Evolution of low-frequency resistance noise during annealing in CoFeB/MgO/CoFeB tunnel junctions

We have studied the evolution of tunneling magnetoresistance (TMR) and resistance noise in magnetic tunnel junctions (MTJs) as a function of annealing time at 425$^{o}$C. Previously, we showed that short annealing times do lead to significant improvement in the MgO crystal structure and crystallization of the CoFeB electrodes, resulting in large TMR values up to 200{\%}. We also observe that the low-frequency resistance noise decreases significantly after annealing for only a few minutes. The resistance noise has a 1/f spectrum and is quantified by a Hooge-like parameter, $\alpha $, given in units of $\mu $m$^{2}$. In unannealed samples $\alpha $ is of order 10$^{-9} \quad \mu $m$^{2 }$and decreases with increasing voltage bias. Upon annealing, $\alpha $ drops to 10$^{-10} \quad \mu $m$^{2 }$and is less dependent on bias, particularly in the parallel configuration. We attribute the decrease in $\alpha $ and its bias dependence, $\alpha $(V), to a reduction of defects in and around the barrier due to annealing. The implications for optimizing the signal to noise ratio of MgO-based MTJ sensors will also be discussed.