A common way to manipulate spin waves (or magnons, their quanta) is by electromagnetic radiation – optical and microwave light. The interaction between light and magnons is strongly enhanced in optical and microwave cavities, and this enhancement, after its experimental realization in the last decade, started the field of cavity magnonics.
I will demonstrate a few theoretical examples of interesting and unusual phenomena which appear in cavity magnonics. I will first talk about Brillouin scattering of light on magnons and show why the experimentally observed asymmetry between the sidebands appears, and how the same mechanism can be used to cool magnons. Then I will turn to interaction of magnons with microwave radiation and discuss phenomena such as uni-directional excitation of magnons in thin films, sub- and superradiance, and level attraction in open cavities. In the last part of the talk, I will show how direct interaction can be induced between a magnet and a qubit and how it can be efficiently used to create non-trivial quantum magnon states. I will also briefly highlight the activities of my group related to two-dimensional van der Waals magnets.