We investigate the effect of Ho and Gd impurities on the femtosecond laser-induced magnetization dynamics of thin permalloy films using the time-resolved magneto-optical Kerr effect. Varying the amount of Ho content from 0% to 8%, we observe a gradual change of the characteristic demagnetization time constant from ~60 fs to ~150 fs. In contrast, Gd concentrations up to 15% do not influence the time scale of the initial photoinduced magnetization loss. We propose a demagnetization mechanism that relies on strong magnetic inertia of the rare-earth dopant which stabilizes the ferrimagnetic ordering of the whole system.
The original publication is available at link DOI: 10.1103/PhysRevLett.102.117201