Influence of Pinned-Layer Dispersion on Magnetic Tunnel Junction Switching Distributions

The influence of pinning field dispersion (PFD) on the coercivity (H_c) and offset field (H_o) of IrMn/NiFeCo/AlOx/NiFeCo magnetic tunnel junctions (MTJ) has been studied by repeated in situ measurement of the resistance as a function of magnetic field, R(H), hysteresis loops of the MTJs after resetting the IrMn/NiFeCo pinned layer. Magneto-thermal magnetic random access memory, MT-MRAM, cells were used to perform this task. Here, the pinned layer is reset by pulsing a current through the tunnel barrier of the MT-MRAM MTJ in the presence of an external magnetic field. This permits rapid field cooling of the pinned layer through the blocking temperature of the IrMn. Repeated measurements of an R(H) loop after a pinned layer reset cycle show only minor variation. R(H) loops measured for different pinned layer reset cycles however, can show large changes in H_c , H_o, and R(H) loop shape. The effect is believed to be due to the random magnetostatic field distribution resulting from the frozen-in PFD of the IrMn/NiFeCo pinning layer. In order to test this conclusion, the evolution of the R(H) loop is studied as a function of reset pulse amplitude. Because of the intrinsic distribution of blocking temperatures of the IrMn layer, PFD should decrease as reset pulse amplitude increases. Results show that H_c increases, while the R(H) loops become more square as the reset pulse amplitude is increased. Ho shows a more complex dependence, which is a competition between ferromagnetic magnetic-roughness-induced Neel coupling and anti-ferromagnetic stray field coupling. A micromagnetic model for studying the dependence of the R(H) loops on the alignment of the IrMn crystallites was developed, and simulated results are in good agreement with the measurements. This work provides a simple explanation for large switching field distributions that can result in arrays of seemingly identical MRAM bits