Effect of vortex handedness on spin momentum torque dynamics in dual-vortex ferromagnetic nanopillar structures

Abstract

An investigation of the dynamics of vortices driven by spin-momentum transfer in magnetic tunnel junction nanopillars containing a vortex in the hard ferromagnetic pinned layer (PL) and a vortex in the soft ferromagnetic free layer (FL) is presented. This dual vortex configuration is interesting because the handedness of the vortex in the PL can be set so that the SMT is either assisted or opposed by the torque due to the Amperean magnetic field produced by the current passing through the nanopillar. It is shown that the handedness of the vortex in the PL controls the dynamics of the nanopillar device. Micromagnetic simulations of the three-dimensional nanopillar structures were performed as a function of PL vortex handedness, spin polarization (η), and nanopillar dimensions. Generally, for positive η, it is found that when the PL vortex is set counter-clockwise (CCW), the FL vortex shows a well-defined switching behavior, where the handedness of the final vortex state in the FL is dependent on the current direction through the nanopillar, and the switching current is decreased as η is increased. In devices where the PL is magnetized clockwise (CW), the FL magnetization dynamics show a more complicated dependence on η. The CW magnetized PL nanopillars show high-current Amperean field-induced vortex switching at low η, chaotic oscillation at intermediate η, and well-defined low-current switching at high η. For negative η, the CCW and CW PL results invert.