8 June 2023

The efficacy of a physical reservoir computer model based on traveling spin waves in a spin-wave delay-line active-ring resonator was demonstrated recently. In the present work, we investigate how this neuromorphic device can be adapted for sensing applications. In this “reservoir computing for sensing” framework, we exploit strong coupling of the physical reservoir to its environment to utilize the reservoir as a sensing element. The dynamics of traveling spin waves in delay-line active rings are strongly dependent on the magnetic field and carrier frequency of those spin waves. Treating the spin-wave frequency as an environmental variable, we excite the active ring into different dynamical states by modulating the carrier frequency of a drive signal of microwave pulses injected into the ring. Training a linear regression on the time-multiplexed output from the ring allows the periodic amplitude patterns of the spin waves to be mapped reproducibly onto two-dimensional trajectories, representing periodic “behavioral” targets. Our work demonstrates the versatility of a magnonic resonator as a multipurpose computing and sensing device.

DOI: 10.1103/PhysRevApplied.19.064029

24 June 2021

In the present paper, we propose and experimentally verify a concept of a magnonic reservoir computer. The system utilizes the nonlinear behavior of propagating magnetostatic surface spin waves in a yttriumiron garnet thin film and the time delay inherent in the active ring configuration to process time-dependent data streams. Higher reservoir dimensionality is obtained through the time-multiplexing method, whereby inputs to the system are multiplied by a mask to drive complex dynamics in the ring and the output is sampled in time to create a series of “virtual” neurons for each sample. The input mask is implemented as a train of microwave pulses injected to the system. To demonstrate the efficacy of the concept, the reservoir computer is evaluated on the short-term memory and parity-check benchmark tasks, and the physical system parameters are tuned to optimize performance. By incorporating a reference line to mix the input signal directly onto the ring-resonator output, both the amplitude and phase nonlinearity of the spin waves can be exploited. The addition of a second spin-wave delay line increases the delay time of the ring and enhances the fading memory capacity of the system. Configuring the second delay line to transmit backward volume spin waves also partly compensates the dispersive pulse broadening that is present because of the large delay time.

DOI: 10.1103/PhysRevApplied.15.064060

28 January 2021

A spin-wave delay-line active-ring oscillator has recently been proposed as a suitable substrate to implement the physical reservoir computing model. The concept displays the required properties of fading memory and nonlinearity characteristic to the model. In this paper, we improve the concept by increasing the signal delay time in the yttrium-iron garnet film by more than four times, and we examine further the improved system by evaluating experimentally the performance on two benchmark classification tasks. The short-term memory (STM) task evaluates the linear memory characteristics of the RC, while the parity-check (PC) task evaluates the nonlinear computing capability. Adequate performance on both is achieved, and the linear memory is shown to be strongly dependent on the synchronization between the reservoir computer (RC) inputs and the active-ring circulation time. The extended delay time and other major improvements result in STM and PC capacities reaching maximum values of 4.68 and 1.74, respectively.

DOI: 10.1063/5.0033292

23 March 2020

We demonstrate the use of propagating spin waves for implementing a reservoir-computing architecture. Our concept utilizes an active-ring resonator comprising a magnetic thin-film delay line with an integrated feedback loop. These systems exhibit strong nonlinearity and delayed response, two important properties required for an effective reservoir-computing implementation. In a simple design, we exploit the electric control of feedback gain to inject input data into the active-ring resonator and use a microwave diode to read out the amplitude of the spin waves circulating in the ring.We employ two baseline tasks, namely the short-term memory and parity-check tasks, to evaluate the suitability of this architecture for processing time-series data.

DOI: 10.1103/PhysRevApplied.13.034057

14 May 2020

The spintronic properties of a palladium thin film have been investigated in the presence of hydrogen gas in cobalt/palladium bilayers. Measurements of the inverse spin Hall effect (ISHE) using cavity ferromagnetic resonance allow estimations of the spin Hall conductivity and spin diffusion length in both nitrogen and hydrogen gas atmospheres. Unwanted spin rectification effects are removed using a simple method of inverting the spin current direction with respect to the measurement setup. Absorption of hydrogen gas in the Pd layer at just 3% concentration results in a reduced ISHE voltage amplitude. Fitting the ISHE voltage against the Pd layer thickness demonstrates that the spin diffusion length decreases by 20% in the presence of hydrogen gas. On the other hand, the results indicate that there is no significant change in the spin Hall conductivity of Pd due to hydrogen absorption.

DOI: 10.1103/PhysRevB.101.174422