Ultrafast dynamics and THz oscillation in [Co/Pd]8 multilayers

•The presession frequency of magnetization is found to be tunable by varying the Co layer thickness but the frequency of the acoustic wave remains constant.•The increase in demagnetizing time with the decrease in Co layer thickness can be attributed to the inc rease in the spin–orbit interaction at the Co/Pd interface, which may affect the phonon mediated Elliott-Yafet type spin-flip scattering.•THz emission is observed following the demagnetization of the MLs and is proposed to be originated from the excitation of the THz transient phonons in the MLs.•A significant variation in the THz bandwidth is achieved by simply changing the magnetic layer thickness (tCo) of the ML which in turn changes the anisotropy of the system whereas the THz peak frequency remains constant around 2.0 THz.•The excitation of THz transient phonons as the origin of emitted THz radiation is proposed.

We report the experimental observation of ultrafast demagnetization associated with a significant terahertz (THz) emission in [Co/Pd]8 multilayers (MLs) with strong perpendicular magnetic anisotropy (PMA) by using a time-resolved magneto-optical Kerr effect (TR-MOKE) magnetometer. The THz emission is associated with subsequent remagnetization which is coupled with a collective picosecond magnetization dynamics and generation of acoustic waves in these multilayers. The demagnetization time increases slightly with decreasing Co layer thickness, while no such clear trend is observed in remagnetization timescales. The anisotropy field and corresponding precession frequency show systematic increase with the decrease of Co layer thickness. However, the acoustic wave frequencies remain unchanged with Co layer thickness. The inverse of the full width at half maximum of the THz radiation varies systematically with the anisotropy field, whereas the THz peak frequency remains almost constant at around 2 THz. The origin of THz radiation is explained in terms of excitation of THz transient phonons.