Laser pulse induced terahertz (THz) emission from Ta/${\left({\mathrm{Co}}_x{\mathrm{Fe}}_{1-x}\right)}_{80}{\mathrm{B}}_{20}$/MgO thin films, with varying compositions $x$ and annealing temperatures, is investigated. With increasing annealing temperature, the THz emission intensity exhibits significant dependence on $x$, with maxima at $x$ of 0.1–0.3. The $x$ dependence of the THz emission originated from the composition dependence of the spin currents induced in ${\mathrm{Co}}_x{\mathrm{Fe}}_{1-x}$ formed by the crystallization of amorphous CoFeB after the annealing. The origin of the laser-induced spin current is qualitatively discussed in terms of a ballistic transport of hot electrons and a spin-dependent Seebeck effect with different compositions.

• Received 25 June 2019
• Revised 17 September 2019

DOI:https://doi.org/10.1103/PhysRevB.100.140406