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

Abstract

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.