Cavity Spintronics: An Early Review of Recent Progress in the Study of Magnon–Photon Level Repulsion
Abstract Light–matter interactions lie at the heart of condensed matter physics, providing physical insight into material behavior 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 (CMP). In this early review of recent work the fundamental behavior of the CMP 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 universal model of coupled oscillators; however, the true origin of hybridization is only revealed by considering a more comprehensive electrodynamic framework. Here both approaches are summarized and an outline of a quantum description through the input–output formalism is provided. Based on this foundation, in depth experimental investigations of the coupled spin–photon system have been performed. For example, it has been found that hybridization will influence spin current generated through the spin pumping mechanism, demonstrating a firm link between spin–photon coupling and spintronics. Furthermore several in situ coupling control mechanisms, which offer both physical insight and a means to develop cavity spintronic technologies, have been revealed. The many recent developments within this field represent only the first steps in what appears to be a bright future for cavity spintronics. Hopefully this early review will introduce new explorers to this exciting frontier of condensed matter research, lying at the crossroads of magnetism and cavity quantum electrodynamics.