Combinatorial optimization for high spin polarization in Heusler alloy composition-spread thin films by anisotropic magnetoresistance effect.

Half-metallic Heusler alloys are promising candidates for spintronic applications due to their high spin polarization. However, the spin polarization strongly depends on the atomic composition, which is time-consuming to optimize from various compositional combinations. Here, we demonstrate a high-throughput compositional optimization method for high spin polarization in Co₂(Mn, Fe)Ge Heusler alloys by combining composition-spread films and anisotropic magnetoresistance (AMR) measurement. Two types of composition-spread films of polycrystalline Co₂(Mn_1−xFe_x)Ge and (Co₂Mn_0.5Fe_0.5)_1−yGe_y are fabricated on SiO₂/Si substrates by combinatorial sputtering deposition, followed by post-annealing. The compositional dependence of AMR shows the largest negative AMR ratio of −0.13% and the smallest temperature dependence of the resistance change of AMR for y = 0.25 in the (Co₂Mn_0.5Fe_0.5)_1−yGe_y composition-spread film, suggesting the highest spin polarization and the closest nature to the ideal half-metal at this composition ratio. To verify this, we also develop a new technique to measure the compositional dependence of spin polarization by measuring the spin accumulation signals of nonlocal spin-valve devices fabricated on the composition-spread films and observe the highest spin polarization of 82% for y = 0.24. This confirms a clear qualitative correlation between the large negative AMR ratio and high spin polarization. Our combinatorial method using the composition-spread films and the AMR measurement proves to be a facile way for optimizing the fabrication conditions of half-metallic Heusler alloys with high spin polarization. [ABSTRACT FROM AUTHOR]