UC Santa Cruz

Recent advances in quantum technologies have garnered significant attention due to their promising applications in future computing, sensing, and various other fields. At the core of these advancements lies the hybrid quantum system, which leverages the advantages of two quanta through their coherent coupling. The key figure of merit for quantifying the strength of the coherent coupling is cooperativity, C, where C > 1 indicates strong coupling. Among the various hybrid systems, spin waves (magnons) stand out due to their tunability in high-frequency applications up to the THz range. However, magnons suffer from a short lifetime. In this context, collective lattice vibrations (phonons) can significantly enhance magnons when coherently coupled to magnons due to their longer lifetime. Recently, Strong coupling in a magnon-phonon hybrid system was experimentally demonstrated using a single polycrystalline nickel (Ni) nanomagnet, showcasing the potential of magnonic hybrid quantum systems coupled to phonons.In this thesis, an engineering strategy to improve hybrid magnon-phonon quantum systems in the technologically relevant cobalt iron (CoFe) material system is presented, achieving a 23 times enhancement compared to a single Ni nanomagnet. First, the materials engineering perspective will be presented by introducing CoFe, a material readily available and commercially used in spintronics applications, such as data storage devices. CoFe possesses a high magnetostriction coefficient, which benefits the strength of magnon-phonon coupling. In order to minimize damping and maximize coherent coupling, epitaxially grown, single-crystal CoFe is chosen. First, optical characterization of the magnetic dynamics in an epitaxially-grown CoFe thin film is presented, showing significantly improved performance compared to polycrystalline films. Next, the development of a nanofabrication process to define high-quality single crystal CoFe nanomagnets is discussed. Using this process, we achieved a cooperativity of ? = 38 in a single CoFe nanomagnet, representing a 23-fold improvement in cooperativity compared to systems using poly-crystalline Ni. This work identifies CoFe as a promising candidate for rare-earth-free magnon-phonon hybrid quantum systems.