Spin Current Generation Driven by Ferromagnetic Resonance

Abstract

Conservation of angular momentum couples spin-currents present in a system with the magnetization dynamics of the ferromagnet. Injecting spin-currents into ferromagnets thereby enables the control of the magnetic state [1]. This forms the basis for many spintronic device applications including spin-transfer torque random access memory [2] and spin torque oscillators [3]. However, the inverse effect where the magnetization generates a spin current also takes place. This can be achieved very efficiently by driving the precession resonantly using an electromagnetic field at GHz frequencies. The same approach has been employed in ferromagnetic resonance spectroscopy experiments for over a hundred years now. Historically ferromagnetic resonance spectroscopy has extensively been used to determine the gyromagnetic ratio, magnetic anisotropies, and exchange stiffness of materials. In recent years the focus has shifted to utilizing broadband ferromagnetic resonance spectroscopy to determine the mechanisms responsible for magnetic damping [4]. In this context the observation that the precession of the magnetization can drive a spin-current into adjacent layers, is known as spin-pumping [5]. In ferromagnetic resonance spectroscopy spin pumping leads to a non-local contribution to the damping in the ferromagnet. This also enables the generation of pure spin currents by exciting the precession of a ferromagnet enabling new functionalities of spintronic devices. Based on this, ferromagnetic resonance spectroscopy can also be used to quantify the relevant parameters governing the spin current generation in these systems, including the interfacial spin mixing conductance [6] and the spin diffusion length [7]. In multilayer systems the spin currents generated at the ferromagnetic resonance frequently also result in the generation of a DC voltage most notably via the inverse spin Hall effect [8]. This has led to the ability to electrically detect ferromagnetic resonance with high sensitivity in small sample volumes, triggering a renewed interest in ferromagnetic resonance spectroscopy. We expect this to continue to enable new insights into the mechanisms foundational to the generation and detection of spin-currents driven by ferromagnetic resonance. **