Effect of Deposition Conditions and Annealing Temperature on Tunnel Magnetoresistance and the Structure of MgO-Based Double-Barrier Magnetic Tunnel Junctions

Tunnel magnetoresistance (TMR) was measured in CoFeB-MgO-based double-barrier magnetic tunnel junctions. The variation of sputtering power density is used to control the relative B content of the middle electrode. MR ratios in both top and bottom junctions are suppressed with respect to the single-barrier case. While the bottom junction shows a saturation of the MR as a function of high-temperature annealing for all sputtering power densities, the top junction exhibits an increase in MR as a function of annealing temperature with higher values for higher sputtering power density. The suppression of high MR is attributed to a lack of strong crystallization in the middle electrode, which is confirmed by cross-sectional transmission electron microscopy. Slight crystallization of the middle electrode is achieved at the highest sputtering power density despite the fact that boron diffusion is suppressed due to the adjacent MgO tunnel barriers. Optimal deposition and postannealing conditions resulted in MR values of 140% and 80% for the top and bottom junctions, respectively.