Vortex dynamics in disks with tailored magnetisations
The fundamental oscillation mode of magnetic vortices in thin-film elements has recently been proposed for designing spin-torque-driven nano-oscillators. 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, Ms, as well as the geometrical confinement of the magnetisation e.g. the diameter and height of a magnetic disk. Micromagnetic simulations have shown that if regions with different saturation magnetisation can be induced in a magnetic disk, multiple precession frequencies can be generated. This work is aimed at partially altering the magnetic properties of the disk containing the magnetic vortex by means of ion irradiation in order to improve frequency tunability within a single disk and thus overcome the main drawback of such structures in view of further integration into potentially commercially-compatible devices. So far, we have designed, fabricated and electrically measured the dynamics of magnetic vortices in confined disks using magnetoresistive detection where we introduced areas of reduced saturation magnetisation in the disks by Chromium ion implantation and investigated the changes in the dynamic properties. The study confirmed that two different output frequencies could be obtained by this method, when having the vortex core precessing in one or the other region.
To achieve these objectives magnetic disks of Permalloy were prepared by means of electron beam lithography followed by electron beam evaporation. A lithography recipe was developed to electrically contact the individual disks. The electrical resistance of a single disk is expected to change based on the relative angle between the magnetisation direction and the applied current (the anisotropic magnetoresistance (AMR) effect). Using the AMR as a detection technique, the dynamics were investigated using the conventional lock-in technique. A concentric disk was exposed within the already existing disk and this exposed resist served as a mask for Chromium ion implantation. Ion implantation parameters were determined and virgin disks were irradiated with Chromium ions in order to reduce the saturation magnetization locally. The high frequency response of the disk was found to change upon irradiation.
The next objective of our work is to analytically model the response of the partially irradiated disks.
Details of the work can be found here: