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Spin-Transfer Torque Induced Vortex Dynamics in Fe/Ag/Fe Nanopillars

We performed experimental and analytical work on spin-transfer torque-induced vortex dynamics in metallic nanopillars with in-plane magnetized layers. Our studies involved nanopillars with a diameter of 150 nm, containing two Fe layers with a thickness of 15nm and 30 nm respectively, separated by a 6nm Ag spacer. The sample geometry is such that it allows for the formation of magnetic vortices in the Fe disks.

As confirmed by micromagnetic simulations, we are able to prepare states where one magnetic layer is homogeneously magnetized while the other contains a vortex. We  show experimentally for this configuration that spin-transfer torque can excite vortex dynamics and analyze their dependence on a magnetic field applied in the sample plane. The centre of gyration is continuously dislocated from the disk centre, and the potential changes its shape according to field strength. The latter is reflected in the field dependence of the excitation frequency.

In a second step, we propose a novel mechanism for the excitation of the gyrotropic mode in nanopillars with a perfectly homogeneously magnetized in-plane polarizing layer. We show analytically that in this configuration the vortex can absorb energy from the spin-polarized electric current if the angular spin-transfer efficiency function is asymmetric. This effect is supported by micromagnetic simulations.


Figure (click to enlarge): Single and double vortex states in a nanopillar with a diameter of 150 nm. Experimental (left) and simulated (right) resistance vs. field dependence of a nanopillar under the influence of a DC current. Stars in the left part indicate high frequency excitations, and the symbols in the right part display the magnetization states in different field ranges.

V. Sluka, A. Kákay, A. M. Deac, D. E. Bürgler, R. Hertel, and C. M. Schneider
Spin-Transfer Torque Induced Vortex Dynamics in Fe/Ag/Fe Nanopillars
J. Phys. D: Appl. Phys. 44, 384002 (2011)