Study of CoFeB thickness and composition dependence in a modified CoFeB/MgO/CoFeB perpendicular magnetic tunnel junction

摘要

We studied the CoFeB thickness and composition dependence of tunneling magnetoresistance (TMR) and resistance-area product (RA) in a modified CoFeB/MgO/CoFeB perpendicular magnetic tunnel junction (MTJ), in which the bottom CoFeB is coupled to an in-plane exchange biased magnetic layer. This stack structure allows us to measure TMR and RA of the MTJs in sheet film format without patterning them, using current-in-plane-tunneling (CIPT) technique. The thickness ranges for both top and bottom CoFeB to exhibit perpendicular magnetic anisotropy are similar to what are seen in each single magnetic film stack. However, CIPT measurement revealed that there exists an optimal thickness for both top and bottom CoFeB to achieve the highest TMR value. Magnetic hysteresis loops also suggest the thickness-dependent coupling between the top and bottom CoFeB layers. We studied MTJs with two CoFeB compositions (Co_(40)Fe_(40)B_(20) and Co_(20)Fe_(60)B_(20)) and found that Co_(20)Fe_(60)B_(20) MTJs give higher TMR and also wider perpendicular thickness range when used at the top layer.