Different from thermally and magnetically induced demagnetization, the laser-induced demagnetization relies on the laser photon field. However, what has been unknown is how the spin moment reduction correlates with the number of photons. Here, our first-principles calculation in ferromagnetic nickel and cobalt clusters shows that the number of photons is not the sole decisive factor for the magnetization change, contrary to earlier belief. For the same number of photons absorbed, the shorter the laser pulse, the larger the induced spin moment reduction. Besides a simple decrease in the magnetic moment, a short pulse also excites a strong coherent spin oscillation, which disappears when using a longer pulse. The longest pulse duration where we observed this oscillation is about 20 fs. Future experiments can directly test our prediction. We show that for our generic uitrafast spin-switching A-process on metallic nanoclusters the electronic correlations constitute a key ingredient, which allows for spin and charge separation. By selectively removing correlational channels, we gradually inhibit the magnetic switching. The dynamics proceeds far from any electronic thermal equilibrium and thus no temperature can be attributed to the system or any subsystem.
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