transfer torque nano-oscillators. Here, we demonstrate how to implement magnetoacoustics to reach this regime, using monochromatic $({f}_{\text{saw}}=450$ MHz) surface acoustic waves traveling on a thin layer of (Ga,Mn)As. By careful tuning of the precession frequency to both ${f}_{\text{saw}}$ and $2{f}_{\text{saw}}$ using the magnetic field and temperature, we evidence clear signatures of a nonlinear magnetoacoustic response of the magnetic dynamics using the time- and space-resolved Kerr effect: (i) frequency and wave-vector doubling in time and space, respectively, (ii) quadratic (sublinear) evolution of the precession amplitude at $2{f}_{\text{saw}}\phantom{\rule{4pt}{0ex}}({f}_{\text{saw}})$ with acoustic amplitude, and (iii) resonance field shift. While (i) can be well reproduced by a parametric resonance model where nonlinearities arise solely from the SAW, we show that features (ii) and (iii) also involve intrinsic magnetic nonlinearities. Understanding the conditions leading to these nonlinearities will mean better control of the acoustic-wave-driven magnetization dynamics, in order to implement optimally the wave properties enabled by this approach.

Spin torque nano-oscillator driven by combined spin injection from tunneling and spin Hall current

Spin-transfer torque nano-oscillators (STNO) are important candidates for several applications based on ultra-tunable microwave generation and detection. The microwave dynamics in these STNOs are induced by spin currents that are typically generated either by spin polarization in an adjacent ferromagnetic layer or through the spin Hall effect. In this paper, a 3-terminal STNO based on a magnetic tunnel junction is excited by both of these spin injection mechanisms. The combination of these two mechanisms excites the free layer into dynamic regimes beyond what can be achieved by each excitation mechanism individually, resulting in enhanced output powers, a key figures of merit for device performance. The system response can be coherently quantified as a function of the total injected spin current density. The experimental data shows an excellent consistency with this simple model and a critical spin current density of 4.52 ± 0.18 × 109ħ/2 e−1 Am−2.

Spin wave modes of two magnetostatic coupled spin transfer torque nano-oscillators

A detailed analytical and numerical study of the spin wave modes of two nanopillar spin torque nano-oscillators coupled by magnetostatic interactions is presented under the macrospin approximation. Results show that the normal modes of the system oscillate with the magnetizations in-phase or anti-phase in both disks. The frequencies and critical current densities necessary to induce auto-oscillations of the spin wave modes of the coupled system depend on the relative position of the nanopillars and the applied magnetic field. If the oscillators are identical, these modes are degenerate at a certain relative position of the nanopillars, while if the oscillators are non-identical, such degeneracy is removed. Then, we can conclude that the magnetostatic coupling between two spin transfer torque nano-oscillators is a powerful mechanism to control the spin wave modes of these systems.

Role of phase breaking processes on resonant spin transfer torque nano-oscillators

Spin transfer torque nano-oscillators (STNOs) based on magnetoresistance and spin transfer torque effects find potential applications in miniaturized wireless communication devices. Using the non-coherent non-equilibrium Green’s function spin transport formalism self-consistently coupled with the stochastic Landau-Lifshitz-Gilbert-Slonczewski’s equation and the Poisson’s equation, we elucidate the role of elastic phase breaking on the proposed STNO design featuring double barrier resonant tunneling. Demonstrating the immunity of our proposed design, we predict that despite the presence of elastic dephasing, the resonant tunneling magnetic tunnel junction structures facilitate oscillator designs featuring a large enhancement in microwave power up to 8μW delivered to a 50Ω load.

Resonant Spin-Transfer-Torque Nano-Oscillators

Spin transfer torque nano-oscillators are potential candidates for replacing the traditional inductor based voltage controlled oscillators in modern communication devices. Typical oscillator designs are based on trilayer magnetic tunnel junctions which are disadvantaged by low power outputs and poor conversion efficiencies. In this letter, we theoretically propose to use resonant spin filtering in pentalayer magnetic tunnel junctions as a possible route to alleviate these issues and present device designs geared toward a high microwave output power and an efficient conversion of the d.c. input power. We attribute these robust qualities to the resulting non-trivial spin current profiles and the ultra high tunnel magnetoresistance, both arising from resonant spin filtering. The device designs are based on the nonequilibrium Green's function spin transport formalism self-consistently coupled with the stochastic Landau-Lifshitz-Gilbert-Slonczewski's equation and the Poisson's equation. We demonstrate that the proposed structures facilitate oscillator designs featuring a large enhancement in microwave power of around $775\%$ and an efficiency enhancement of over $1300\%$ in comparison with typical trilayer designs. We also rationalize the optimum operating regions via an analysis of the dynamic and static device resistances. This work sets stage for pentalyer spin transfer torque nano-oscillator device designs that extenuate most of the issues faced by the typical trilayer designs.

Micromagnetic simulation of electric field-modulation on precession dynamics of spin torque nano-oscillator

Understanding electric field effects on precession dynamics is crucial to the design of spin transfer torque devices for improving the performance in nano-oscillator. In this letter, the precession dynamics of a CoFeB/MgO multi-layer structured nano-oscillator under externally applied electric field is predicted using a micromagnetic simulation. It is revealed that the electric field can modify the range of oscillation spectra in single frequency mode. With the increase in electric field, there is a red-shift of the resonant frequency. When a positive electric field pulse is applied, a phase lag of the spin precession is induced, which is proportional to the pulse amplitude and duration. The present work is expected to stimulate future experimental efforts on designing devices with electric-field modulated spin transfer torque nano-oscillators.

