Electrical detection of spin-microwave coupling in a cavity

Abstract

Via exploring the spin dynamics and transport properties at interfaces of magnetic material/non-magnetic metal bilayers, spintronics advanced many techniques on generation, detection, and manipulation of spin currents, which laid down a foundation for designing spintronic devices. New ideas and methods for manipulating spin are required in the spintronics community. Merging with cavity quantum electrodynamics, magnetic resonance in ferromagnets inside of a microwave cavity, realizes a strong magnon-microwave coupling at room temperature, opening an avenue to manipulate spin and spin current for the spintronics community. Spin-microwave coupling strength was significantly improved by replacing a few spins in a paramagnetic spin system with magnetization in ferromagnetic materials. Thus, a strongly coupled magnon-microwave system offers a hybrid quantum platform with many measures to tune the coupled system, such as an exchange magnetic field, an anisotropy field, controlled coupling strength, controlled damping parameters, and so on. Many devices were proposed and demonstrated with potential applications. In this review, we briefly introduce the concept and mechanism of magnon-microwave coupling, a classical electrodynamical coupling model for the coupling, spin currents produced by the coupled magnon, and manipulation of the spin current via the strong coupling. In ferromagnets, magnons can couple to a microwave mode as a magnon-polariton propagating in the materials. The strongly coupled magnon-microwave system is a magnon-polariton in a cavity where the microwave was confined as a resonance mode with a high quality factor. Using the Landau-Lifshitz-Gilbert equation and Maxwell equations, we introduce the classical electrodynamical coupling model and reveal that the magnon is driven by a torque produced by the microwave magnetic field on the magnetization and the microwave is feedback via Faraday’s law. Therefore, the coupling strength of the magnon-microwave system can be controlled by changing the torque. Magnon-microwave coupling is experimentally studied using many techniques, such as microwave transmission, Brillouin light scattering, Faraday rotation measurement, and spin pumping electrical detection. Spin pumping is well explored in the spintronics community as a spin current produced by magnon and detected as a voltage via inverse spin Hall effect in a novel metal. Combining with microwave transmission of the coupled microwave subsystem, spin pumping electrical detection by directly detecting the spin current produced by the magnon subsystem in the coupled system, illustrates a more complete profile of the coupled magnon-microwave system. In addition, the strongly coupled magnon-microwave system offers a new technique to manipulate spin currents. We experimentally demonstrate that spin currents produced by the strongly coupled system are correlated with each other mediated by the microwave cavity, and can be manipulated remotely. Two almost identical magnet samples are used to couple to a cavity and the coupling strengths between each magnet and the cavity are tunable individually. By using electrical detection to locally detect each magnet sample respectively, we find that the spin currents produced by two magnet samples are coherently correlated. By tuning the coupling strength of one magnet sample to the cavity, the spin current produced by the second magnet sample is manipulated distantly over a few centimeters, which is far longer than the distance of spin-orbit interaction or exchange interaction. This distant control is only limited by the coherence length of the microwave and the dimension of the cavity. This flexibility opens the door to improve spin current generation and manipulation for cavity spintronic devices.