Size dependence of domain wall mediated switching dynamics of perpendicular magnetic tunnel junctions in the presence of reference layer stray field

Recent investigations on spin-transfer-torque-induced switching dynamics of perpendicular magnetic tunnel junctions (MTJ) have revealed different switching anomalies. Here, the influence of stray field from a synthetic anti-ferromagnet (SAF) based reference layer on the domain wall (DW) mediated switching of the free-layer magnet is studied via finite temperature micro-magnetic simulations for varying MTJ diameters. For larger diameters (∼80 – 120 nm), a stray field gives rise to persistent back-and-forth oscillation of the unswitched domain, delaying the switching process and causing increased write errors. For smaller diameters (∼30 – 56 nm), quasi-coherent switching occurs, as expected. For the intermediate ranges of MTJ diameters (∼60 – 70 nm), another switching mode emerges where a bubble-like feature is observed to evolve during the switching process, causing a very rapid change in magnetization. These paths are observed to originate from DWs partly in Bloch and partly in Néel configuration. We find that at the intermediate ranges of device size, the stray field becomes stronger for a given SAF configuration, and the energy difference between the Néel and Bloch configurations is also lowered. Hence, a stronger stray field could easily distort the DW propagation in the Walker breakdown regime, leading to such magnetization behavior. Our findings present interesting insights into DW-mediated switching modes in perpendicular MTJ structures caused by unoptimized stray fields from the SAF.