Self-induced spin pumping and inverse spin Hall Effect in single FePt thin films

Self-induced spin Hall effect and self-torque hold great promise in the field of spintronics, offering a path toward highly efficient spin-to-charge interconversion, a pivotal advancement for data storage, sensing devices, or unconventional computing. In this study, we investigate the spin-charge current conversion characteristics of chemically disordered ferromagnetic single FePt thin films by spin-pumping ferromagnetic resonance experiments performed on both a resonance cavity and on patterned devices. We clearly observe a self-induced signal in a single FePt layer. The sign of a single FePt spin pumping voltage signal is consistent with a typical bilayer with a positive spin Hall angle layer such as that of Pt on top of a ferromagnet (FM), substrate//FM/Pt. Structural analysis shows a composition gradient due to natural oxidation at both FePt interfaces, with the Si substrate and with the air. The FePt-thickness dependence of the self-induced charge current produced allowed us to obtain λ_FePt = (1.5 ± 0.1) nm and self-induced θ_self-FePt = 0.047 ± 0.003, with efficiency for reciprocal effects applications θ_self-FePt × λ_FePt = 0.071 nm which is comparable to that of Pt, θ_SH-Pt × λ_Pt = 0.2 nm. The spin pumping voltage is also observed in a symmetrical stacking, Al/FePt/Al with a lower overall efficiency. Moreover, by studying bilayer systems such as Si//FePt/Pt and Si//Pt//FePt we independently could extract the individual contributions of the external inverse spin Hall effect of Pt and the self-induced inverse spin Hall effect of FePt. Notably, this method gives consistent values of charge currents produced due to only self-induced inverse spin Hall effect in FePt layers. These results advance our understanding of spin-to-charge interconversion mechanisms in composite thin films and pave the way for the development of next-generation spintronics devices based on self-torque.