Strong coupling of microwaves and magnons in YIG microstructures

Georg Schmidt, Martin Luther Univ. Halle

Strong coupling between microwaves and magnons has already been demonstrated. So, magnon modes in an yttrium iron garnet (YIG) sphere of several hundred micrometer diameter were successfully coupled to the microwave modes of a large microwave cavity[1]. Also coupling of magnons in YIG to phonons[2] or to optical photons[3] has already been shown. All these experiments have in common that rather macroscopic pieces of YIG were used. This is unfavorable if the effect is to be integrated for device purposes, both in terms of size and technology.
On the other hand coupling between magnetic microstructures and superconducting resonators has been reported making use of ferromagnetic metals that can easily be patterned[4,5]. Nevertheless, the lifetime of spin waves in ferromagnetic metals is rather small and although strong coupling could be demonstrated, it would be desirable to use microscopic YIG resonators instead.
We have realized coupling between microwave photons in superconducting lumped element resonators and magnons in Permalloy and YIG nanostructures, respectively. With the metallic ferromagnet we realize an unusual coupling behavior because of the strong shape anisotropy in an elongated structure. With YIG, we are able to reach the strong coupling regime. This is possible because of an optimized lumped element resonator that concentrates the magnetic field in the magnetic microstructure.

[1] e.g. Y. Tabuchi et al., Phys. Rev. Lett. 113, 083603 (2014)
[2] X. Zhang et al., Science Advances 2, e1501286
[3] A. Osada et al., Phys. Rev. Lett. 116, 223601 (2016)
[4] Y. Li et al., Phys. Rev. Lett. 123, 107701 (2019)
[5] J.T. Hou et al., Phys. Rev. Lett. 123, 107702 (2019)

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