Microscopic theory of spin-wave excitations in ferromagnetic nanorings

A microscopic theory is developed for the spin-wave excitations in ferromagnetic nanorings where the physical dimensions (inner and outer diameters and height) may be of the same order. Both the dipole-dipole and the exchange interactions are included in the Hamiltonian, together with single-ion anisotropy terms and an external magnetic field assumed to be applied either parallel or perpendicular to the ring axis. The equilibrium configurations of the systems are obtained by minimizing numerically the energy functional with respect to the spin orientations. The nanorings can be in bottleneck, twisted, bidomain, or vortex types of configuration. A Green's function method is employed to calculate the dispersion spectra and other spin-wave properties. Applications to real systems, such as nickel nanorings, are made by choosing the parameters appropriately. (C) 2006 American Institute of Physics.