Nonlinear Behavior and Mode Coupling in Spin-Transfer Nano-Oscillators

By investigating thoroughly the tunable behavior of coupled modes, we highlight how it provides a means to tune the properties of spin-transfer nano-oscillators. We first demonstrate that the main features of the microwave signal associated with coupled vortex dynamics, i.e., frequency, spectral coherence, critical current, and mode localization, depend drastically on the relative vortex core polarities. Second, we report a large reduction of the nonlinear linewidth broadening obtained by changing the effective damping through the control of the core configuration. Such a level of control on the nonlinear behavior reinforces our choice to exploit the microwave properties of collective modes for applications in advanced spintronics devices for integrated telecommunication concerns.

Figure 1

(a) Schematic of an hybrid magnetic tunnel junction: a Cu-based spin-valve system with the two-vortex Py layer (8 nm at the top, 20 nm at the bottom) above a 1-nm MgO barrier and a CoFeB-based synthetic antiferromagnet. (b),(c) Field dependence of the low-frequency “excited” mode and the high-frequency “damped” mode for the APc (b) and Pc (c) configuration at $I_{\mathrm{dc}}=-16\mathrm{mA}$. Inset: Frequency spectrum of the output emitted signal at $H_z=0\mathrm{kA}/\mathrm{m}$ for the APc (b) and Pc (c) configuration.

Figure 2

Frequency (a) and integrated output emitted power (b) current dependency for the APc (red dot) and Pc (blue dot) configurations at zero applied field.

Figure 3

Frequency vs coupling for the lowest excited mode in the parallel core (blue line) and antiparallel core (red line) for $I_{\mathrm{dc}}=-16\mathrm{mA}$ at zero field. (b) The ratio of gyration radii between thin and thick layers for the lowest frequency mode in the parallel (blue line) and antiparallel (red line) configurations. [Filled (unfilled) circles represent the (expected) experimental points.] (c) Micromagnetic simulations representing gyration radii $({\rho }_1,{\rho }_2)$ for the Pc and APc configurations at zero field, $I_{\mathrm{dc}}=-16\mathrm{mA}$, and 0 K. Dot edges (black lines) present a 50-nm difference due to ion etching [see Fig. 1].

Figure 4

Spectral linewidth of the fundamental and the first harmonics divided by two (filled symbols) and four (open symbols) for the APc (a) and Pc (b) configuration measured at zero applied magnetic field. The autocorrelation function of power fluctuations for both configurations APc (c) and Pc (d) at zero field for $I_{\mathrm{dc}}=-17\mathrm{mA}$.