We have investigated the effect of Cr substitution for Fe on the spin polarization, P, of Co2 Crx Fe1-x Si alloys using the point contact Andreev reflection method. A steep increase from P=0.57 to P=0.64 occurs by a small substitution of Cr for Fe of x=0.02, thereafter P values are constant for higher Cr concentrations up to x=0.4. The increase in spin polarization can be attributed to both the improved L 21 ordering found experimentally as well as the half-metallicity predicted by ab initio calculations in the composition range of 0x0.5. We propose the composition range of 0.02x0.1 is the most promising for ferromagnetic electrodes of magnetic tunneling junctions to achieve high tunneling magnetoresistance.
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