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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 (CoFe) lead to a finite DOS in the half-metallic gap of CFGG. Fe antisites defects in Co-sites (FeCo) 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−αFe1+α) (Ga1−βGeβ)1+γ with small positive γ (=0.09–0.29). It turns out that Co1.75Ge or Fe1.7Ge secondary phase often forms in the films for β ≥ 0.69 in Fe-deficient (α ≤ 0.21) and excess (α ≥ 0.49) compositions. This secondary phase can be suppressed by tuning the Ge and Fe compositions, and the L21-phase pure film was found in Co39.4Fe29.3Ga13.4Ge17.9 (α = 0.28 , β = 0.57 , γ = 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. [ABSTRACT FROM AUTHOR]

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(Mn1−xFex)Ge and (Co2Mn0.5Fe0.5)1−yGey 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 (Co2Mn0.5Fe0.5)1−yGey 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]

Abstract: Polycrystalline bilayer thin film of multiferroic [Ba(Zr0.2Ti0.8)O3‐0.5(Ba0.7Ca0.3)TiO3]/CoFe2O4([BZT‐0.5BCT]/CFO) has been deposited on Pt/Si (100) substrate using a pulsed laser deposition technique. The dielectric analysis reveals a significant change in the dielectric constant (~39% at a typical frequency of 100 Hz) at room temperature when a magnetic field is applied, in addition to a substantial improvement in the saturation polarization. A low leakage current density (~ 5 × 10−7 A/cm2) and a high magnetoelectric coupling coefficient (αE) both in the transverse (~2.085 V/Oe cm) as well as in the longitudinal (~0.708 V/cm Oe) directions, indicate in‐principle usability of this system for multifunctional device applications in thin film form. [ABSTRACT FROM AUTHOR]