Strong coupling between quantized magnon modes in a YIG microstructure and microwaves in a superconducting resonator

Strong-coupling experiments based on magnons enable the exploration of on-chip demonstrations involving numerous long-lived excitations. Yttrium iron garnet has been considered for decades as a gold standard material for magnonics due to its low-loss magnonic properties. While YIG has successfully demonstrated strong-coupling in macroscopic device geometries, the strong coupling of magnons in truly sub-10 μm YIG structures to date has not yet been realized. This is due to the difficulty of producing the necessary effective magnonic mode volume, primarily due to limitations of device fabrication techniques. Here, we demonstrate the use of a microplatelet of YIG, manufactured from a single crystal via focused ion beam techniques, placed on a constricted inductive line of an optimized superconducting lumped element LC resonator to achieve strong coupling between numerous magnon modes and the LC resonator photons. These experimental findings are qualitatively backed by micromagnetic simulations and quantitatively supported by analytical calculations to identify the magnon modes corresponding to the experimentally observed anti-crossings in the microwave transmission signal. Further, we show that these anti-crossings remain even at incredibly low device input powers (≤1 pW), while indications of magnon–photon coupling remain visible at powers ≤10 fW. The fabrication techniques and device geometry enable the deterministic use of numerous confined magnon modes in micrometer-scale YIG structures for various magnetic field strengths and orientations at substantially reduced device powers. The results here establish a foundational path forward to achieving efficient magnon-based strong-coupling experiments in micrometer-scale YIG magnetic elements for effective on-chip studies.