The ideal substrate for yttrium iron garnet films in quantum magnonics

Abstract Quantum magnonics leverages the quantum properties of magnons to advance nanoscale quantum information technologies. Ferrimagnetic yttrium iron garnet (YIG), valued for its exceptionally low magnetic damping, is typically grown as thin films on gadolinium gallium garnet (GGG) for lattice matching. However, paramagnetic GGG introduces additional damping at low temperatures due to substrate magnetization, limiting quantum applications. Here, we study magnetic damping in a 150 nm-thick YIG film grown on yttrium scandium gallium aluminum garnet (YSGAG), a newly developed diamagnetic alternative to GGG. Ferromagnetic resonance spectroscopy down to 30 mK reveals that YIG/YSGAG maintains low damping from room temperature to millikelvin temperatures, with a room-temperature Gilbert damping of 4.29 × 10 −5 , comparable to state-of-the-art YIG/GGG films and bulk YIG. Crucially, no low-temperature damping increase is observed. By eliminating paramagnetic substrate-induced dissipation, YSGAG enables consistently low damping in YIG films across the entire temperature range, establishing it as an ideal substrate for quantum magnonics and paving the way for spin-wave-based quantum technologies.