Correlation of tunnel magnetoresistance with the magnetic properties in perpendicular CoFeB-based junctions with exchange bias

We investigate the dependence of magnetic properties on the post-annealing temperature/time, the thickness of soft ferromagnetic electrode and Ta dusting layer in the pinned electrode as well as their correlation with the tunnel magnetoresistance ratio, in a series of perpendicular magnetic tunnel junctions of materials sequence Ta/Pd/IrMn/CoFe/Ta$(textit{x})$/CoFeB/MgO$(textit{y})$/CoFeB$(textit{z})$/Ta/Pd. We obtain a large perpendicular exchange bias of 79.6$,$kA/m for $x=0.3,$nm. For stacks with $z=1.05,$nm, the magnetic properties of the soft electrode resemble the characteristics of superparamagnetism. For stacks with $x=0.4,$nm, $y=2,$nm, and $z=1.20,$nm, the exchange bias presents a significant decrease at post annealing temperature $T_textrm{ann}=330,^{circ}$C for 60 min, while the interlayer exchange coupling and the saturation magnetization per unit area sharply decay at $T_textrm{ann}=340,^{circ}$C for 60 min. Simultaneously, the tunnel magnetoresistance ratio shows a peak of $65.5%$ after being annealed at $T_textrm{ann}=300,^{circ}$C for 60 min, with a significant reduction down to $10%$ for higher annealing temperatures ($T_textrm{ann}geq330,^{circ}$C) and down to $14%$ for longer annealing times ($T_textrm{ann}=300,^{circ}$C for 90 min). We attribute the large decrease of tunnel magnetoresistance ratio to the loss of exchange bias in the pinned electrode.