Core-Core Interaction in Spin-Torque Double-Vortex Oscillators

Magnetic vortex states are determined by two binary parameters: The vorticity refers to the sense of rotation of the curling in-plane magnetization; the polarity determines the up or down orientation of the out-of-plane magnetized vortex core [1]. Due to their unique nano-scale properties, magnetic vortices may be used in a wide range of applications. However, to exploit their full potential, ways must be sought to measure the core polarity. Due to the smallness of the vortex core, this has so far been a difficult task requiring elaborate imaging techniques [2-4].
Here we present a combined experimental and numerical study on double vortex oscillators. Our samples are all-metallic nanopillars 150 nm in diameter that contain a Fe(30)/Ag(6)/Fe(15) pseudo spin valve. The geometry of the Fe disks is chosen as to stabilize configurations with two stacked vortices. By applying d.c. cpp-currents, we excite magnetization dynamics corresponding to gyrotropic vortex motion. We show that the coupled dynamics of the two vortices split into a fine structure of modes. As confirmed by micromagnetic simulations, the different frequencies correspond to different vorticity-polarity combinations of the double vortex system. In particular, changes in relative core polarity between the two vortices lead to frequency differences of the order of 100 MHz. Our results therefore suggest a way to measure vortex core polarities by electrical means.
[1] E. Feldtkeller, and H. Thomas, Phys. kondens. Mat. 4 (1965) 8.
[2] T. Shinjo et al., Science 289 (2000) 930.
[3] A. Wachowiak et al., Science 298 (2002) 577.
[4] A. Vansteenkiste et al., Nature Phys. 5 (2009) 332.