Spin wave eigenmodes excited by spin transfer torque in circular nanopillars: influence of lateral size and Oersted field studied by micromagnetic simulations

Micromagnetic simulations are exploited to analyse the spectrum of low-amplitude dynamical modes excited by spin-transfer torque in the free layer of a cylindrical nanopillar, with either in-plane, out-of-plane or tilted magnetization. It is found that the frequency and the spatial character of the specific subset of excited eigenmodes appreciably depend on the pillar diameter (either 100 or 300 nm), as well as on the direction of the free-layer magnetization and the current polarization. Moreover, the spatial symmetry and the frequency of the excited modes are substantially influenced by the Oersted field generated by the injected current. These results demonstrate that a comprehension of the incipient dynamics of spin-torque oscillators, just above the current threshold for persistent precession, is only possible taking into account the influence of the Oersted field as well as the spatial inhomogeneity of the excited eigenmodes.