Magnetization and exchange-stiffness constants of Fe-Al-Si alloys at finite-temperatures: A first-principles study

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.