Cavity-free nano-magnomechanics with a thin-film magnon waveguide supported by cantilever and bridge geometries

While photons have been widely explored for the development of platforms that couple quantum objects together, there are fundamental constraints that limit their applications. Typically, these photons populate cavities whereby supported resonant frequencies are dependent on the geometry of the device, and there is little scope offered to change this in-situ. Magnons, on the other hand, can couple to a variety of subsystems such as photons and phonons and support a broad frequency range dependent on an applied biasing field. The interaction between magnons and phonons, parametric in nature, has been explored experimentally prior using a sphere of yttrium-iron-garnet and a copper cavity, although the device implementation is bulky and incompatible with on-chip realisations. To this end, this thesis examines the possibility of a ’cavity-free’ realisation of parametric magnon-phonon coupling, in which the device need not be situated within a physical cavity, rather driven by a superconducting wave-guide. For this, magnons supported in thin films of yttrium-iron-garnet are coupled to the phonon modes of the bulk substrates on which it is grown to mediate coupling. This offers both a nano-scale implementation compatible with chip designs as well as the promise of a versatile device in which substrates of a smaller mechanical damping could easily be integrated.

The theory of coupling between phonons and magnons is derived assuming that the bulk substrate can be treated as an Euler-Bernoulli beam and that the thin magnetic film does not impact the elastic dynamics. Analysis finds that a single magnon-phonon coupling rate some three to four orders of magnitude larger than that previously reported using the YIG sphere and copper cavity device setup, and that cooperativities on the order of 100s to 1000s should be attainable using this newly proposed device. This thesis also considers a number of bench-marking phenomena to establish where the device sits in the larger landscape of coupled systems. For a pre-cooled device at currently attainable cryogenic temperatures, accessing the mechanical ground state should be feasible. The emergence of magnomechanically induced transparency windows is also considered, and the strength of the magnon-phonon interaction leads to behaviour of the mechanical sidebands previously unseen. Lastly, the magnon spring effect is found to permit significant spring
hardening of the phonons almost 3 orders of magnitude larger than previously reported even in the resolved-sideband regime.