Charge and spin current pumping by ultrafast demagnetization dynamics

The authors predict here that nonperiodic magnetization dynamics in the field of ultrafast demagnetization, where a femtosecond laser pulse forces the magnetization vector to shrink without rotating, acts as the second (in addition to light itself) nonequilibrium drive on conduction electrons, leading to charge current pumping. The electromagnetic radiation by such pumped current contains proper spectral features at THz frequencies, as often observed experimentally but not fully understood microscopically. Since the time-dependent quantum transport formalism unraveling this effect also accounts for the well-known spin current pumping by periodic magnetization precession, the present study unifies, from a common microscopic viewpoint, such ``low energy'' effect with demagnetization dynamics as ``higher energy'' effect in spintronics.