We investigate the effect of Ho, Dy, Tb, and Gd impurities on the femtosecond laser-induced magnetization dynamics of thin Permalloyud films using the time-resolved magneto-optical Kerr effect. Varying theud amount of Ho, Dy, Tb content from 0% to 8%, we observe a gradualud change of the characteristic demagnetization time constant fromud approximate to 60 to approximate to 150 fs. In contrast, Gdud concentrations up to 15% do not influence the time scale of theud initial photoinduced magnetization loss. We propose a demagnetizationud mechanism that relies on strong magnetic inertia of the rare-earthud dopant which stabilizes the ferrimagnetic ordering and thereby delaysud the demagnetization.
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