  We investigated the effect of a Mg-Al layer insertion at the bottom interface
of epitaxial Fe/$MgAl_{2}O_{4}$/Fe(001) magnetic tunnel junctions (MTJs) on
their spin-dependent transport properties. The tunnel magnetoresistance (TMR)
ratio and differential conductance spectra for the parallel magnetic
configuration exhibited clear dependence on the inserted Mg-Al thickness. A
slight Mg-Al insertion (thickness < 0.1 nm) was effective for obtaining a large
TMR ratio above 200% at room temperature and observing a distinct local minimum
structure in conductance spectra. In contrast, thicker Mg-Al (> 0.2 nm) induced
a reduction of TMR ratios and featureless conductance spectra, indicating a
degradation of the bottom-Fe/$MgAl_{2}O_{4}$ interface. Therefore, a minimal
Mg-Al insertion was found to be effective to maximize the TMR ratio for a
sputtered $MgAl_{2}O_{4}$-based MTJ.
