Spin-torque switching mechanisms of perpendicular magnetic tunnel junction nanopillars

Understanding the spin-transfer magnetization switching mechanisms of perpendicular magnetic tunnel junction nanopillars is critical to optimizing their performance in memory devices. Here, we use micromagnetics to study how the free layer's exchange constant affects its switching dynamics. Switching is shown to generally occur by (1) growth of the magnetization precession amplitude in the element center; (2) an instability in which the reversing region moves to the element edge, forming magnetic domain wall(s); and (3) the motion of the domain wall(s) across the element. For small exchange and large element diameters, step 1 leads to a droplet with a fully reversed core that experiences a drift instability (step 2). While in the opposite case (large exchange and small diameters), the central region of the element is not fully reversed before step 2 occurs. The origin of the micromagnetic structure is shown to be the free layer's non-uniform demagnetization field. More coherent, energy-efficient, and faster switching is associated with larger exchange, showing that increasing the exchange interaction strength leads to improvements in device performance.