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

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 L2(1) 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 L2(1) or B2 structures are half-metallic. Thin films of the quaternary Co2Fe(Ge0.5Ga0.5) alloy grown on MgO(001) substrates order to the L2(1) structure upon annealing at 500 degrees 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 similar to 0.008. All these indicate that the Co2Fe(Ge0.5Ga0.5) alloy is promising as a spin polarized current source for spintronics devices. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.