  Magnetic tunnel junctions with Fe1+xCo2-xSi (0 < x < 1) electrodes and MgO
barrier were prepared on MgO substrates by magnetron co-sputtering. Maximum
tunnel magnetoresistance (TMR) ratios of 262 % at 15 K and 159 % at room
temperature were observed for x=0.75. Correlations of the annealing temperature
dependent atomic ordering and TMR amplitude are discussed. The high TMR for an
intermediate stoichiometry is ascribed to the adjustment of the Fermi energy
within a minority spin pseudo gap.
