Micromagnetic simulation for tunnel junctions with synthetic antiferromagnetic pinned layers annealed at different external fields

Bottom-pinned tunnel junctions with synthetic antiferromagnetic (SAF) pinned layers require an annealing step near the blocking temperature under an applied magnetic field to set the exchange direction. Experimental results (anneal at $\text{270\hspace{0.05em}°}\mathrm{C},$ for a MnIr/SAF structure) show that the tunnel magnetoresistance signal is strongly reduced (3%) for anneals under an applied field of 2000 Oe, when compared with large tunnel magnetoresistance signals $\text{(>20\%)}$ obtained for samples annealed either at low (250 Oe) or high (5000 Oe) fields. To clearly understand this behavior, a micromagnetic simulation was performed to demonstrate the magnetization evolution of the SAF layer during the different field anneals. The simulated magnetization configurations indicate that the SAF layer net moment aligns parallel to the applied field (high field anneal) or antiparallel to the applied field (low field anneal), leading in both cases to high TMR signal. After annealing at intermediate field, the SAF net moment rotates almost orthogonal to the annealing field direction, resulting in the measured low TMR signal.