Ultrafast light-driven magneto-optical nonlinearity in ferromagnetic heterostructures.

The dynamic control of magnetization by short laser pulses has recently attracted interest. The transient magnetization at the metallic magnetic interface has been investigated through second-harmonic generation and the time-resolved magneto-optical effect. However, the ultrafast light-driven magneto-optical nonlinearity in ferromagnetic heterostructures for terahertz (THz) radiation remains unclear. Here, we present THz generation from a metallic heterostructure, Pt/CoFeB/Ta, which is ascribed to an ∼6-8% contribution from the magnetization-induced optical rectification and an ∼94-92% contribution from both spin-to-charge current conversion and ultrafast demagnetization. Our results show that THz-emission spectroscopy is a powerful tool to study the picosecond-time-scale nonlinear magneto-optical effect in ferromagnetic heterostructures.