Amplification of spin waves in ultra-thin Yttrium Iron Garnet microwaveguides by the spin-orbit torque

Abstract

The significant advantage of spin-orbit torque (SOT) for the emerging field of magnonics utilizing spin waves as a signal carrier in nano-circuits, is the possibility to exert spin-transfer torque over extended areas providing an opportunity to control the propagation length of spin waves. Another important advantage of SOT is the possibility to implement spin-torque devices based on low-damping insulating magnetic materials, such as Yttrium Iron Garnet (YIG). We studied the effects of SOT on the propagation of spin waves in 1 micrometer wide waveguides prepared from a 20 nm thick YIG film grown by the pulsed laser deposition and covered by an 8 nm thick layer of Pt. Propagating spin waves were detected with the submicrometer spatial resolution by using micro-focus Brillouin light scattering (BLS) spectroscopy, which allows the direct characterization of the spatial decay of propagating spin waves. The results of the measurements showed that, by using SOT, the propagation length of spin waves can be increased by nearly a factor of 10. This variation corresponds to the SOT-induced increase of the spin-wave intensity at the output of a 10 micrometer long transmission line by three orders of magnitude. Additionally, the high efficiency of our system allowed us to study the regime, where the damping is overcompensated by SOT and the true amplification of spin waves is expected. We show that, in this regime, the spin system of the YIG film is strongly overdriven, which results in the suppression of the spin-wave amplification. Our observations should stimulate both the development of advanced magnonic devices and theoretical work to deepen the understanding of the interaction of spin currents with spin waves