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The ultrafast demagnetization dynamics of 3d and 4f spins, respectively, in FeCo and Tb of TbFeCo alloy film are studied independently by employing a dual-color time-resolved magneto-optical Kerr spectroscopy. The demagnetization dynamics of 3d and 4f spins are independently probed, respectively, by 800 and 400 nm light. Two-step demagnetization dynamics are observed for both the 3d and 4f spins under the excitation of 800 nm laser. In particular, the onset of 4f spin dynamics presents a delayed time with respect to the one of 3d spin dynamics. Those results clearly reveal a strong inter-atomic 3d–5d–4f exchange coupling which drives the first-step subpicosecond ultrafast demagnetization process of 4f spins, and a spin(4f)-lattice coupling which drives the second-step slower demagnetization process of 4f spins. A numerical calculation based on four temperature model reproduces the coupling characteristics in the demagnetization dynamics, and reveals the energy evolution dynamics among the different subsystems. These results provide a direct demonstration of strong coupling dynamics between the two spin subsystems in rare earth-transition metal alloy occurring within subpicosecond timescale, and show a new approach for ultrafast control of 4f spins via an indirect excitation.

Ultrafast dynamics of 4f electron spins in TbFeCo film driven by inter-atomic 3d–5d–4f exchange coupling