Bursts of efficient terahertz radiation with saturation effect from metal-based ferromagnetic heterostructures

We report the broadband terahertz (THz) radiation in the metallic ferromagnetic (FM) heterostructures, upon irradiation of a femtosecond laser pulse at room temperature. The origin of THz generation from FM heterostructures can be interpreted using two terms: the transient demagnetization (a local modification of spin order of the FM metal) and electric-dipole radiation resulting from a non-local spin current pulses. Here, we show that the THz emission is dominated by the photo-excited transient charge current, which is converted from the spin current with inverse spin Hall effect. We tailor the metallic heterostructures with different non-magnetic thin layer (Pd or Ru) and FM materials (CoFeB or CoFe), to shape the THz transients. Moreover, we find that a saturation effect of THz radiation for CoFeB/Pd is less compared to CoFeB/Ru. THz emission spectroscopy can be used to qualitatively visualize the spin accumulation in the heterostructures.