Switching-field stabilization against effects of high-temperature annealing in magnetic tunnel junctions using thermally reliable NiFe100-x/Al-oxide/Ta free layer

We examined the effects of the capping layers of Ta, Al-oxide (AlO), and Ru on the magnetic properties of ultrathin (1-20 nm) NiFe free layers and investigated thermal degeneration of the switching field (H-SW) in magnetic tunnel junctions (MTJs) using these caps in the cell structure of magnetoresistive random access memory (MRAM). The magnetization reversal of the free layer was found to be sensitive to the capping layer that affected various magnetic anisotropies. For postannealing temperatures of up to 350 degrees C, the H-SW of a NiFe(2 nm)/AlO/Ta free layer was thermally stable, whereas that of conventional NiFe(3 nm)/Ta seriously deteriorated. This is because the AlO capping layer completely prevented the thermal interdiffusion between the NiFe and Ta layers even after annealing at 400 degrees C. Moreover, a free layer with a small magnetostriction reduced the influence of the stress-relaxation on the free layer caused by postannealing. which also resulted in a thermally stable H-SW in MTJs.