Generation of coherent spin-wave modes in Yttrium Iron Garnet microdiscs by spin-orbit torque

TL;DR

This paper demonstrates coherent spin-wave auto-oscillations in micron-sized, 20 nm thick YIG discs driven by spin-orbit torque, highlighting the role of quasi-degenerate modes and size reduction for damping compensation.

Contribution

It provides the first clear evidence of coherent auto-oscillations in thin YIG microdiscs induced by spin-orbit torque, emphasizing the importance of mode degeneracy and size effects.

Findings

Coherent auto-oscillations observed in 20 nm thick YIG discs.

Quasi-degenerate spin-wave modes increase the threshold current.

Size reduction enables damping compensation in YIG microdiscs.

Abstract

Spin-orbit effects [1-4] have the potential of radically changing the field of spintronics by allowing transfer of spin angular momentum to a whole new class of materials. In a seminal letter to Nature [5], Kajiwara et al. showed that by depositing Platinum (Pt, a normal metal) on top of a 1.3 $\mu$m thick Yttrium Iron Garnet (YIG, a magnetic insulator), one could effectively transfer spin angular momentum through the interface between these two different materials. The outstanding feature was the detection of auto-oscillation of the YIG when enough dc current was passed in the Pt. This finding has created a great excitement in the community for two reasons: first, one could control electronically the damping of insulators, which can offer improved properties compared to metals, and here YIG has the lowest damping known in nature; second, the damping compensation could be achieved on very large objects, a particularly relevant point for the field of magnonics [6,7] whose aim is to use spin-waves as carriers of information. However, the degree of coherence of the observed auto-oscillations has not been addressed in ref. [5]. In this work, we emphasize the key role of quasi-degenerate spin-wave modes, which increase the threshold current. This requires to reduce both the thickness and lateral size in order to reach full damping compensation [8] , and we show clear evidence of coherent spin-orbit torque induced auto-oscillation in micron-sized YIG discs of thickness 20 nm.