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Magnetization Dynamics in STOs: Vortex State versus Uniform State


STT_compare.jpg

Figure: STT induced excitation of qualitatively different oscillatory modes in a nanodisk. (a) After preparation of a uniform state, a standing-wave mode with a transition from blue-to-red shift is excited. (b) The gyrotropic mode is excited after preparation of the vortex state. Note that the microwave output power generated by the gyrating vortex for a given DC current in (b) is much higher than for the standing-wave mode in (a).

We undertook experimental studies of current-driven high-frequency (HF) excitations of STOs for two fundamental magnetization states of the free layer, namely, the vortex state and uniform in-plane magnetization. Our ability to switch between the two states in a given STO enables a direct comparison to be made of the critical currents, agility, power, and linewidth of the HF output signals. We found that the vortex state has some superior properties; in particular, it maximizes the emitted HF power and shows a wider frequency tuning range at a fixed magnetic field.

R. Lehndorff, D. E. Bürgler, S. Gliga, R. Hertel, P. Grünberg, and C. M. Schneider
Magnetization dynamics in spin torque nano-oscillators: Vortex state versus uniform state
Phys. Rev. B 80, 054412 (2009).