Phase stability and half-metallic character of off-stoichiometric Co2FeGa0.5Ge0.5 Heusler alloys

We investigate the effects of off-stoichiometric compositional variations from the Co2Fe(Ga0.5Ge0.5) (CFGG) full-Heusler alloy on its half-metallic electronic structure. First-principles calculations predict that the Co antisite defects that occupy Fe-sites (Co-Fe) lead to a finite DOS in the half-metallic gap of CFGG. Fe antisites defects in Co-sites (Fe-Co) introduced by excessing Fe composition, which could suppress the formation of CoFe, preserves the half-metallic gap but reduces spin polarization because the Fermi level shifts to the lower energy. We found that, in Fe-excess CFGG, Ge-excess has an important role to enhance the spin polarization by lifting up the Fermi level position and suppressing the formation of CoFe. To confirm the effect of the Fe and Ge-excess off-stoichiometric composition on spin polarization and phase-purity experimentally, we fabricated CFGG epitaxial thin films with various composition ratios (Co2-alpha Fe1+alpha) (Ga1-beta Ge beta)(1+gamma). with small positive gamma (=0.09-0.29). It turns out that Co1.75Ge or Fe1.7Ge secondary phase often forms in the films for beta >= 0.69 in Fe-deficient (alpha < 0.21) and excess (alpha > 0.49) compositions. This secondary phase can be suppressed by tuning the Ge and Fe compositions, and the L2(1)-phase pure film was found in Co39.4Fe29.3Ga13.4Ge17.9 (alpha = 0.28, beta = 0.57, gamma = 0.25). The measurements of conventional magnetoresistance effects qualitatively indicate higher spin polarization in the Co39.4Fe29.3Ga13.4Ge17.9 film compared to other Co-excess and Ge-deficient films, which evidences the benefit to make Fe- and Ge-excess off-stoichiometric CFGG for obtaining the half-metallic nature of CFGG. Published under an exclusive license by AIP Publishing.