Micromagnetic study of the above-threshold generation regime in a spin-torque oscillator based on a magnetic nano-contact magnetized at an arbitrary angle

The nature and the above-threshold behavior of spin-wave modes excited by spin-polarized direct current in a spin-torque auto-oscillator based on a magnetic nanocontact was studied by a micromagnetic simulation in the case when the external bias magnetic field was rotated from the in-plane to perpendicular-to-plane orientation. In qualitative agreement with the weakly nonlinear analytical theory, it was found that, at a certain critical angle, an abrupt switching from the self-localized nonlinear “bullet” mode to a propagating quasilinear Slonczewski mode takes place and is accompanied by an upward jump in generated microwave frequency. It was also found that the analytical theory overestimates the magnitude of a critical magnetization angle corresponding to the mode switching and that the magnitude of the frequency jump caused by the mode switching is inversely proportional to the nanocontact radius. Numerical modeling performed in the region of large above-threshold currents, which is not accessible to analytical theory, allowed us to determine the intervals of the bias current variation corresponding to excitation of propagating and self-localized spin-wave modes, and reveal hysteretic and nonhysteretic scenarios of microwave generation with the variation of bias current.