The author reports here a thorough investigation of structural and magnetic properties of Co 2FeAl 0.5Si 0.5 Heusler alloy films, and the tunnel magnetoresistance effect for junctions with Co 2FeAl 0.5Si 0.5 electrodes, spin injection into GaAs semiconductor from Co 2FeAl 0.5Si 0.5, and spin filtering phenomena for junctions with CoFe 2O 4 ferrite barrier. It was observed that tunnel magnetoresistance ratio up to 832%(386%) at 9 K (room temperature), which corresponds to the tunnel spin polarization of 0.90 (0.81) for the junctions using Co 2FeAl 0.5Si 0.5 Heusler electrodes by optimizing the fabrication condition. It was also found that the tunnel magnetoresistance ratio are almost the same between the junctions with Co 2FeAl 0.5Si 0.5 Heusler electrodes on Cr buffered (1 0 0) and (1 1 0) MgO substrates, which indicates that tunnel spin polarization of Co 2FeAl 0.5Si 0.5 for these two direction are almost the same. The next part of this paper is a spin filtering effect using a Co ferrite. The spin filtering effect was observed through a thin Co-ferrite barrier. The inverse type tunnel magnetoresistance ratio of -124% measured at 10 K was obtained. The inverse type magnetoresistance suggests the negative spin polarization of Co-ferrite barrier. The magnetoresistance ratio of -124% corresponds to the spin polarization of -0.77 by the Co-ferrite barrier. The last part is devoted to the spin injection from Co 2FeAl 0.5Si 0.5 into GaAs. The spin injection signal was clearly obtained by three terminal Hanle measurement. The spin relaxation time was estimated to be 380 ps measured at 5 K.