Ferromagnetic nanorings

Abstract

Ferromagnetic metal rings of nanometre range widths and thicknesses exhibitfundamentally new spin states, switching behaviour and spin dynamics, whichcan be precisely controlled via geometry, material composition and applied field.Following the discovery of the ‘onion state’, which mediates the switching toand between vortex states, a range of fascinating phenomena has been found inthese structures. In this overview of our work on ring elements, we first show howthe geometric parameters of ring elements determine the exact equilibrium spinconfiguration of the domain walls of rings in the onion state, and we show howsuch behaviour can be understood as the result of the competition between theexchange and magnetostatic energy terms. Electron transport provides an extremelysensitive probe of the presence, spatial location and motion of domain walls, whichdetermine the magnetic state in individual rings, while magneto-optical measurementswith high spatial resolution can be used to probe the switching behaviour of ringstructures with very high sensitivity. We illustrate how the ring geometry hasbeen used for the study of a wide variety of magnetic phenomena, including thedisplacement of domain walls by electric currents, magnetoresistance, the strength of thepinning potential introduced by nanometre size constrictions, the effect of thermalexcitations on the equilibrium state and the stochastic nature of switching events.