We report the spin polarization of Co2FeAlxSi1−x (x=0.0, 0.3, 0.5, 0.7) bulk alloys measured by the point contact Andreev reflection method. All the Co2FeAlxSi1−x alloys had an L21 structure along with A2- and B2-type disorder. Several off-stoichiometric alloys (CoxFeyAl0.5Si0.5) were prepared to understand the effect of the compositional deviation from the stoichiometry on the spin polarization. By substituting Al for Si, the spin polarization changed from 0.57±0.01 for x=0.0 to a maximum value of 0.60±0.01 for x=0.5. The off-stoichiometric alloys had spin polarizations of 0.57−0.60±0.01. Ab initio calculations were performed to interpret the effect of Al addition as well as the effect of disorder on the magnetic properties and on the electronic structure.

We report the transport properties of a current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) device with Co2Fe(Al0.5Si0.5) (CFAS) Heusler alloy ferromagnetic layers and a Ag spacer layer. The CPP-GMR devices showed relatively high ΔRA values and MR ratios up to 17 m Ω μm2 and 80% at 14 K, and 8 m Ω μm2 and 34% at 290 K. The spin diffusion length ∼3 nm and the bulk spin asymmetry ∼0.77 for the CFAS alloy at 14 K were estimated by the Valet–Fert model, indicating a large contribution of the interfacial scattering.

Using a newly developed highly spin-polarized Heusler alloy, Co2Fe(Ga0.5Ge0.5) (CFGG), as ferromagnetic layers, we have fabricated a current-perpendicular-to-plane pseudospin valve with large resistance change-area product (ΔRA) of 9.5 mΩ μm2 and magnetoresistance (MR) ratio (100×ΔR/R) of 41.7% at 300 K. These values were further enhanced to ΔRA=26.4 mΩ μm2 and MR=129.1% at 10 K. The large MR values are attributed to the high spin polarization of the CFGG alloy confirmed by point contact Andreev reflection measurements.

Spin-dependent tunneling spectroscopy has been studied in fully epitaxial magnetic tunnel junctions with full-Heusler Co 2 FeAl 0.5 Si 0.5 ͑CFAS͒ alloys. We fabricated CFAS/MgO/CFAS structures with L2 1 -and B2-ordered CFAS layers and measured the bias voltage dependence of differential conductance G. We found for L2 1 -CFAS/ MgO/ L2 1 -CFAS structure symmetrical conductance curves with respect to polarity of the bias voltage for parallel ͑P͒ and antiparallel ͑AP͒ magnetization configurations and two characteristic crossovers in G between P and AP accompanied with a flat feature within Ϯ0.6 V in G ͑P͒. On the other hand, only one crossover was observed at a negative-bias voltage for L2 1 -CFAS/ MgO/ B2-CFAS structure. The direct tunneling that reflects the specific spin-dependent density of states of the half-metallic L2 1 -CFAS is proposed as a possible transport mechanism leading to the notable crossovers.