Strong on-Chip Microwave Photon−Magnon Coupling Using Ultralow-Damping Epitaxial Y₃Fe₅O₁₂ Films at 2 K

Yttrium iron garnet (Y₃Fe₅O₁₂, YIG) is one of the best magnetic materials for magnon-based quantum information science (QIS) because of its extremely low damping loss. For scalable on-chip QIS devices, ultralow-damping YIG films at mK temperatures are desired. However, almost all epitaxial YIG films are grown on rare-earth-containing garnet substrates which cause very large damping at low temperatures, hindering the use of YIG films for QIS studies. We report ultralow damping at 2 K in epitaxial Y₃Fe₅O₁₂ thin films grown on a diamagnetic Y₃Sc₂Ga₃O₁₂ (YSGG) substrate that contains no rare-earth elements. The extremely low damping of the YIG epitaxial films on diamagnetic YSGG substrates at very low temperatures is highly promising for QIS studies. As an initial step in this regard, we integrate the YIG/YSGG films with superconducting resonators for the study of coupling between magnons in a YIG film and microwave photons emitted by a superconducting coplanar waveguide resonator. We observe strong coupling between magnons in patterned YIG thin films and microwave photons in a superconducting Nb resonator at 2 K. This is the first demonstration that ultralow-damping YIG epitaxial films on YSGG can be integrated with superconductor resonators to achieve strong microwave photon-magnon coupling at few Kelvin temperatures. Such ultralow-damping YIG films offer advantages over metallic ferromagnets for on-chip hybrid quantum systems that incorporate magnonic conduits, microwave superconductor resonators, and superconductor qubits for QIS applications operating in the mK regime.