Z. Cui

We present the results of a study of the magnetic properties of an array of 34-nm thick Co(100) epitaxial ring magnets, with inner and outer diameters of d(in) = 1.3 microm and d(out) = 1.6 microm, respectively. Magnetic measurements and micromagnetic simulations show that a two step switching process occurs at high fields, indicating the existence of two different stable states. In addition to the vortex state, which occurs at intermediate fields, we have identified a new bi-domain state, which we term the onion state, corresponding to opposite circulation of the magnetization in each half of the ring. The onion state is stable at remanence and undergoes a simple and well characterized nucleation free switching.

Fluorescent CdTe nanocrystal–polymer transparent composites have been fabricated by a combination of aqueous synthesis of nanocrystals, styrene extraction using polymerizable surfactants, and radical polymerization of monomer mixture containing composite nanocrystals. The Figure shows the photoluminescence of transparent CdTe–polymer composites excited by an ultraviolet lamp.

We present a simple method to control the direction of the circulation of the magnetization in mesoscopic ring magnets, using a uniform magnetic field only. The method is based on the nucleation free switching which occurs when the rings switch from the near-saturated state, referred to as the “onion state,” to the flux-closed vortex state. Two possible onion states, forward or reverse magnetized, are possible for a given direction of the magnetic field. Going from the forward or the backward onion state, both local scanning Kerr microscopy measurements and micromagnetic simulations show that the clockwise or the counterclockwise vortex state, respectively, can be selected due to asymmetric pinning of the two domain walls that are present in the onion state.

Using nonintrusive imaging techniques we have directly observed the nanoscopic details of the magnetization configurations of epitaxial and polycrystalline mesoscopic ring structures with a 15-nm resolution. We have found head to head domain walls with different spin structures depending on ring width. Further, we can classify the geometry dependent switching processes according to the number of transitions (single, double, triple) that a ring undergoes in a hysteresis cycle. In the case of triple switching we find a state with a complete vortex core present in the ring.

The magnetic nanostructure of epitaxial fcc Co/Cu(001) circular elements $(\ensuremath{\sim}1.7\ensuremath{\mu}\mathrm{m}$ in diameter) has been imaged with scanning electron microscopy with polarization analysis. The disks are obtained by ultrahigh vacuum deposition of the metal films onto a prepatterned Si(001) substrate. The Si structures are 700 nm high, ensuring that the continuous background film and that of the circular structures are not physically connected. A closed flux configuration (a quadrant configuration) is observed for some of the disks, characteristic of systems with cubic anisotropy. The measured width of the $90\ifmmode^\circ\else\textdegree\fi{}$ domain wall varies from $70\ifmmode\pm\else\textpm\fi{}25\mathrm{nm}$ close to the vortex core, up to $150\ifmmode\pm\else\textpm\fi{}25\mathrm{nm}$ at a normalized distance ${r/r}_{d}\ensuremath{\approx}0.625$ from the vortex core (where ${r}_{d}$ is the domain wall length from the vortex core to the disk periphery), i.e., significantly exceeding the bulk domain wall width, and increasing further with increasing distance from the vortex core. Such a wide domain wall is a consequence of the geometrical constraints imposed by the element, thus defining a geometrically constrained domain wall. This view is supported by detailed micromagnetic simulations that also show that the domain wall width increases dramatically with radial position from the disk center.