Effect of off-stoichiometric composition on half-metallic character of Co2Fe(Ga,Ge) investigated using saturation magnetization and giant magnetoresistance effect

We investigated the Ge-composition (gamma) dependence of the saturation magnetization of Co2Fe(Ga, Ge) (CFGG) thin films and the magnetoresistance (MR) ratio of CFGG-based current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices together with first-principles calculations of the electronic states of CFGG. Theoretical calculations showed that spin polarization is highest at the stoichiometric composition gamma = 0.56 in Co2Fe1.03Ga0.41Ge (gamma) and that it decreases in off-stoichiometric CFGG, mainly due to the formation of Co-Fe antisites for Ge-deficient compositions and Fe-Co antisites for Ge-rich compositions, where Co-Fe (Fe-Co) indicates that Co (Fe) atoms replace the Fe (Co) sites. The saturation magnetic moment (mu (s)) per formula unit decreased monotonically as gamma increased from 0.24 to 1.54 in Co2Fe1.03Ga0.41Ge (gamma) . The mu (s) was closest to the Slater-Pauling value predicted for half-metallic CFGG at the stoichiometric composition gamma = 0.56, indicating that stoichiometric CFGG has a half-metallic nature. This is consistent with the result for the theoretical spin polarization. In contrast, the MR ratio of CFGG-based CPP-GMR devices increased monotonically as gamma increased from 0.24 to 1.10 and reached an MR ratio of 87.9% at the Ge-rich composition gamma = 1.10. Then, the MR ratio decreased rapidly as gamma increased from 1.10 to 1.48. Possible origins for the slight difference between the Ge composition at which the highest MR ratio was obtained (gamma = 1.10) and that at which the highest spin polarization was obtained (gamma = 0.56) are improved atomic arrangements in a Ge-rich CFGG film and the reduction of effective Ge composition due to Ge diffusion in the GMR stacks.

Automated summary

The dependence of saturation magnetization and magnetoresistance ratio on Ge-composition (gamma) in Co2Fe(Ga, Ge) (CFGG) thin films and CFGG-based current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices was investigated. Theoretical calculations showed that stoichiometric CFGG with gamma = 0.56 has the highest spin polarization and half-metallic nature. In contrast, the MR ratio of CPP-GMR devices based on CFGG increased with increasing gamma, reaching a maximum at gamma = 1.10 and then rapidly decreasing.