Vortex dynamics in disks with tailored magnetisations

Vortex dynamics in disks with tailored magnetisations

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


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]. Micromagnetic simulations [4] have shown that if regions with different saturation magnetisation can be induced in a magnetic disk, multiple precession frequencies can be generated. Ion implantation is a promising method to fabricate such devices [5].

Disks with different radii- 0.5 µm to 4 µm and thicknesses- 25 nm and 30 nm were prepared using electron beam lithography followed by electron beam evaporation to study the formation of magnetic vortices with respect to size and thickness. The single disks were contacted by gold leads to study the interaction of spin polarized current on the magnetic vortex. The presence of vortex is verified by magneto optic Kerr effect (MOKE) and X-ray magnetic circular dichroism (XMCD).

Magnetotransport measurements on electrically contacted disks (Figure 1 (a)) show the presence of anisotropic magnetoresistance (AMR) in different disks with varying thickness (Figure 1 (b)). The resonance frequencies measured using a lock-in technique on 3 µm and 4 µm radii disks with 25 nm permalloy are 40.9 MHz and 29.5 MHz respectively. Modification of the resonance frequency by ion irradiation will be presented.

Keywords: frequency tunability; magnetic vortex

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