UC Santa Cruz

The spin structure in a magnetic dot is studied as a function of the exchange coupling strength and dot size within the semiclassical approximation on a discrete lattice. As the exchange coupling is decreased or the size is increased, the ground state undergoes a phase change from a homogeneous single-domain ferromagnet (HSDF) to a spin vortex. The line separating these two phases has been calculated numerically for small system sizes. The dipolar interaction has been fully included in our calculations. Magnon frequencies in such a dot have also been calculated in both phases by the linearized equation of motion method. These results have also been reproduced from the Fourier transform of the spin autocorrelation function. From the magnon density of states (DOS), it is possible to identify the magnetic phase of the dot, as well as to compute their finite temperature magnetization or vorticity. Furthermore, the magnon modes have been characterized for both the homogeneous ferromagnetic and the vortex phase, and the magnon instability mechanism leading to the vortex-HSDF transition has also been identified. (c) 2007 Elsevier B.V. All rights reserved.