Cavity Spintronics: An Early Review of Recent Progress in the Study of Magnon-Photon Level Repulsion

Light-matter interactions lie at the heart of condensed matter physics, providing physical insight into material behaviour while enabling the design of new devices. Perhaps this is most evident in the push to develop quantum information and spintronic technologies. On the side of quantum information, engineered light-matter interactions offer a powerful means to access and control quantum states, while at the same time new insights into spin-photon manipulation will benefit the development of spintronic technologies. In this context the recent discovery of hybridization between ferromagnets and cavity photons has ushered in a new era of light-matter exploration at the crossroads of quantum information and spintronics. The key player in this rapidly developing field of cavity spintronics is a quasiparticle, the cavity-magnon-polariton. In this early review of recent work, the fundamental behaviour of the cavity-magnon-polariton is summarized and related to the development of new spintronic applications. In the last few years a comprehensive theoretical framework of spin-photon hybridization has been developed. On an intuitive level many features can be described by a model of coupled oscillators, however the origin of hybridization is only revealed by considering a comprehensive electrodynamic framework. Here both approaches are summarized and a quantum description using the input-output formalism is outlined. Based on this foundation, in depth experimental investigations of the coupled spin-photon system have been performed. For example, the influence of hybridization on spin current generation has been revealed and several in-situ coupling control mechanisms have been developed. The many recent developments within this field represent only the first steps in what appears to be a bright future for cavity spintronics.