Microwave magnon damping in YIG films at millikelvin temperatures

Magnon systems used in quantum devices require low damping if coherence is to be maintained. The ferrimagnetic electrical insulator yttrium iron garnet (YIG) has low magnon damping at room temperature and is a strong candidate to host microwave magnon excitations in future quantum devices. Monocrystalline YIG films are typically grown on a gadolinium gallium garnet (GGG) substrate. In this work, comparative experiments made on YIG waveguides with and without GGG substrates indicate that the substrate plays a significant role in increasing the damping at low temperatures. Measurements reveal that damping due to temperature-peak processes is dominant above 1K and that damping due to the two-level fluctuators (TLFs) is present below 1K. Upon saturating TLFs in the substrate-free YIG at 20mK, linewidths of ~1.4MHz are achievable, lower than the linewidths measured in the same film at room temperature. These results provide a key perspective on the potential of implementing solid-state quantum technologies based on YIG films.