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.

The objective of our work is to

  • design, fabricate and electrically measure the dynamics of magnetic vortices in confined disks using magnetoresistive detection
  • introduce areas of reduced saturation magnetisation in the disks by Chromium ion implantation and investigate the changes in the dynamic properties

To achieve these objectives magnetic disks of Permalloy are prepared by means of electron beam lithography followed by electron beam evaporation. A lithography recipe is 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 are investigated using the conventional lock-in technique. A concentric disk will be exposed within the already existing disk and this exposed resist will serve as a mask for Chromium ion implantation. Ion implantation parameters will be determined and virgin disks will be irradiated with Chromium ions in order to reduce the saturation magnetisation. The high frequency response of the disk is expected to change upon irradiation and the modified dynamics will be electrically detected again via the AMR effect.

magnetic vortex ©Copyright: Ramasubramanian, Lakshmi