Ultrafast spin dynamics: role of laser-induced modification of exchange parameters

Induced by an ultra-short laser pulse, the electronic structure of a material undergoes strong modifications leading to a fast demagnetization in magnetic materials. Induced spin-flip transitions are one of the reasons for demagnetization, that is associated in the literature with a Stoner-like mechanism. On the other hand, demagnetization due to transverse spin fluctuations is usually discussed on the basis of the Heisenberg Hamiltonian and hardly accounts for the modification of the electronic structure. In this work we demonstrate a strong impact of the laser-induced electron transitions, both spin-flip and spin-conserving, on the exchange coupling parameters. For this, a simple two-step scheme is suggested. As a first step, the electronic structure time evolution during the ultra-short laser pulse is described accurately within time-dependent density-functional theory calculations. As a next step, the information on the time-dependent electronic structure is used for calculations of the parameters of the Heisenberg Hamiltonian. A strong modification of the exchange coupling parameters is found on the femtosecond time scale as a consequence of the depopulation of the electronic states in response to the applied ultra-short laser pulse. Due to the relatively slow attenuation of these changes after the laser pulse, taking place on the subpicosecond/picosecond time scale, we expect an appreciable impact on the magnon energies and that way on the demagnetization/relaxation processes. The same concerns the spin-lattice interactions playing a central role for the relaxation process. A strong impact of the laser-induced modification of the electronic structure on the spin-lattice coupling parameters is also shown in this work.