Enhancement of the Spin Hall Angle by Interdiffusion of Atoms in Co2FeAl0.5Si0.5 / n - Ge Heterostructures

Abstract

Realizing efficient spin-charge conversion is an important issue in spin-based electronics, especially for the development of low-power-consumption magnetization switching. A spin-charge conversion is generally induced in a nonmagnetic (NM) layer by the inverse spin Hall effect in the bulk or by the inverse Rashba-Edelstein effect at surfaces or interfaces. In the bulk, the spin-charge conversion efficiency is called the spin Hall angle, ${\ensuremath{\theta}}_{\mathrm{SHE}}$, and it is limited by the spin-orbit interaction of the bulk layer. Thus, artificially enhancing ${\ensuremath{\theta}}_{\mathrm{SHE}}$ to greater than its intrinsic value is difficult. Here, using spin-pumping measurements on a well-controlled interface between a ferromagnetic (FM) ${\mathrm{Co}}_{2}{\mathrm{Fe}\mathrm{Al}}_{0.5}{\mathrm{Si}}_{0.5}$ (CFAS) layer and a NM n-type $\mathrm{Ge}$ layer, we demonstrate that ${\ensuremath{\theta}}_{\mathrm{SHE}}$ can be strongly enhanced by annealing the sample and the resulting interdiffusion of atoms within only about 3.7 nm around the CFAS/n-$\mathrm{Ge}$ interface. Because of this annealing process, ${\ensuremath{\theta}}_{\mathrm{SHE}}$ is increased from 0.0058--0.0079 to 0.015--0.019, which is much larger than the intrinsic value of ${\ensuremath{\theta}}_{\mathrm{SHE}}$ reported for $\mathrm{Ge}$ (0.00096--0.002) and is comparable to ${\ensuremath{\theta}}_{\mathrm{SHE}}$ values reported for $\mathrm{Pt}$. This enhancement is attributed to strong scattering of the spin current in the intermixed layer formed at the interface by annealing. Our results indicate that ${\ensuremath{\theta}}_{\mathrm{SHE}}$ is strongly influenced by the atomic structure of the FM/NM interface, which suggests an efficient method to control spin-charge conversion by precise control of the FM/NM interface.