Mechanisms of ultrafast demagnetization and reverse spin Hall effect in cobalt-based terahertz thin-film emitters

The effect of the cobalt layer thickness on the magnetic characteristics in a series of spintronic terahertz emitters is analyzed. Both single-layer Co($d$) films, where $d$ is a variable thickness from 3 to 10 nm, and two-layer structures of Co($d$)/W(3 nm) and W(3 nm)/Co($d$) with uniaxial magnetic anisotropy are considered. The results emphasize the critical role of structural parameters, especially in thin layers, where an increase in magnetic stiffness and coercive field is observed due to an increase in the density of defects at the interfaces. It is shown that optimal magnetic characteristics and energy efficiency are achieved in ferromagnetic films with a thickness of about 10 nm, which is important for the development of THz emitters with high polarization controllability. The study demonstrates that the structure of Co(10 nm)/W(3 nm) provides a better balance between the efficiency of THz emission and the coercive field, due to a combination of mechanisms of ultrafast demagnetization and the reverse spin Hall effect