  We investigated the magnetic properties of Sendust (Fe-Al-Si) alloys not only
at 0 K but also at finite-temperatures by means of the first-principles
calculations assuming A2, B2, and DO3 structures. We confirmed that the
itinerant characteristics of 3d electrons of Fe are not negligible for A2 and
B2 structures and a significantly small exchange stiffness constant exists at
zero-temperature in a B2 structure. However, the calculated Curie temperatures
are in the same order for all structures; this indicates that the Curie
temperature cannot be determined only by the exchange interactions at
zero-temperature in itinerant electron systems. Temperature dependence of the
exchange interaction, namely spin configuration dependence, also might be
important for determining it. In addition, this property might also be related
to the unique behavior of the temperature dependence of the exchange stiffness
constant for the B2 structure, which does not decrease monotonically as
temperatures increase, contrary to the behavior expected from the Heisenberg
model. In addition, we investigated composition dependence on the exchange
stiffness constant at zero-temperature and confirmed that the substitution of
Si with Al could improve the amplitude of the exchange stiffness constant at
zero-temperature for all structures.
