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Vortex dynamics in disks with tailored magnetisations

Ramasubramanian, L.; Fowley, C.; Kákay, A.; Yildirim, O.; Matthes, P.; Böttger, R.; Lindner, J.; Fassbender, J.; Gemming, S.; Schulz, S. E.; Deac, A. M.

Abstract

The fundamental oscillation mode of magnetic vortices in thin-film elements has recently been proposed for designing spin-torque-driven nano-oscillators [1]. Commercial applications require tuning of the output frequency by external parameters, such as applied fields or spin-polarized currents. However, the tunability of vortex-based devices is limited, since the gyrotropic frequency is specific to the individual sample design [2, 3].

Using micromagnetic simulations, [4] we show that if regions with different saturation magnetisation can be induced in a magnetic disk, multiple precession frequencies can be generated. Experimentally we employ ion implantation as a promising method to fabricate such devices [5].

Disks with different radii- 0.5 µm to 4 µm, thicknesses- 25 nm and 30 nm and lateral electrical contacts were prepared using electron beam lithography followed by electron beam evaporation to study the formation of magnetic vortices with respect to size and thickness.

Magnetotransport measurements (Fig. 1(a)) show the presence of anisotropic magnetoresistance (AMR. The resonance frequencies measured using a lock-in technique on 25 nm thick permalloy disks are shown in Fig. 1(b). The disks were subsequently subjected to partial ion irradiation and the induced modification of the resonance frequency will be presented.

The Nanofabrication Facilities Rossendorf at the IBC is gratefully acknowledged.
Funding : Helmholtz Young Investigator Initiative Grant No. VH-N6-1048.

Keywords: frequency tunability; chromium implantation in permalloy; electrically detected dynamics

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