When a laser pulse excites a ferromagnet, its spin undergoes a dramaticchange. The initial demagnetization process is very fast. Experimentally, it isfound that the demagnetization time is related to the spin moment in thesample. In this study, we employ the first-principles method to directlysimulate such a process. We use the fixed spin moment method to change the spinmoment in ferromagnetic nickel, and then we employ the Liouville equation tocouple the laser pulse to the system. We find that in general the dependence ofdemagnetization time on the spin moment is nonlinear: It decreases with thespin moment up to a point, after which an increase with the spin moment isobserved, followed by a second decrease. To understand this, we employ anextended Heisenberg model, which includes both the exchange interaction andspin-orbit coupling. The model directly links the demagnetization rate to thespin moment itself and demonstrates analytically that the spin relaxes moreslowly with a small spin moment. A future experimental test of our predictionsis needed.
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