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$(\text{x})$/CoFeB/MgO$(\text{y})$/CoFeB$(\text{z})$/Ta/Pd. We obtain a large perpendicular exchange bias of 79.6kA/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 $T_{\text{ann}}=330{}^{\circ }$C for 60 min, while the interlayer exchange coupling and the saturation magnetization per unit area sharply decay at $T_{\text{ann}}=340{}^{\circ }$C for 60 min. Simultaneously, the tunnel magnetoresistance ratio shows a peak of $65.5\%$ after being annealed at $T_{\text{ann}}=300{}^{\circ }$C for 60 min, with a significant reduction down to $10\%$ for higher annealing temperatures ($T_{\text{ann}}\ge 330{}^{\circ }$C) and down to $14\%$ for longer annealing times ($T_{\text{ann}}=300{}^{\circ }$C for 90 min). We attribute the large decrease of tunnel magnetoresistance ratio to the loss of exchange bias in the pinned electrode.