Structural and magnetic properties of He⁺ irradiated Co₂MnSi Heusler alloys

We have investigated the atomic disorder induced by a 150 keV He⁺ ion irradiation in a 40 nm thick Co₂MnSi Heusler alloy. Disorder parameters on each atomic site are deduced from normal and anomalous x-ray diffraction measurements with Co and Cu K α sources. While the film grows mainly in the L2₁ phase with inclusion of B2 grains, we observe an increase of both the Mn–Si and Co–Mn exchanges with the ion fluence. HAADF-STEM analysis demonstrates that the increase in Mn–Si disorder corresponds to a growing size of the B2 grains while the Co–Mn exchange is accounted for a D0₃ disorder type in the L2₁ matrix. These structural modifications are shown to decrease the average magnetization of the alloy, which is due to D0₃ disorder and local defects induced by irradiation. (paper)