Manipulation of precession modes in all-permalloy nanostripe-nanopillar structured spin torque nano-oscillator driven by direct current.

We investigated the dynamical behaviors of an all-permalloy structured spin-torque nano-oscillator (STNO) composed of a nanostripe with in-plane magneto-anisotropy and a free magnetized nanopillar in the absence of a non-magnetic spacer via a micro-magnetic simulation. It is found the newly-devised STNO exhibits two stable precession modes of magnetization in the nanopillar: out-of-plane precession mode and in-plane precession mode under varying applied DC current densities. The switching between the two modes is generated in a certain current density, depending on geometries of the nanopillar as well as the nanostripe. Given a special nanopillar geometry, both modes demonstrate stable oscillation properties in a certain range of current densities. Pulsed magnetic field can effectively realize transformation of the two modes under application of a proper current density. The realization of synchronous oscillations to significantly enhance the output power is verified in this new type of STNO by etching plenty of nanopillars on the nanostripe to build STNOs array.