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

TL;DR

This study explores how magnetic properties and tunnel magnetoresistance in perpendicular CoFeB-based magnetic tunnel junctions are affected by annealing conditions, layer thicknesses, and material composition, revealing a strong correlation between exchange bias and TMR ratio.

Contribution

It provides detailed insights into the dependence of magnetic properties and TMR ratio on annealing parameters and layer structures in perpendicular magnetic tunnel junctions, highlighting the loss of exchange bias as a key factor.

Findings

Maximum exchange bias of 79.6 kA/m at specific layer thickness.

TMR ratio peaks at 65.5% after optimal annealing.

TMR ratio significantly decreases with higher annealing temperatures.

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.