Spin polarization and Gilbert damping of Co2Fe(GaxGe1−x) Heusler alloys

Abstract

The spin polarization (P) of ferromagnetic Heusler alloys, Co2Fe(GaxGe1−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 Co2FeGe and Co2FeGa alloys are 0.58 and 0.59, respectively, the corresponding value for Co2Fe(Ge0.5Ga0.25) is as high as 0.69. Co2Fe(Ge0.5Ga0.5) alloy shows a strong tendency for L21 ordering and a high Curie temperature of 807°C. Ab initio calculations indicate that the band structures of the Co2FeGe and Co2Fe(Ge0.5Ga0.5) alloys with L21 or B2 structures are half-metallic. Thin films of the quaternary Co2Fe(Ge0.5Ga0.5) alloy grown on MgO(001) substrates order to the L21 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 Co2Fe(Ge0.5Ga0.5) alloy is promising as a spin polarized current source for spintronics devices.