Tunnel Magnetoresistance Effect in $\hbox{CoFeB/MgAlO}_{\rm x}/\hbox{CoFeB}$ Magnetic Tunnel Junctions

Magnetic tunnel junctions (MTJs) with the core structure of
${\rm CoFeB/MgAlO}_{\rm x}/{\rm CoFeB}$
were fabricated using magnetron sputtering technique. The
${\rm MgAlO}_{\rm x}$
tunnel barrier was obtained by plasma oxidation of an Mg/Al bilayer in an
${\rm Ar}+{\rm O}_{2}$
atmosphere. Series of MTJs were fabricated with different Mg layer thicknesses
$(t_{\rm Mg})$
, and Al layer thickness was fixed at 1.3 nm. The annealing effect on the tunneling magnetoresistance (TMR) ratio was investigated, and TMR ratio of 65% at room temperature (RT) was shown when it was annealed at 375
$^{\circ}{\rm C}$
with the
$t_{\rm Mg}=0.5\ {\rm nm}$
. The temperature dependence of conductance can be fit by the magnon-assisted tunneling model by adding spin independent tunneling contribution for the samples investigated here, and the spin independent conductance
$G_{SI}$
varies with
$t_{\rm Mg}$
, possibly due to less oxidation for thicker Mg layer.