Tunnel magnetoresistance in double barrier magnetic tunnel junctions with different free layer deposition conditions

Double barrier magnetic tunnel junctions (DB-MTJs) allow for operation at higher bias voltages than their single barrier counterparts, but their total tunnelling magnetoresistance (TMR) ratio is still less than in the single barrier case.[1] Here, we prepare CoFeB/MgO-based DB-MTJs with differing free layer deposition conditions. The deposition conditions for the outer CoFeB electrodes and the MgO barriers were kept the same. The middle CoFeB layer was deposited at differing sputtering power densities (from 1.3 to 4.4 W/cm2) to vary the B concentration.[2] Contributions of the upper and lower junction to the total TMR were compared as a function of sputtering power density and annealing temperature. As the sputtering power density of the free layer is increased the TMR response of the upper and lower junctions is opposite, indicating that the growth of both MgO on CoFeB as well as CoFeB on MgO is sensitive to B content. This is attributed to the suppression of B diffusion which is confirmed by transmission electron microscopy analysis. [1] T. Nozaki et al., Appl. Phys. Lett., 86, 082501 (2005). [2] H.D. Gan et al., IEEE Trans. Magn. 47, 1567 (2011).