Understanding and controlling the magnetic properties of small ferromagnetic elements is a major challenge in the rapidly evolving field of nanoscale science. These structures not only allow for the investigation of fundamental physical properties but also have important applications in, for example, magnetic memory and sensor devices. One key issue is to understand and control the magnetic switching precisely, and to achieve this well defined and reproducible remanent states are required, as well as a simple and reproducible switching process. A geometry that fulfils these criteria is the ring geometry, and in particular narrow ferromagnetic rings, which have recently become the focus of intense interest. The ring geometry is well suited to the investigation of fundamental magnetic properties such as domain wall trapping and current-induced domain wall displacement, and it has also been suggested for technological applications such as magnetic random access memory (MRAM) and bio-detection.
Fig. 1. Schematics of the onion and vortex states in a ferromagnetic ring.
Magnetic measurements and micromagnetic simulations have shown the existence of two magnetic states (Fig. 1), the ?onion? state, accessible reversibly from saturation and characterized by the presence of opposite head-to-head and tail-to-tail domain walls, and the flux-closure ?vortex? state. Nanoscale magnetic imaging and micromagnetic simulations have revealed that the onion state exists with transverse or vortex walls as shown in Fig. 2. The onion-to-vortex state switching has been found to be a very fast nucleation-free domain wall propagation process. What is more, micromagnetic simulations suggest that magnetization reversal from onion to reverse onion state in narrow cobalt rings can occur on application of a field pulse in the film plane, with switching times of the order of 400 ps.
Fig. 2. Onion state in a wide and in a narrow epitaxial 34-nm fcc Co ring, outer diameters 1.7 micrometres. Scanning electron microscopy with polarization analysis images of the wide (a) and the narrow (c) ring. Corresponding micromagnetic simulations of the wide (b) and the narrow (d) ring, showing the vortex- and transverse-type domain walls.
In addition to showing interesting intrinsic magnetic properties, rings are a useful geometry for the investigation of fundamental domain wall properties. Geometrically confined domain walls in which the spin structure of a wall can be controlled via the lateral dimensions and film thickness have become the focus of intense research recently. To investigate the pinning of domain walls at notches, rings with multiple non-magnetic contacts have been fabricated as shown in Fig. 3. The different properties of transverse and vortex domain walls have been established and magneto-resistance effects associated with the domain walls were found.
Fig. 3. Scanning electron micrograph of a narrow cobalt ring (outer diameter 1.5 micrometres, inner diameter 1.3 micrometres). Clearly visible are six non-magnetic contacts and two notches.
We are currently involved in a joint project with the Massachusetts Institute of Technology (MIT) to develop prototype magneto-electronic devices using the ring geometry. The Cambridge-MIT Institute funds this project, and some further details can be found on their website.
The goal is to fabricate ring-shaped spin valves and tunnel junctions with useful magneto-resistance and controllable magnetic switching properties. A crucial issue for applications is controlling the sense of the switching between the onion and vortex states, and in this context the magneto-optic Kerr effect techniques we have developed are invaluable, since they allow the observation of magnetization reversal in individual micrometre-sized rings and the subsequent determination of the circulation of the vortex states accessed during an applied field cycle. Fig. 4 shows simulated and experimental hysteresis loops for a 2 mm-diameter Permalloy ring and the corresponding spin configurations as determined by micromagnetic simulation.
Fig. 4. Simulated and experimental hysteresis loops from a 2 micrometre-diameter polycrystalline Permalloy ring.
Click here to view a movie of the switching from onion to opposite onion state in a simple macroscopic ring. (You will need the Quicktime player.)
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