Spin polarization and Gilbert damping of Co ₂Fe(Ga _xGe _1-x) Heusler alloys

The spin polarization (P) of ferromagnetic Heusler alloys, Co ₂Fe(Ga _xGe _1-x) (x = 0, 0.25, 0.5, 0.75, and 1), is investigated by point contact Andreev reflection (PCAR) measurements. While the P values of the ternary Co ₂FeGe and Co ₂FeGa alloys are 0.58 and 0.59, respectively, the corresponding value for Co ₂Fe(Ge _0.5Ga _0.25) is as high as 0.69. Co ₂Fe(Ge _0.5Ga _0.5) alloy shows a strong tendency for L2 ₁ ordering and a high Curie temperature of 807 °C. Ab initio calculations indicate that the band structures of the Co ₂FeGe and Co ₂Fe(Ge _0.5Ga _0.5) alloys with L2 ₁ or B2 structures are half-metallic. Thin films of the quaternary Co ₂Fe(Ge _0.5Ga _0.5) alloy grown on MgO(0 0 1) substrates order to the L2 ₁ structure upon annealing at 500 °C, giving rise to a high P of 0.75. This is the highest P value reported for Heusler alloy thin films using the PCAR technique. Ferromagnetic resonance measurements show that the Gilbert damping constant of the film is ∼0.008. All these indicate that the Co ₂Fe(Ge _0.5Ga _0.5) alloy is promising as a spin polarized current source for spintronics devices.