Influence of annealing on tunnelling magnetoresistance of perpendicular magnetic tunnel junctions

We have shown for the first time that the magnetometry on the free layer of fully perpendicular magnetic tunnel junctions (pMTJ) can be used to optimize the annealing conditions, and, subsequently, the transport properties of these devices. pMTJ stacks with Co/Pd multilayer (ML) synthetic antiferromagnet (SAF)-based pinned layer and thin Co31.5Fe58.5B10 free layer were patterned into devices and then rapid thermally annealed (RTA) in high vacuum at temperatures ranging from 350 to 500 degrees C. The soaking time at these RTA temperatures ranged from 3 to 12 min. Under annealing condition of 500 degrees C for 8 min yielded a tunnelling magnetoresistance (TMR) of 50% at a transport measurement temperature of 300K and 59% at 4.2 K. Further increase in annealing time to 12 min degraded the TMR at all temperatures tested. Analysis of the minor M-H loops of the free layer of these devices showed a clear transition from partially perpendicular to fully perpendicular to in-plane anisotropy as the annealing time was increased at each temperature. These minor loops correlate strongly with the transport measurements, indicating that the optimization of perpendicular magnetic anisotropy of the free layer by annealing leads to optimized TMR as well.