Magnon Damping in Yttrium Iron Garnet Films at Millikelvin Temperatures*

Magnon-based quantum devices require low damping materials to maintain coherence. One of the most promising candidates is yttrium iron garnet (YIG). A ferrimagnetic insulator, YIG has the narrowest known linewidth of any practical material at room temperature. Bulk YIG has been shown to have low damping at millikelvin (mK) temperatures. However, there is some debate about whether this is the case in YIG films. Much of the uncertainty centres around the fact that the low-temperature magnetic behaviour of the substrate upon which high-quality samples are grown -- gadolinium gallium garnet (GGG) -- is not fully understood.

In this work, we study magnon damping in YIG films with and without GGG substrates by measuring the resonance linewidth at low temperatures. We observe that GGG contributes to significantly increased damping at low temperatures. Further measurements up to 9K reveal that temperature-peak processes are active above 1K and the presence of additional damping at mK temperatures which we attribute to the two-level fluctuators. Upon saturating the fluctuators, linewidths of 1.4 MHz are achievable in substrate-free YIG films at 20 mK, lower than the measured linewidth in the same films at room temperature.