Ultrafast demagnetization times in ferromagnetic alloys*

Upon laser excitation, a ferromagnet demagnetizes. However, it remains
unclear whether two different elements in the same ferromagnetic alloy
have the same or different demagnetization times. Experimental results
show that at least in permalloy, Ni delays Fe by 10-20 fs, but results
from other materials are inconclusive. In this presentation, we take
L10-FeNi as an example and employ the time-dependent Liouville
density-matrix functional theory to compute laser-induced ultrafast
demagnetization. We find that Ni demagnetizes a lot more than Fe. Fe
delays its demagnetization with respect to Ni. We then use FeNi₃ to
carry out further investigation, and we find a similar result. Our
results point out a more complex picture. Additional investigation is
necessary.