Effect of off-stoichiometric composition on half-metallic character of Co 2 Fe(Ga,Ge) investigated using saturation magnetization and giant magnetoresistance effect

• Published in: Journal of physics. D, Applied physics Applied physics, 2022-08, Vol.55 (34), p.345003
• Main Authors: Chikaso, Yuki, Inoue, Masaki, Tanimoto, Tessei, Kikuchi, Keita, Yamanouchi, Michihiko, Uemura, Tetsuya, Inubushi, Kazuumi, Nakada, Katsuyuki, Shinya, Hikari, Shirai, Masafumi
• Format: Article
• Language: English
• ISSN: 0022-3727; 1361-6463
• DOI: 10.1088/1361-6463/ac73c1

We investigated the Ge-composition ( γ ) dependence of the saturation magnetization of Co 2 Fe(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 γ = 0.56 in Co 2 Fe 1.03 Ga 0.41 Ge γ 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 ( μ s ) per formula unit decreased monotonically as γ increased from 0.24 to 1.54 in Co 2 Fe 1.03 Ga 0.41 Ge γ . The μ s was closest to the Slater–Pauling value predicted for half-metallic CFGG at the stoichiometric composition γ = 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 γ increased from 0.24 to 1.10 and reached an MR ratio of 87.9% at the Ge-rich composition γ = 1.10. Then, the MR ratio decreased rapidly as γ 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 ( γ = 1.10) and that at which the highest spin polarization was obtained ( γ = 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.