Harnessing GHz Frequency Spin‐Orbit Torque for Spin‐Wave Amplification in Nanomagnets

Abstract Spin‐orbit torque (SOT) has many applications in magnetism. Here, a new application is reported – the tunable amplification and spectral filtering of selected spin‐wave (SW) modes in a 2D periodic array of nanomagnets (a “magnonic crystal”) using alternating current SOT (ac‐SOT). Ultrashort laser pulses are used to excite the natural or intrinsic SW modes of the magnonic crystal which is placed in contact with a heavy metal nanostrip into which an alternating charge current is injected to produce an alternating spin current (ASC) in the nanomagnets via the spin Hall effect. The charge current frequency is tuned to the frequency of a specific intrinsic SW mode of the magnonic crystal to amplify the latter by resonant transfer of energy from the ASC to the SW. The amplification can be varied by varying the ac charge current amplitude, resulting in more than ten‐fold increment in the amplitude of some mode(s). Simultaneously, the linewidth of the spectrum for the amplified mode is narrowed. All these features can be explained by coupled mode theory. They improve the signal‐to‐noise ratio, and benefit GHz‐frequency information transmission and analog signal processing with SWs.