High TMR Ratio in ${\rm Co}_{2}{\rm FeSi}$ and ${\rm Fe}_{2}{\rm CoSi}$ Based Magnetic Tunnel Junctions

Magnetic tunnel junctions with Fe
_1+z
Co
_2-x
Si (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.