Arrays Of Spin-torque Nano-oscillators Research Articles

Mutual synchronization of spin-torque oscillators within a ring array

An array of spin torque nano-oscillators (STNOs), coupled by dipolar interaction and arranged on a ring, has been studied numerically and analytically. The phase patterns and locking ranges are extracted as a function of the number N, their separation, and the current density mismatch between selected subgroups of STNOs. If Nge 6 for identical current densities through all STNOs, two degenerated modes are identified an in-phase mode (all STNOs have the same phase) and a splay mode (the phase makes a 2pi turn along the ring). When inducing a current density mismatch between two subgroups, additional phase shifts occur. The locking range (maximum current density mismatch) of the in-phase mode is larger than the one for the splay mode and depends on the number N of STNOs on the ring as well as on the separation. These results can be used for the development of magnetic devices that are based on STNO arrays.

Designing Large Arrays of Interacting Spin-Torque Nano-Oscillators for Microwave Information Processing

Arrays of spin-torque nano-oscillators are promising for broadband microwave signal detection and processing, as well as for neuromorphic computing. In many of these applications, the oscillators should be engineered to have equally-spaced frequencies and equal sensitivity to microwave inputs. Here we design spin-torque nano-oscillator arrays with these rules and estimate their optimum size for a given sensitivity, as well as the frequency range that they cover. For this purpose, we explore analytically and numerically conditions to obtain vortex spin-torque nano-oscillators with equally-spaced gyrotropic oscillation frequencies and having all similar synchronization bandwidths to input microwave signals. We show that arrays of hundreds of oscillators covering ranges of several hundred MHz can be built taking into account nanofabrication constraints.

Phase Locking of Spin Transfer Nano-Oscillators Using Common Microwave Sources

In this paper, we study typical nonlinear phenomenon of phase-locking or synchronization in spin-torque nano oscillators (STNOs). To start with the oscillators are considered as uncoupled but interlinked through either a common microwave current or a microwave field. We identify the phase locking of an array of STNOs (first for two and then for 100 oscillators) by means of injection locking which represents locking the oscillations to a common alternating spin current or a common microwave magnetic field. We characterize the locking of STNOs through both first and second harmonic lockings in an array. We find that second harmonic lockings takes lesser value of microwave current and field when compared with the first harmonic lockings. Our results also show that oscillating microwave current can induce integer harmonic locking while microwave field can induce both integer and several fractional harmonic lockings. We also extend our analysis to study locking behavior of two STNOs by introducing time delay feedback and coupling through a current injection and bring out the associated locking characteristics. Finally, we have also analyzed the stability of synchronization of identical array of STNOs with current coupling by using master stability function formalism.

Spin Torque Nano-Oscillators Directly Integrated on a mosfet

We propose spin torque nano-oscillators (STNOs) directly integrated on a metal-oxide-semiconductor field-effect-transistor ( mosfet ). In this model, we consider an array of STNOs, where the STNOs are synchronized via magnetodipolar interaction. We found that the ac voltage generated by magnetization precession of STNOs directly integrated on a mosfet can be amplified by the normally-on mosfet . We also found that the load resistance plays an important role with respect to increasing the output voltage of synchronized STNOs compared to a single STNO and in relation to the gate voltage of the mosfet . By using numerical calculation, we found that the emitted microwave power is greatly enhanced by amplifying the ac voltage generated by synchronized STNOs. It is anticipated that the results will be verified with existing experimental techniques.

Enhanced synchronization in an array of spin torque nano-oscillators in the presence of oscillating external magnetic field

We demonstrate that the synchronization of an array of electrically coupled spin torque nano-oscillators (STNO) modelled by Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation can be enhanced appreciably in the presence of a common external microwave magnetic field. The applied microwave magnetic field stabilizes and enhances the regions of synchronization in the frequency determining parameter space. We have identified two regions of synchronization in the parameter space of our analysis, where the oscillators are exhibiting synchronized oscillations thereby emitting improved microwave power. To characterize the synchronized oscillations we have calculated the locking range in the domain of external source frequency.

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.

Spin wave excitation patterns generated by spin torque oscillators

Spin torque nano-oscillators (STNO) are nanoscale devices that can convert a direct current into short wavelength spin wave excitations in a ferromagnetic layer. We show that arrays of STNO can be used to create directional spin wave radiation similarly to electromagnetic antennas. Combining STNO excitations with planar spin waves also creates interference patterns. We show that these interference patterns are static and have information on the wavelength and phase of the spin waves emitted from the STNO. We describe a means of actively controlling spin wave radiation patterns with the direct current flowing through STNO, which is useful in on-chip communication and information processing and could be a promising technique for studying short wavelength spin waves in different materials.

Anisotropic spin-wave patterns generated by spin-torque nano-oscillators

Spin-wave excitations due to spin-momentum transfer in ferromagnetic thin films will enable new types of information processing and memory storage. Here, we show how arrays of spin-torque nano-oscillators (STNOs) can be used to create anisotropic spin-wave interference patterns, which can be used for information processing. We consider STNO arrays contacting a thin ferromagnetic film. Contacts to the film (including the STNOs themselves) can be used to detect the spin-waves and then, when coupled to a simple circuit, can create new excitation patterns. The propagating spin-wave patterns can be generated by pulsing transponders. Arrangements of transponders create resonant (reverberating) spin-wave activity—that may be the basis of polychronous wave computation of the arithmetic and Boolean functions as well as information storage.

Spin-wave interference patterns created by spin-torque nano-oscillators for memory andcomputation

Magnetization dynamics in nanomagnets has attracted broad interest since it was predictedthat a dc current flowing through a thin magnetic layer can create spin-wave excitations.These excitations are due to spin momentum transfer, a transfer of spin angularmomentum between conduction electrons and the background magnetization,that enables new types of information processing. Here we show how arrays ofspin-torque nano-oscillators can create propagating spin-wave interference patterns ofuse for memory and computation. Memristic transponders distributed on thethin film respond to threshold tunnel magnetoresistance values, thereby allowingspin-wave detection and creating new excitation patterns. We show how groupsof transponders create resonant (reverberating) spin-wave interference patternsthat may be used for polychronous wave computation and information storage.