Angular dependence of the microwave-generation threshold in a nanoscale spin-torque oscillator

It is shown that in a spin-torque microwave oscillator based on a magnetic nanocontact, the nature of the microwave spin wave mode generated at the threshold critically depends on the angle between the external bias magnetic field and the plane of the free layer. When the external bias field is rotating from normal to in-plane orientation, an abrupt transition from a propagating cylindrical wave with the frequency higher than the frequency of the linear ferromagnetic resonance (FMR) to a self-localized standing nonlinear spin wave "bullet" with the frequency lower than the FMR frequency takes place at a certain intermediate angle theta(cr). This transition manifests itself as an abrupt jump (of the order of several gigahertz) in the generated microwave frequency. This mechanism of mode switching might explain abrupt jumps of the generated microwave frequency observed in recent experiments on spin-torque oscillators.