We report the spin polarization of Co2 FeAlx Si1-x (x=0.0, 0.3, 0.5, 0.7) bulk alloys measured by the point contact Andreev reflection method. All the Co2 FeAlx Si1-x alloys had an L 21 structure along with A2 - and B2 -type disorder. Several off-stoichiometric alloys (Cox Fey Al0.5 Si0.5) were prepared to understand the effect of the compositional deviation from the stoichiometry on the spin polarization. By substituting Al for Si, the spin polarization changed from 0.57±0.01 for x=0.0 to a maximum value of 0.60±0.01 for x=0.5. The off-stoichiometric alloys had spin polarizations of 0.57-0.60±0.01. Ab initio calculations were performed to interpret the effect of Al addition as well as the effect of disorder on the magnetic properties and on the electronic structure.
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