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We have successfully grown both L 2 1 polycrystalline Co 2 Cr Al and epitaxial L 2 1-structured Co 2 Fe Al films onto GaAs (001) substrates under an optimized condition. Both structural and magnetic analyses reveal the detailed growth mechanism of the alloys, and suggest that the Co 2 Cr Al film contains atomically disordered phases, which decreases the magnetic moment per fu, while the Co 2 Fe Al film satisfies the generalized Slater–Pauling behavior. By using these films, magnetic tunnel junctions (MTJs) have been fabricated, showing 2% tunnel magnetoresistance (TMR) for the Co 2 Cr Al MTJ at 5 K and 9% for the Co 2 Fe Al MTJ at room temperature (RT). Even though the TMR ratio still needs to be improved for future device applications, these results explicitly include that the Co 2 (Cr, Fe) Al full Heusler alloy is a promising compound to achieve half-metallicity at RT by controlling both disorder and surface …