Synchronization Of Spin Torque Nano-oscillators Research Articles

Realization of mutual synchronization of spin torque nano-oscillators under room temperature by noise reduction technique

The pragmatic use of Spin Torque Nano-Oscillators (STNOs) in real electronic systems is severely hindered due to their low output power and poor noise figure. The most accepted and promising way to increase the output power and decrease the signal noise of STNOs is through their mutual synchronization. However, it is confused that the mutual synchronization of STNOs is very difficult to achieve at room temperature although the non-linear nature of STNOs contributes to the large frequency range of injection locking. In this work, first, it is revealed that the difficulty of STNOs' mutual synchronization stems from the high output signal noise of STNOs. Based on this observation, a noise reduction technique is invented, which introduces a Band Pass Filter in the coupling loop of STNOs. Using the noise reduction technique, it is demonstrated that even non-identical STNOs can be mutually synchronized at room temperature.

Phase-Locking of Spin-Torque Nano-Oscillator Pairs by Magnetic Dipolar Coupling in Electrical Serial Connection

Synchronization of spin-torque nano-oscillators (STNOs) by magnetic dipolar interactions in a parallel electrical connection (PC) introduces the technological possibility for an enhancement of output signal. In this study, we consider two identical or nonidentical STNOs in serial electrical connection (SC). By using a pendulum-like approximation model, we analytically predict the dipolar coupling strength, critical current and frequency of phase-locking precession. The numerical calculations of those analytical expressions are confirmed by macrospin simulations, revealing that the dipolar coupling strength responsible for the phase-locking in SC case is three times larger than that in the PC case for the two STNOs with the same separation, which results in the enlarged and symmetric phase-locking window of frequency versus current. These results suggest that the SC of multiple STNOs could be more promising for the enhancement of output power.

Mutual synchronization of spin torque nano-oscillators through a long-range and tunable electrical coupling scheme

The concept of spin-torque-driven high-frequency magnetization dynamics, allows the potential construction of complex networks of non-linear dynamical nanoscale systems, combining the field of spintronics and the study of non-linear systems. In the few previous demonstrations of synchronization of several spin-torque oscillators, the short-range nature of the magnetic coupling that was used has largely hampered a complete control of the synchronization process. Here we demonstrate the successful mutual synchronization of two spin-torque oscillators with a large separation distance through their long range self-emitted microwave currents. This leads to a strong improvement of both the emitted power and the linewidth. The full control of the synchronized state is achieved at the nanoscale through two active spin transfer torques, but also externally through an electrical delay line. These additional levels of control of the synchronization capability provide a new approach to develop spin-torque oscillator-based nanoscale microwave-devices going from microwave-sources to bio-inspired networks.

Synchronization of spin torque nano-oscillators

Synchronization of spin torque nano-oscillators (STNOs) has been a subject of extensive research as various groups try to harness the collective power of STNOs to produce a strong enough microwave signal at the nanoscale. Achieving synchronization has proven to be, however, rather difficult for even small arrays while in larger ones the task of synchronization has eluded theorists and experimentalists altogether. In this work we solve the synchronization problem, analytically and computationally, for networks of STNOs connected in series. The procedure is valid for networks of arbitrary size and it is readily extendable to other network topologies. These results should help guide future experiments and, eventually, lead to the design and fabrication of a nanoscale microwave signal generator.

Synchronization of Spin Torque Nano-Oscillators in a Parallel Circuit: Effects of Direct Electrical Coupling Between Free Layers

The synchronization of spin torque nano-oscillators (STNOs) is studied by the parallel circuit model for the giant magnetoresistance pillar. Employing a macrospin simulation approach, we obtain a synchronization range in the plane of the frequency and the resistance for various capacitances. The region of the synchronization is found to be extended by introducing capacitors and to be robust against magnetic anisotropy field fluctuations. We discuss calculated results to improve the synchronization between STNOs.

Synchronization of spin torque nano-oscillators through dipolar interactions

In an array of spin-torque nano-oscillators (STNOs) that combine a perpendicular polarized fixed layer with strong in-plane anisotropy in the free layers, magnetic dipolar interactions can effectively phase-lock the array, thus further enhancing the power of the output microwave signals. We perform a qualitative analysis of the synchronization of an array based on the Landau-Lifshitz-Gilbert equation, with a spin-transfer torque that assumes strong in-plane anisotropy. Finally, we present the numerical results for four coupled STNOs to provide further evidence for the proposed theory.

Synchronization of spin-torque-driven nano-oscillators for point contacts on a quasi-one-dimensional nanowire: Micromagnetic simulations

In this paper we present numerical simulation studies of the synchronization of two coupled spin-torque nano-oscillators (STNO) in the quasi-one-dimensional (1D) geometry: magnetization oscillations are induced in a thin NiFe nanostripe by a spin-polarized current injected via square-shaped CoFe nanomagnets on the top of this stripe. In a sufficiently large out-of-plane field, a propagating oscillation mode appears in such a system. Due to the absence of the geometrically caused wave decay in 1D systems, this mode is expected to enable a long-distance synchronization between STNOs. Indeed, our simulations predict that synchronization of two STNOs on a nanowire is possible up to the intercontact distance $\ensuremath{\Delta}L=3\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ (for the nanowire width $w=50\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$). However, we have also found several qualitatively important features of the synchronization behavior for this system, which make the achievement of a stable synchronization in this geometry a highly nontrivial task. In particular, there exists a minimal distance between the nanocontacts, below which a synchronization of STNOs cannot be achieved. Further, when the current value in the first contact is kept constant, the synchronized oscillation power depends nonmonotonously on the current value in the second contact. Finally, for one and the same current values through the contacts, there might exist several synchronized states (with different frequencies), depending on the initial conditions.