Ultrafast spin dynamics: role of laser-induced modification of exchange parameters
arxiv(2024)
摘要
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 discussed in the literature as 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 (TD-DFT)
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
in response to the applied ultra-short laser pulse. The most important reason
for this modification is played by the laser induced repopulation of the
electronic states. Although the changes of the exchange parameters are most
prominent during the laser pulse, they may be important also for the magnetic
relaxation. 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.
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