Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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, i.e. the diameter and height of the 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. We show that ion implantation [5] is a novel route to fabricate such devices.
Permalloy (Py) disks of various diameters and thicknesses were prepared using electron beam lithography followed by electron beam evaporation. Individual disks were contacted by gold leads to study the interaction of spin-polarized current with 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 of the vortex position 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 show the presence of anisotropic magnetoresistance (AMR) in different disks with varying thickness (Figure 1 (c)).
Using a conventional lock-in technique, the resonance frequencies are measured for disks with different radii as shown in Figure 2 (a), with the inset showing the scanning electron microscope image of an electrically contacted disk. In order to modify the magnetisation within a single disk and to achieve two different oscillation frequencies, we implant chromium in different regions of the disk (inner and outer). Cr-implantation leads to a decrease in the Curie temperature and thus a reduction in the magnetic moment [6]. The reduction of M_s as a function of Cr fluence was optimised on extended Py films using a vibrating sample magnetometer – superconducting quantum interference device (VSM-SQUID), see figure 2 (b). A clear drop in M_s with increasing the chromium ion fluence is observed. Concentric donut-like structures were then implanted with Cr and the modification of dynamics as a function of magnetic field was investigated. An example of Cr implantation in a 3 µm radius disk at 30 keV with a fluence of 1.2×〖10〗^16 ions/cm^2 is shown in Figure 2 (c). The vortex core is shifted between the two different magnetisation regions by applying an external in-plane field. The vortex nucleates in the irradiated region at – 2.281 mT, leading to a resonance frequency of 30.2 MHz (shown in orange in Figure 2 (c)). Further increasing the external field pushes the vortex core to the non-irradiated region where the resonance frequency is 42.3 MHz (shown in green), corresponding to a field of + 1.597 mT. The results show that ion implantation is a novel way to obtain multiple frequencies from a single disk.