Spin-torque-induced switching in a perpendicular GMR nanopillar with a soft core inside the free layer

Considerable reduction of the switching current is observed on micromagnetic simulation in a perpendicularly magnetized giant magnetoresistive (GMR) nanopillar with a soft nanocore inside the free layer. In this paper, an analytical model based on the single-domain assumption for both the hard and the soft regions is developed to deeply understand the nanocore effect. Combining the analytical solutions with the micromagnetic simulation results, we could interpret well the essential features of the spin-torque-driven magnetization switching in such GMR pillars with perpendicular anisotropy. The asymmetric critical switching current is attributed to the stray field caused by the fixed layer together with the intrinsic difference in the spin-torque efficiency associated with the current direction. However, such an asymmetric switching could be compensated partially by an asymmetric reduction in the critical current after a soft core is inserted into the free layer. In addition, a slight jump observed in the simulated antiparallel-to-parallel switching curve could also be explained by this model.