Generation of out-of-plane polarized spin current in (permalloy, Cu)/EuS interfaces

The generation of out-of-plane polarized spin current is crucial for efficiently manipulating perpendicularly magnetized systems used in high-density magnetic recording. Here, we demonstrate the generation of out-of-plane polarized spin current at room temperature using an insulator, EuS. By employing angle-resolved spin-torque ferromagnetic resonance, we find a large unconventional out-of-plane torque conductivity, ${\sigma }_{\mathrm{DL}}^z=-0.13\times {10}^5(\hslash /2e){\left(\Omega m\right)}^{-1}$ in the $\mathrm{Py}(={\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19})/\mathrm{EuS}$ bilayer, which is comparable to the conventional in-plane dampinglike torque conductivity, ${\sigma }_{\mathrm{DL}}^y$. Additionally, a giant in-plane fieldlike torque (${\sigma }_{\mathrm{FL}}^z$) with a magnitude of 27 times larger than that of the conventional out-of-plane fieldlike torque (${\sigma }_{\mathrm{FL}}^y$) is also observed in the Py/EuS bilayer. The unconventional torques due to the out-of-plane polarized spin current (${\sigma }_{\mathrm{DL}}^z$ and ${\sigma }_{\mathrm{FL}}^z$) persist even by inserting a 10-nm-thick Cu layer between Py and EuS. Our findings demonstrate that the unconventional torques in these systems originate from the interfaces through spin swapping and/or spin-orbit precession mechanisms.