Vortex dynamics in disks with tailored magnetisations: from single frequency to multiple frequencies

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. Indeed, the fundamental frequency is known to be determined by the saturation magnetisation, M_s , as well as the geometrical confinement of the magnetisation e.g. the diameter and height of a magnetic disk [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].
To study the formation of magnetic vortices with respect to size and thickness, 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. Moreover, to allow for electrical measurements, 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), X-ray magnetic circular dichroism (XMCD) and magnetotransport measurements.
The magnetic field dependence can be tuned by the disk size as shown by XMCD (Figure 1 (a)). Higher magnetic stability due to larger annihilation fields can be achieved by smaller disk diameters whereas larger field sensitivity is present in larger disks (Figure 1 (b)).
Magnetotransport measurements on electrically contacted disks (Figure 2 (a)) show the presence of anisotropic magnetoresistance (AMR) in different disks with varying thickness (Figure 2 (b)). Additionally, to the shown static DC measurements, AC measurements are accessible by a lock-in technique and the resonance frequencies measured for 3 µm and 4 µm radii disks with 25 nm permalloy are 40.9 MHz and 29.5 MHz respectively. In order to alter the saturation magnetisation within defined volumes and to achieve two different oscillation frequencies, a concentric region is irradiated within the actual disk (Figure 2 (c)). After ion irradiation, modified magnetic vortex dynamics are investigated by ac-magnetotransport measurements and the results are supported by micromagnetic simulations.
Acknowledgements:
This work is supported by the Helmholtz Young Investigator Initiative Grant No. VH-N6-1048. Support of the Nanofabrication Facilities of Rossendorf at the Ion Beam Centre is gratefully acknowledged (Dr. Artur Erbe, Bernd Scheumann).