Ferromagnetic Nanorings

Abstract

Ferromagnetic metal rings of nanometre range widths and thicknesses exhibit fundamentally new spin states, switching behaviour and spin dynamics, which can be precisely controlled via geometry, material composition and applied f eld. Following the discovery of the 'onion state', which mediates the switching to and between vortex states, a range of fascinating phenomena has been found in these structures. In this overview of our work on ring elements, we f rst show how the geometric parameters of ring elements determine the exact equilibrium spin conf guration of the domain walls of rings in the onion state, and we show how such behaviour can be understood as the result of the competition between the exchange and magnetostatic energy terms. Electron transport provides an extremely sensitive probe of the presence, spatial location and motion of domain walls, which determine the magnetic state in individual rings, while magneto-optical measurements with high spatial resolution can be used to probe the switching behaviour of ring structures with very high sensitivity. We illustrate how the ring geometry has been used for the study of a wide variety of magnetic phenomena, including the displacement of domain walls by electric currents, magnetoresistance, the strength of the pinning potential introduced by nanometre size constrictions, the effect of thermal excitations on the equilibrium state and the stochastic nature of switching events.

Effects of Interlayer Coupling in Elongated $\hbox{Ni}_{80}\hbox{Fe}_{20}/\hbox{Au/Co}$ Nanorings

“…Digital Object Identifier 10.1109/TMAG.2009.2038799 domain walls [13]. In this work we systematically investigate the magnetization reversal processes in multilayer elongated nanorings as a function of the thickness of Au by means of MOKE measurements.…”

Strain-Mediated Electrical Control of Magnetization in Micron-Scale Nickel Ring on PMN-Pt

“…It should be noted that there exist multiple magnetic states other than the magnetic onion state, such as the vortex state and further multi-domain states [11]. The width and thickness of the ring largely determine its magnetic state at the micron-scale and nanoscale [6,11]. Crystal anisotropy energy and Zeeman energy are assumed to be zero because (1) the evaporated Ni structure used for experiments is polycrystalline, and (2) no external magnetic field exists after the first initialization of the magnetic onion state [12].…”

“…One interesting magnetic state available to the ring geometry is the magnetic "onion" state, which is observed at the micron-or submicron-scale (Figure 2 (b)). A remnant onion state can be formed under certain geometric and material constraints by applying a saturating magnetic field along an in-plane direction and then releasing the applied field [6]. The onion state possesses two opposite circular magnetizations, and their direction depends on the previous magnetization process.…”

Magnetoresistive system with concentric ferromagnetic asymmetric nanorings

“…The above properties have been confirmed by direct measurements with several experimental techniques. 1,[10][11][12] In electrical spin injection in semiconductors, ferromagnetic electrodes are used to create and detect spin polarized currents in a semiconductor channel. 13 Unlike spin valves and magnetic tunnel junctions, which use multi-layered systems and currents perpendicular to the planes, spin injection devices are usually fabricated in lateral geometries like the Datta-Das spin transistor.…”