Synchronized, periodic, and chaotic dynamics in spin torque oscillator with two free layers

Abstract

A phase diagram of the magnetization dynamics is studied by numerically solving the Landau-Lifshitz-Gilbert (LLG) equation in a spin torque oscillator consisting of asymmetric two free layers that are magnetized in in-plane direction. We calculated the dynamics for a wide range of current density for both low and high field cases, and found many dynamical phases such as synchronization, auto-oscillation with different frequencies, and chaotic dynamics. The observation of the synchronization indicates the presence of a dynamical phase which has not been found experimentally by using the conventional electrical detection method. The auto-oscillations with different frequencies lead to an oscillation of magnetoresistance with a high frequency, which can be measured experimentally. The chaotic and/or periodic behavior of magnetoresistance in a high current region, on the other hand, leads to a discontinuous change of the peak frequency in Fourier spectrum.

Modulation and phase-locking in nanocontact vortex oscillators

“…However, the role of vortex core reversal [20] in this context has remained largely unexplored. Indeed, in nanocontact-based systems, core reversal can give rise to more complex states such as rich modulation patterns but also a chaotic dynamic [21][22][23][24][25][26]. Because of the sensitivity to initial conditions, chaos is potentially useful for information processing as a large number of patterns can be generated rapidly [27], and therefore be used as random number generator, in symbolic dynamics or even neuromorphic computing.…”

Computational capability for physical reservoir computing using a spin-torque oscillator with two free layers

“…We use the macrospin LLG equation based on the model in Ref. [20], where the accuracy of the macrospin model was verified by the comparison with the experiment [16]. The results will be compared to those of STOs with two free layers in Secs.…”

“…A critical difference in the magnetization dynamics between a GMR/TMR device with a single free layer and a device with two free layers is the appearance of chaos in the latter structure because of the increased dynamical degrees of freedom [19][20][21][22]. In particular, devices with two free layers might be applicable to physical reservoir computing [23][24][25][26][27], which is another new application of spintronics technology.…”

Input-driven chaotic dynamics in vortex spin-torque oscillator

“…The ability to excite chaotic dynamics in nanomagnets [1][2][3][4][5][6][7][8][9] would potentially have practical applications, such as encoding and random number generators 10 . So far, however, it has proven difficult to achieve; chaos is a nonperiodic and sustainable motion of a physical body maintained by energy injected into a nonlinear system, whereas the magnetization dynamics induced by applying magnetic field and/or current are mainly in the form of switching or a periodic oscillation [11][12][13][14][15][16] .…”

“…Here, several techniques have recently been proposed and experimentally demonstrated with the goal of exciting and identifying chaos, including ones using a delayed-feedback circuit 5,7,9 and ones that add another ferromagnet [2][3][4][5][6]8 . Another possible stage on which to demonstrate chaotic dynamics is an input-driven dynamical system 17 , where a series of time-dependent stimuli drive the nonlinear dynamics of a physical system.…”