Characteristics of the Al2O3 barrier with CoFeB pinned layer in magnetic tunnel junctions

Amorphous FM electrode, CoFeB, was used in MTJs to improve the property of the insulating layer interface. The tunnel junctions with a structure of Ta(50 nm)/NiFe(8 nm)/IrMn(10 nm)/pinned layer(4 nm)/Al₂O₃(Al 1.1 nm)/CoFeB(3 nm)/NiFe(15 nm) were prepared in a DC magnetron sputtering system. The junctions were made by a photolithographic method and the samples were annealed from 200 °C to 340 °C. The depth profiles of the insulating barrier before and after annealing were analyzed by XPS and SIMS. MR ratio and junction resistance (RA) products of the MTJs were measured with annealing temperature. The maximum MR ratio were observed at 260 °C annealing. RA products increased up to 380 °C for CoFeB pinned junctions while it increased up to a 280 °C and decreased over this temperature in the tunnel junctions with CoFe pinned layer.