High power and low critical current density spin transfer torque nano-oscillators using MgO barriers with intermediate thickness

Reported steady-state microwave emission in magnetic tunnel junction (MTJ)-based spin transfer torque nano-oscillators (STNOs) relies mostly on very thin insulating barriers [resulting in a resistance × area product (R × A) of ~1 Ωμm2] that can sustain large current densities and thus trigger large orbit magnetic dynamics. Apart from the low R × A requirement, the role of the tunnel barrier in the dynamics has so far been largely overlooked, in comparison to the magnetic configuration of STNOs. In this report, STNOs with an in-plane magnetized homogeneous free layer configuration are used to probe the role of the tunnel barrier in the dynamics. In this type of STNOs, the RF modes are in the GHz region with integrated matched output powers (Pout) in the range of 1–40 nW. Here, Pout values up to 200 nW are reported using thicker insulating barriers for junctions with R × A values ranging from 7.5 to 12.5 Ωμm2, without compromising the ability to trigger self-sustained oscillations and without any noticeable degradation of the signal linewidth (Γ). Furthermore, a decrease of two orders of magnitude in the critical current density for spin transfer torque induced dynamics (JSTT) was observed, without any further change in the magnetic configuration.

Spin transfer torque nano-oscillators based on synthetic ferrimagnets: Influence of the exchange bias field and interlayer exchange coupling

A comprehensive numerical study of the spin toque driven dynamic states is presented for a synthetic ferrimagnet. For this, the Landau-Lifshitz-Gilbert equation has been solved simultaneously for the two coupled layers of the synthetic ferrimagnet in a macrospin approach including the spin transfer torque term from an external polarizer for one of them. It is shown that a large variety of dynamic modes (in-plane precession (IPP) and out-of-plane precession) can be established, upon varying the strength of the exchange bias field that pins one of the layers of the SyF as well as the Ruderman-Kittel-Kasuya-Yosida interlayer coupling strength. The current—field state diagrams are presented as well as the frequency current dependencies of the most important mode which is the IPP mode. A characteristic feature of the IPP mode for the coupled system (as compared to single layer excitations) is the change, increase or decrease of the frequency, with current upon increasing field. It is shown that this strongly depends on the asymmetry of the internal fields that the two layers experience, upon varying either their thickness or the exchange bias field.

Wireless current sensing by near field induction from a spin transfer torque nano-oscillator

We demonstrate that spin transfer torque nano-oscillators (STNO) can act as wireless sensors for local current. The STNO acts as a transducer that converts weak direct currents into microwave field oscillations that we detect using an inductive coil. We detect direct currents in the range of 300–700 μA and report them wirelessly to a receiving induction coil at distances exceeding 6.5 mm. This current sensor could find application in chemical and biological sensing and industrial inspection.

Ferromagnetic Multilayers: Magnetoresistance, Magnetic Anisotropy, and Beyond

Obtaining highly sensitive ferromagnetic, FM, and nonmagnetic, NM, multilayers with a large room-temperature magnetoresistance, MR, and strong magnetic anisotropy, MA, under a small externally applied magnetic field, H, remains a subject of scientific and technical interest. Recent advances in nanofabrication and characterization techniques have further opened up several new ways through which MR, sensitivity to H, and MA of the FM/NM multilayers could be dramatically improved in miniature devices such as smart spin-valves based biosensors, non-volatile magnetic random access memory, and spin transfer torque nano-oscillators. This review presents in detail the fabrication and characterization of a few representative FM/NM multilayered films—including the nature and origin of MR, mechanism associated with spin-dependent conductivity and artificial generation of MA. In particular, a special attention is given to the Pulsed-current deposition technique and on the potential industrial applications and future prospects. FM multilayers presented in this review are already used in real-life applications such as magnetic sensors in automobile and computer industries. These material are extremely important as they have the capability to efficiently replace presently used magnetic sensors in automobile, electronics, biophysics, and medicine, among many others.

Mechanisms of nonlinear spin-wave emission from a microwave-driven nanocontact

We present a micromagnetic study of linear and nonlinear spin-wave modes excited in an extended permalloy thin film by a microwave driven nanocontact. We show that the linear mode having the frequency equal to the excitation frequency ($f$) is driven by the ac Oersted field component perpendicular to the static external field (applied in-plane of the sample). The nonlinear mode with the frequency $f$/2 is excited as an independent eigenmode within a parametric longitudinal pumping process (due ac Oersted field component parallel to the bias field). Spectral positions of those modes are determined both in the space and phase domain. The results are important for the transfer of information coded into spin waves between nanocontacts, and for synchronization of spin transfer torque nano-oscillators.

Amplitude and phase noise of magnetic tunnel junction oscillators

The microwave emission linewidth of spin transfer torque nano-oscillators is closely related to their phase and amplitude noise that can be extracted from the magnetoresistive voltage signal V(t) using single shot time domain techniques. Here we report on phase and amplitude noise studies for MgO based magnetic tunnel junction oscillators. The analysis of the power spectral densities allows one to separate the linear and nonlinear contributions to the phase noise, the nonlinear contribution being due to the coupling between phase and amplitude. The coupling strength as well as the amplitude relaxation rate can be directly extracted.