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Quenched Slonczewski windmill in spin-torque vortex oscillators

We present a combined analytical and numerical study on double-vortex spin-torque nano-oscillators and describe a mechanism that suppresses the windmill modes. The magnetization dynamics is dominated by the gyrotropic precession of the vortex in one of the ferromagnetic layers. In the other layer, the vortex gyration is strongly damped. The dominating layer for the magnetization dynamics is determined by the sign of the product between sample current and the chiralities. Measurements on Fe/Ag/Fe nanopillars support these findings. The results open up a new perspective for building high quality-factor spin-torque oscillators operating at selectable, well-separated frequency bands.


Figure: Experimental resistance vs. field measurements of a Fe(25 nm)/Ag(6 nm)/Fe(15 nm) nanopillar with a diameter of 230 nm for (a) increasing field at +21 mA and (b) decreasing field at -21 mA. The low-resistance region around B=0 indicates the existence of the double-vortex state. Simultaneously recorded high-frequency spectra yields the excitation frequencies shown in (c) and (d). The frequency ranges are clearly different and well separated for the two current polarities.

V. Sluka, A. Kákay, A. M. Deac, D. E. Bürgler, R. Hertel, and C. M. Schneider
Quenched Slonczewski windmill in spin-torque vortex oscillators
Phys. Rev. B 86, 214422 (2012)