Effect of thermal annealing on the ac impedance of Co(75)/Al2O3(2.3)/Co(5.0)/Al2O3(2.3)/Co(50) double-barrier MTJs

Double-barrier magnetic tunnel junctions (DBMTJs) were prepared from Co(75 nm)/Al2O3(2.3 nm)/Co(5 nm)/Al2O3(2.3 nm)/Co(50 nm) sputtering pentalayer films. The ac electrical properties of as-deposited DBMTJs and those annealed in a vacuum at 100-350 °C for 30 min were then investigated using a complex impedance spectroscopic technique. The ac impedance responses as a function of annealing temperature were further analyzed based on Maxwell's layered dielectric barrier and Maxwell-Wagner capacitor models after considering the DBMTJs as having double-capacitor-type structures. The effect of thermal annealing on the ac transport behavior of the DBMTJs was interpreted by examining the equivalent electric circuits fitted to Nyquist plots of each different sample. The effects were found to be due to changes in the structural characteristics in both bulk and interface morphologies of Co and Al2O3 layers. The structural morphology determined the different ac transport modes that occurred in the DBMTJs.