THz emission manipulation and thermal robustness in CoFeB films via boron doping

Spintronic terahertz emitters (STEs) based on ferromagnet/nonmagnet heterostructures present a compelling alternative to conventional THz sources. In this work, we systematically investigate the effects of boron (B) doping and post-deposition annealing on spin transport and THz emission in W/(CoFe)_1−xB_x/Al trilayers with B concentrations from 0% to 20%. Time-domain THz emission spectroscopy reveals a monotonic increase in THz field amplitude with increasing B content, despite the reduction in saturation magnetization. This enhancement is attributed to more efficient spin current injection, evidenced by increased spin mixing conductance (g ^↑↓) extracted from time-resolved magneto-optical Kerr effect measurements, and reduced THz reabsorption stemming from lower THz conductivity in B-rich samples. Furthermore, thermal annealing also reveals a modulation effect induced by B doping, where the THz emission varies nonmonotonically with annealing temperature and peaks near 300°C. Below this critical temperature, B precipitation improves magnetic ordering and boosts THz emission, whereas higher temperatures degrade spin transport due to interlayer diffusion and increase THz absorption, primarily as a result of crystallization-induced conductivity enhancement. Compared to undoped CoFe, B-doped alloy films exhibit both superior emission intensity and enhanced thermal stability, demonstrating great potential for efficient and robust STE applications.