Co₂MnGa and Co₂MnIn are ideal half-metallic materials for spin-injection in nanostructured semiconductor devices.Magneto-optical Kerr effect and electrical transport measurements demonstrate that although these alloys can be integrated with semiconductor devices, the metallic phase dominates over the intrinsic, minority spin phase of the alloy. Co₂MnGa : GaAs(001) has a weak uniaxial magnetic anisotropy with the easy axis along the [0,–1,1] in-plane direction. Co₂MnIn : GaAs(001) has no in-plane anisotropy. Metallic behavior is concomitant with ferromagnetic behavior, as the anisotropic magnetoresistance tends to zero at the same film thickness (100 Å) that the residual resistivity ratio extrapolates to 1. Co₂MnIn has an AMR of 0.5%, however, an AMR as large as 6% is observed in Co₂MnGa at 300 K. This demonstrates the potential for nanostructured devices, although the first 100 Å of alloy growth will determine the spin-injection efficiency.