  Giant tunnel magnetoresistance (TMR) ratios of 417% at room temperature (RT)
and 914% at 3 K were demonstrated in epitaxial Fe/MgO/Fe(001) exchanged-biased
spin-valve magnetic tunnel junctions (MTJs) by tuning growth conditions for
each layer, combining sputter deposition for the Fe layers, electron-beam
evaporation of the MgO barrier, and barrier interface tuning. Clear TMR
oscillation as a function of the MgO thickness with a large peak-to-valley
difference of ~80% was observed when the layers were grown on a highly
(001)-oriented Cr buffer layer. Specific features of the observed MTJs are
symmetric differential conductance (dI/dV) spectra for the bias polarity and
plateau-like deep local minima in dI/dV (parallel configuration) at |V| =
0.2~0.5 V. At 3K, fine structures with two dips emerge in the plateau-like
dI/dV, reflecting highly coherent tunneling through the Fe/MgO/Fe. We also
observed a 496% TMR ratio at RT by a 2.24-nm-thick-CoFe insertion at the
bottom-Fe/MgO interface.
