Magnons—the quanta of spin waves—propagating in magnetic materials with wavelengths at the nanometer-scale and carrying information in the form of an angular momentum, can be used as data carriers in next-generation, nano-sized low-loss information processing systems. In this respect, artificial magnetic materials with properties periodically varied in space, known as magnonic crystals, are especially promising for controlling and manipulating the magnon currents. In this article, different approaches for the realization of static, reconfigurable, and dynamic magnonic crystals are presented along with a variety of novel wave phenomena discovered in these crystals. Special attention is devoted to the utilization of magnonic crystals for processing of analog and digital information. Magnonic crystals for data processing 2

https://iopscience.iop.org/article/10.1088/1361-6463/aa6a65/pdf

Magnonic crystals for data processing

The time reversal of pulsed signals or propagating wave packets has long been recognized to have profound scientific and technological significance. Until now, all experimentally verified time-reversal mechanisms have been reliant upon nonlinear phenomena such as four-wave mixing. In this paper, we report the experimental realization of all-linear time reversal. The time-reversal mechanism we propose is based on the dynamic control of an artificial crystal structure, and is demonstrated in a spin-wave system using a dynamic magnonic crystal. The crystal is switched from an homogeneous state to one in which its properties vary with spatial period a, while a propagating wave packet is inside. As a result, a linear coupling between wave components with wave vectors k≈π/a and k′=k−2ππ/a≈−π/a is produced, which leads to spectral inversion, and thus to the formation of a time-reversed wave packet. The reversal mechanism is entirely general and so applicable to artificial crystal systems of any physical nature.

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All-linear time reversal by a dynamic artificial crystal

Magnonics: Experiment to prove the concept

The propagating spin wave spectroscopy (PSWS) technique is applied for the first time to metallic thin film patterns, allowing to measure their magnetostatic wave modes. Using micrometer scale local excitation and detection, we show that magnetostatic surface waves propagate on a distance as long as 40 μm in a continuous permalloy film. In micrometer wide permalloy stripes, we observe both quantized spin wave modes and quasi-saturation modes, well explained by existing models. In addition, low-frequency additional modes were observed which, we suggest, are generated in the magnetically inhomogeneous edge regions.

https://iopscience.iop.org/article/10.1209/epl/i2001-00583-2/pdf

Spin waves propagation and confinement in conducting films at the micrometer scale

Parallel pumping for magnon spintronics: Amplification and manipulation of magnon spin currents on the micron-scale

A solution is obtained for the general problem of the nonstationary interaction of backward volume magnetostatic waves in films of yttrium-iron garnet with local parametric pumping. In the case of a large pump region, l≫λ, where λ is the wavelength of the backward volume magnetostatic waves, the problem reduces to a system of truncated equations for two packets of counter propagating waves. In the opposite case, l<λ, the exact problem of parametric interactions of the eigenmodes of a ferrite film (both counterpropagating and in the same direction) is solved numerically. Both cases are studied experimentally and good qualitative and quantitative agreement is obtained with the theory. For the first time, the reversal of a wave front and the time reversal of the shape of backward volume magnetostatic wave pulses are observed and a change in the propagation time for the peak of the signal pulse and a reduction in its width owing to pumping are recorded. Two operating regimes are identified for a nonstationary parametric backward volume magnetostatic wave amplifier with local pumping, which differ in the ratio of the duration of the pump pulse to the transit time for the wave through the local pump region, and the effect of the parametric excitation of two-dimensional spin waves on the interaction of backward volume magnetostatic waves with a local nonstationary parametric pump is determined.

Parametric interaction of magnetostatic waves with a nonstationary local pump

The effects of wave front reversal and inversion of the time profile are observed experimentally for a dipolar spin wave pulse (carrier frequency ${\ensuremath{\omega}}_{1}$) interacting in a yttrium-iron garnet film with a weakly localized nonstationary parametric pumping (carrier frequency ${\ensuremath{\omega}}_{p}\phantom{\rule{0ex}{0ex}}\ensuremath{\cong}2{\ensuremath{\omega}}_{1}$) in a three-wave process with conservation law ${\ensuremath{\omega}}_{p}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}{\ensuremath{\omega}}_{1}+{\ensuremath{\omega}}_{2}$, where ${\ensuremath{\omega}}_{2}$ is the carrier frequency of the reversed pulse. Theoretical analysis based on the solution of the system of equations for envelopes of interacting wave packets by means of a Green's function formalism gives a quantitative description of the observed phenomena.

Wave front reversal of a dipolar spin wave pulse in a nonstationary three-wave parametric interaction

A new phenomenon of momentum relaxation reversal has been discovered experimentally and explained theoretically for dipolar spin waves in magnetic garnet films. It is shown that the process of momentum relaxation, caused by the scattering of a signal wave on defects, can be reversed, and the signal can be restituted after it left the scattering region. The reversal of momentum relaxation is achieved by frequency-selective parametric amplification of a narrow band of scattered waves having low group velocities and frequencies close to the frequency of the original signal wave. The phenomenon can be used for the development of a new type of active microwave delay lines.

Reversal of momentum relaxation.

The interaction of a propagating wave packet (carrier frequency $\ensuremath{\omega}$, wave number $k)$ with nonadiabatic parametric pumping, localized in a region of size the order of the carrier wavelength $L\ensuremath{\sim}\ensuremath{\lambda}=2\ensuremath{\pi}/k$, is studied experimentally in a system of dipolar spin waves in a ferrite film. It is shown that the three-wave parametric interaction $\ensuremath{\omega}+{\ensuremath{\omega}}^{\ensuremath{'}}={\ensuremath{\omega}}_{p}$ leads to the formation of both contrapropagating ${(k}^{\ensuremath{'}}=\ensuremath{-}k)$ and copropagating ${(k}^{\ensuremath{'}}=k)$ idle wave packets of carrier frequency ${\ensuremath{\omega}}^{\ensuremath{'}}$. A system of equations derived for the packet envelopes gives a quantitative description of the observed random modulation of the output signal caused by the interference of the input and copropagating idle wave packets.

Nonadiabatic interaction of a propagating wave packet with localized parametric pumping.

Phase conjugation of magnetostatic waves by a local longitudinal pump in yttrium iron garnet films is observed experimentally. Theoretical expressions are obtained which describe the experimental curves well.

V. S. Tiberkevich

Papers

Parametric interaction of magnetostatic waves with a nonstationary local pump

Wave front reversal of a dipolar spin wave pulse in a nonstationary three-wave parametric interaction

Reversal of momentum relaxation.

Nonadiabatic interaction of a propagating wave packet with localized parametric pumping.

Phase conjugation of linear signals and solitons of magnetostatic waves