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 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 lightmatter 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 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 cavityspintronic 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.

Steering between level repulsion and attraction: broad tunability of two-port driven cavity magnon-polaritons

“…In figure 1(c), we illustrate the CMP by a system of two coupled harmonic oscillators with individual loss parameters κ c for the cavity photons and κ m for the magnons and corresponding coupling losses due to the coupling to the external microwave feedline (see [7]). Now, such an introduction of a second input in the experimental setup for the study of the CMP, requires the modification of the standard reflection parameter S 11 (ω).…”

“…However, in order to include a control of the coupling strength via the additional torque which can compensate for the dissipation in the system, the above Hamiltonian needs to be non-Hermitian. Also, considering the 2×2 matrix by modeling the CMP, for instance, by two coupled harmonic oscillators where the off-diagonal elements representing the coupling terms [7], level merging is only possible if the product of the off-diagonal terms is negative. It cannot be a positive, real valued quantity because the interaction potential would be repulsive.…”

“…The phenomena of (strong) coupling of magnons-the associated quanta of collective spin wave excitations-to microwave cavity photons resulting in cavity magnon-polaritons (CMPs) has been the subject of numerous works in the past few years [1][2][3][4][5][6][7][8][9]. The ability to couple magnons to different physical systems, through magnetooptical [10][11][12] to optical, or by magnetostrictive interaction to mechanical [13] and cavity photons simultaneously makes CMPs highly interesting for various applications [8].…”

“…Rather, g 0 is determined by the photon and magnon mode overlap η, the resonance frequency ω 0 , and mode volume V mode of the chosen cavity resonator mode. The observation of such an avoided crossing (anticrossing) is a characteristic feature of cavity magnon-polaritons (CMPs) and it enables the study of properties of said systems [7].…”

Microscopic origin of level attraction for a coupled magnon-photon system in a microwave cavity

“…Microwave cavity resonators are useful for coupling microwave photons to various excitations including mechanical [1], acoustic [2], and magnetic degrees of freedom [3]. In magnetism, hybridization between cavity modes and ferromagnetic resonances in the form of cavity magnon-polaritons has been demonstrated and studied in a number of systems [4][5][6][7][8][9][10][11][12].…”