Picosecond Nonlinear Spintronic Dynamics Investigated By Terahertz Emission Spectroscopy

We demonstrate a control of spin-polarized currents and a nonlinear response to the excited state of magnetic films through measuring the terahertz waves generated by illuminating a W/CoFeB/Pt structure with a pair of femtosecond laser pulses. By changing the delay time of the two laser pulses, the waveforms and spectra of the terahertz waves are efficiently modulated. The terahertz wave generated by one pulse is obviously influenced by the other. The experimental results can be explained by a microscopic three-temperature model, which serves as proof for spin-flip scattering contributing to ultrafast demagnetization. Our experiment offers a new method for the investigation of ultrafast magnetism and terahertz spintronics.