Study of spin pumping damping and THz emission in CoFe/Ti_xW_1−x bilayers with various Ti concentrations

Laser-induced magnetization dynamics and terahertz (THz) emission in CoFe/Ti_xW_1−x bilayers with varying Ti concentrations are systematically investigated using the time-resolved magneto-optical Kerr effect and time-domain THz emission spectroscopy. The incorporation of Ti into heavy metal W leads to a significant reduction in spin pumping damping, particularly for Ti concentrations below 50%. Similarly, the THz emission peak amplitude decreases with increasing Ti concentration. Both effects are attributed primarily to the reduced spin current transmittance at the CoFe/Ti_xW_1−x interface, caused by the substantially decreased electrical conductivity of the Ti_xW_1−x layer. Interestingly, while spin pumping damping continues to decrease, the THz emission amplitude starts to increase at x = 63%, where the THz signal approaches zero due to the opposite spin Hall angles of W and Ti. This behavior underscores the distinct yet correlated mechanisms governing spin pumping damping and THz emission, reflecting their specific dependences on spin current reflection, propagation, dissipation, and spin-to-charge conversion in ferromagnet/nonmagnetic metal bilayer systems. These findings enhance our understanding of ultrafast spin dynamics and spin transport properties, offering valuable insights for advancing the development of miniaturized and high-speed spintronic devices.