Spin Pumping into two-dimensional systems

Abstract

In this review, we present recent theoretical developments on spin transport phenomena probed by ferromagnetic resonance (FMR) modulation in two-dimensional systems coupled to magnetic materials. We first address FMR linewidth enhancements induced by spin pumping at interfaces, emphasizing their potential as sensitive probes of superconducting pairing symmetries in two-dimensional superconductors. We then examine FMR modulation due to spin pumping into two-dimensional electron gases formed in semiconductor heterostructures, where the interplay of Rashba and Dresselhaus spin-orbit interactions enables gate-controlled spin transport and persistent spin textures. Finally, we investigate spin pumping in monolayer transition-metal dichalcogenides, where spin-valley coupling and Berry curvature effects lead to valley-selective spin excitations and a spin-current Hall effect. These developments demonstrate that the spin pumping technique provides a versatile tool for probing spin transport and spin-dependent phenomena in low-dimensional systems, offering a basis for future spintronics applications.

Figures (11)

• Figure 1: A schematic illustration of spin pumping in a paramagnetic metal/ferromagnet bilayer system. Ferromagnetic resonance is induced by microwave irradiation, leading to the injection of a spin current into the metal via interfacial spin exchange interactions. As a back-action of spin pumping, the spin dynamics of the ferromagnet is modulated. This modulation encodes information about the spin excitation of conduction electrons, allowing the determination of their dynamic spin susceptibility in the adjacent metal. Simultaneously, a magnon carrying angular momentum $-\hbar$ is excited in the magnetic system, flipping the conduction electron spin from $+\hbar/2$ to $-\hbar/2$ via interfacial interaction. This process serves as a universal mechanism for the generation of spin currents.
• Figure 2: (a) Broadening of the FMR linewidth due to spin transfer at the interface between 25-nm-thick permalloy (Py) and single-layer graphene. Measurements were performed at room temperature and microwave frequency $9.62~\mathrm{GHz}$. Adapted from Ref. Tang2013. (b) Dependence of the effective damping enhancement on the Co$_3$FeB thickness ($t_{\mathrm{FM}}$) in a WS$_2$/Co$_3$FeB heterostructure. Adapted from Ref. Husain2022.
• Figure 3: Enhanced Gilbert damping $\delta\alpha_{\mathrm{G}}$ as a function of temperature $T$. (a) and (c) show $\delta\alpha_{\mathrm{G}}$ in the $s$-wave SC in the low- and high-frequency cases, respectively. (b) and (d) show $\delta\alpha_{\mathrm{G}}$ in the $d$-wave SC in the low- and high-frequency cases, respectively. $\delta\alpha_{\mathrm{G},n}$ is the normal-state value. Adapted from RefOminato2022.
• Figure 4: The enhanced Gilbert damping for (a),(b) Chiral $p$-wave SC and (c),(d) Helical $p$-wave SC. $\delta\alpha_{\mathrm{G},n}=S_0J^2D_{\mathrm{n}}(\epsilon_{\mathrm{F}})/(N_{\mathrm{FI}}k_{\mathrm{B}}T_c)$ is the characteristic value in the normal state. Adapted from Ref Ominato2022b.
• Figure 5: Spin dynamics of the superconducting NbN thin films probed via spin pumping. The normalized four-probe resistance (a) and Gilbert damping (b) as a function of the temperature for the samples of NbN (2)/GdN (5)/NbN (2) and NbN (10)/GdN (5)/NbN (10), respectively. Adapted from Ref Yao2018.