The influence of dynamic interlayer interactions on the spin torque drivenand damped excitations are illustrated for a three layer macrospin model systemthat corresponds to a standard spin-torque oscillator. The free layer and asynthetic antiferromagnetic (SyF) pinned layer of the spin-torque oscillatorare in-plane magnetized. In order to understand experimental results, numericalsimulations have been performed considering three types of interlayerinteractions: exchange interaction between the two magnetic layers of the SyF,mutual spin torque between the top layer of the SyF and the free layer anddipolar interaction between all three magnetic layers. It will be shown thatthe dynamic dipolar coupling plays a predominant role. First, it leads to ahybridization of the free layer and the SyF linear modes and through this givesrise to a strong field dependence of the critical current. In particular, thereis a field range of enhanced damping in which much higher current is requiredto drive the modes into steady state. This results in a gap in the excitationspectrum. Second, the dynamic dipolar interaction is also responsible for thenon-linear interaction between the current driven steady state mode and thedamped modes of the system. Here one can distinguish: (i) a resonantinteraction that leads to a kink in the frequency-field and frequency-currentdispersions accompanied by a small hysteresis and a reduction of the linewidthof the steady state mode and (ii) a non-resonant interaction that leads to astrong frequency redshift of the damped mode. The results underline the strongimpact of interlayer coupling on the excitation spectra of spin-torqueoscillators and illustrate in a simple three mode model system how in thenon-linear regime the steady state and damped modes influence each other.
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